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Zebra chip may sound like a fun new black-and-white striped snack, but it’s not. Zebra chip is a bacterial disease that attacks potatoes.  Zebra chip disease is more pronounced on fried potatoes. (USU Extension) Like most bacteria, Candidatus Liberibacter solanacearum doesn’t move around very well on its own. Instead, it lives in the gut of potato psyllids. Potato psyllids are tiny, sap-sucking pests. As they feed, the bacteria move from the insect to the plant, infecting the vascular tissue in both the plant and its tubers.  Potato psyllid adults and nymphs. Adults vector the zebra chip disease bacterium. (USU Extension) Symptoms of zebra chip There are no aboveground symptoms of zebra chip, but potato psyllid feeding causes foliage to turn yellow or purple. It can also cause pink or red discoloration of leaves.
The real symptoms are visible only after you cut into a tuber. The zebra chip bacteria cause potatoes to store sugar, rather than starch. That might sound like a great idea for a new dessert food, but the presence of sugars cause vascular tissue to turn into ugly brown lines. When cooked, these brown lines turn black, hence the name. This condition also reduces crop size by 20 to 50%. Healthy appearing potatoes from plants affected by zebra chip are more likely to sprout while in storage. Seed pieces taken from infected plants either do not sprout at all, or they produce weak, infected plants.
Controlling zebra chip Since zebra chip is carried in by potato psyllids, that’s where you must work to break this disease triangle. Potato psyllids can be trapped with yellow sticky sheets and spinosad can be used to reduce potato psyllid populations. These treatments won’t get rid of all the psyllids, but they will help. Be sure to inspect potato, bean, and pepper plants regularly for signs of psyllids. In commercially grown potato fields, where potato psyllids have been identified, a type of systemic neonicotinoid neurotoxin, called imidacloprid, is applied. [While not yet noted in California, resistance to imidacloprid has been documented in Texas.] Zebra chips might sound like a fun new brand of potato chips, but what they really mean is you need to be on the lookout for potato psyllids as you work and play in the garden.
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In the short days of winter, many of your fruit trees look as though they aren’t doing much of anything. Other than collecting chill hours and working to stay alive, that would be mostly true. As the days begin to lengthen, leaf and flower buds start to swell. But, sometimes, those swellings are something else entirely. Also known as the almond and plum bud gall mite (Acalitus phloecoptes), this pest is native to Europe and the Middle East. As of January 2019, it made its way to California, threatening tens of thousands of plum, pluot, almond, apricot, and many other fruit and nut trees.  Plum stem exhibiting plum bud gall mite galls. Photo courtesy of Susan Casner-Kay. What are plum bud gall mites? Plum bud gall mites are a type of eriophyid mite. Eriophyid mites are a family of microscopic plant parasites. These pests enter stems and buds through lenticels and injury points, and then overwinter under the bark. Very little information is available about this new pest, but knowing what to look for can help you to stop it from spreading.  Microscopic image of eriophyid mite (USU Extension) Plum bud gall mite identification In late winter, galls begin to form around these tiny invaders. By spring, adults emerge from their protective galls. At 1/100th of an inch in length, these mites are too tiny to see with the naked eye. If you have a 20x hand lens, you may be able to see them, if you look very closely. They can be a translucent yellow, pink, white, or purple, with two pairs of legs up near the head. You are more likely to see galls on new shoots and fruit spurs that plants produce in response to these invaders. Galls are warty, bumpy growths that don’t look like normal tissue.  Dissected plum bud gall mite gall. Photo courtesy of Susan Casner-Kay. Controlling plum bud gall mites
Treating your trees with wettable sulfur in March or April, when plum bud gall mites first start to emerge from their protective galls, has been effective in controlling these pests in other regions. Treatments may need to be repeated, depending on the level of infestation. Note that apricot leaves are very sensitive to sulfur, so you can only treat apricot trees with sulfur before leaves emerge. Because these particular eriophyid mites are new to the region, we do not yet know what sort of an impact native predatory insects will have on controlling plum bud gall mite populations. If you happen to see this new pest on your trees, please contact your County Extension Office right away.
Host plants While it might be easier to list the plants not susceptible to beet armyworms, you need to know where to look for these pests. In addition to beets, the list of potential beet armyworm hosts includes beans, celery, cilantro, citrus, cole crops, cucurbits, lettuces, parsley, peppers, strawberries, and tomatoes. Beet armyworms also attack alfalfa and cotton. Beet armyworm lifecycle Female moths lay pale, pinkish or greenish striated eggs in clusters of more than 100 eggs, often on the upper sides of leaves. These clusters look fuzzy, due to hairlike scales left behind by the moth. After they hatch, larvae begin feeding on nearby leaves, slowly dispersing throughout the plant. As larvae get older, they also feed on fruit. After defoliating your plant, the mature larva drops to the ground, where it pupates in a shallow depression in the soil, or in a pocket excavated just below the soil surface. An adult moth emerges, and the whole process begins again. This cycle is completed in one month, so there can be multiple generations each year.  Newly hatched beet armyworm larvae. (UFL) Beet armyworm description Larvae are smooth, pale green caterpillars, with several pale, wavy lines down the back and a broad stripe down either side. You may also see a dark spot above the second pair of legs. Other color variations can occur, depending on the food source and developmental stage. After 2 or 3 weeks of feeding, caterpillars will reach 1.25 inches in length. Adult moths are mottled brown and grey, with a 1-inch wingspan.  Adult beet armyworm moth (UFL) Damage caused by beet armyworms Beet armyworms can destroy seedlings in only minutes. When feeding begins, the damage appears as clusters of circular or irregularly shaped holes in leaves. It can also cause flagging, a condition that slows or halts growth on one side of a plant. Larvae will feed on the crown of lettuce plants, killing them. As caterpillars get bigger, they can skeletonize all the leaves on a plant. Most fruit feeding occurs on or near the surface, and can be cut away, assuming other pathogens haven’t entered the fruit, causing disease or decay. Of course, you will want to wash the fruit thoroughly, to get rid of caterpillar feces. If beet armyworms feed on floral buds, the buds will abort.  Mature beet armyworm larva found feeding inside a pepper fruit. (UCANR) How to control beet armyworms
In the home garden, natural predators are your plants’ best defense against beet armyworms. Predatory wasps will parasitize beet armyworm larvae, while big-eyed bugs, and minute pirate bugs will feed on the eggs. Spiders, damsel bugs, assassin bugs, tachinid flies, and lacewings will also feed on beet armyworms, so avoid using broad spectrum pesticides. In severe cases, you can apply spinosad or a specific type of Bacillus thuringiensis (ssp. aizawai). Prevent beet armyworm invasions by monitoring nearby weeds, especially lambsquarters, goosefoot, and pigweeds for signs of egg clusters. Harvesting your crops as soon as they are ready can also interrupt the lifecycle of these pests. Beet armyworms have been known to travel as far as 10 feet during a night, putting most of your garden plants at risk. Monitoring for signs of beet armyworm infestation can help you prevent the problem from spreading. Bare, dormant stems begin to swell in spring, transforming from green to red tips, from which tight clusters of pink blossom buds emerge. Those buds will bloom, drop their petals, and generate fruit, assuming they have been pollinated. That is, of course, unless blossom brown rot has taken hold.  Blossom brown rot on apricot (UCANR) Blossom brown rot (Monilinia laxa), also known as brown rot blossom blight, is a fungal disease of almonds, apricots, cherries, and other stone fruits. Similar to brown rot (Monilinia fructicola), blossom brown rot can affect flowers from pink bud stage through petal fall. All parts of the flower are susceptible. Symptoms of blossom brown rot The first sign of blossom brown rot is the death of young blossoms. What should be a colorful, flower-laden tree, buzzing with pollinators, looks more like clusters of brown, dried up tissue paper. [That would be an extreme case.] More often, infected flowers are intermittent (at first).  Twig cankers (dark, discolored areas) caused by blossom brown rot (UCANR) Gum may ooze from the base of infected flowers and cankers may form on twigs. Those cankers will have tan centers and dark edges. Blossom spurs and their leaves may collapse. Under humid conditions, you may be able to see tan to grey spore masses.  Blossom brown rot spore masses (UCANR) Blossom brown rot lifecycle Fungal spores overwinter in twig cankers, on mummified fruit, and on any diseased flowers that remain attached to the tree. As temperatures rise in spring, fungal spores begin populating nearby twigs and other blossoms, causing twig and branch dieback, along with blossom losses. Spores are airborne, and spread by irrigation and rain water splash, and by insects.  Leaves remain attached to shoots killed by blossom brown rot (UCANR) How to control blossom brown rot This fungi thrives in rainy weather with temperatures in the 70s. High humidity can also encourage spore development. In fact, this fungi’s growth is almost directly related to humidity and temperature, both of which are difficult to control in the home garden. Proponents of compost tea recommend foliar sprays as a treatment for blossom brown rot, but research has shown that compost tea either has no effect, or that it worsens the condition.  Fruit mummies are a source of fungal disease infection. (UCANR) Unless you want to apply chemical fungicides, you are best off selecting varieties that are resistant to this disease in the first place. In the world of almond trees, the following species are most susceptible to blossom brown rot: Butte, Carmel, Drake, Ne Plus Ultra, Winters, and Wood Colony.
You can also reduce the likelihood of blossom brown rot by removing all mummies, as soon as they are seen, and disposing of them in the garbage. Pruning and training for better air flow can also reduce the amount of time blossoms take to dry. Curly dwarf may sound like the punchline from a bad joke, but this viral disease can ruin your artichoke plants.
Curly dwarf is spread by insects, and can be fatal, so knowing what it looks like can help you keep it from spreading to uninfected plants. While only found on artichokes, in the field, cardoons, sunflowers, and zinnias have been infected in laboratory tests. [Unfortunately, I was unable to find a single image of an artichoke plant infected with curly dwarf, but I will keep looking. Please let us know if you have one!] Curly dwarf, also known as artichoke curly dwarf, is caused by the artichoke curly dwarf virus (ACDV). While very little is currently known about this particular virus, we do know that it is almost found in tandem with another virus (Artichoke latent virus), which seems to have no disease symptoms. Symptoms of curly dwarf Severe stunting, leaf curling, and reduced bud production, with buds remaining small and often misshapen, is a clear indication that your plant has become infected with curly dwarf. Leaves may also have dark, dead areas. Preventing curly dwarf We do not yet know which insects spread curly dwarf, but we do know that it can be transmitted to uninfected plants. For this reason, it is important to remove any infected plants as soon as they are identified. The curly dwarf virus is commonly spread when infected plants are divided for propagation purposes, so only use certified disease-free plants. Since the virus also lives on milk thistle (Silybum marianum), keeping those weeds away from your artichoke plant may reduce the chance of infection. Winter months are an excellent time to prune fruit and nut trees. Naked and dormant, it is easy to see each tree’s structure. This is also a good time to inspect for common pests, such as scale insects and European red mites. While you will certainly want to get rid of any San Jose scale, walnut scale, Italian pear scale, or frosted scale insects you see, you should leave the European red mites where they are.  European red mite adult female (E. Beers, WSU) Why in the world would you want to leave pests on your trees?
Females measure in at 1/72 inch. Males are 1/80 inch, which means you could line up 4 of them on the edge of a dime. Females can lay eggs without mating, but these offspring will all be male. [This is called arrhenotokous parthenogenesis. Most parthenogenic offspring are female, as with aphids, so this is different.]
Heavy feeding can bronze leaves. Bronzing may be fine for baby shoes, but it makes photosynthesis impossible.  Apple foliage bronzing (right) by European red mite (L. Hull, WSU) Whereas other mites produce webbing and cause leaf drop, the European red mite produces little or no webbing and no leaf drop.
Persistent, heavy mite feeding can also cause transpiration burn (leaf blackening), reduced fruit size and quality, shoot growth, trunk and limb growth, and root growth. If populations of European red mites become significant, you can apply delayed dormant horticultural oil, but that oil may cause sunburn damage. It’s a tough call. Since European red mites have demonstrated resistance to miticides (a type of pesticide geared toward mites), it is better to avoid chemical sprays. Spraying these pests with a hose does nothing. If your garden or landscape has a lot of biodiversity, odds are pretty good that there will be enough predators to control European red mite populations. Also, keeping plants dust-free makes the environment less hospitable to these pests. Exocortis is a virus-like disease of citrus tree bark. I say virus-like because it is caused by a particle, not a virus, called the Citrus Exocortis viroid (CEVd). Viroids are the smallest known infectious pathogens, made up of a single, naked strand of RNA. Other diseases caused by viroids include potato tuber spindle disease, avocado sunblotch, and peach latent mosaic.
 Stunting is one symptom of citrus exocortis (UCANR) For one thing, you may also see gum droplets under the loose bark, or stunting. Stunting occurs because nutrients are having a difficult time moving through damaged or exposed vascular bundles. Sunburn damage generally does not cause stunting or gummosis.
Dealing with exocortis You can’t cure exocortis and it is highly contagious. That being said, it probably won’t kill your tree. What it will do is reduce production and make your tree susceptible to other pest and disease problems. Unless you are ready to commit to complete sanitation of shoes, tools, and anything else that might come into contact with an infected tree, its removal is your best option, if only to protect neighboring trees. You may love calico cats (I do!), or have fond memories of calico dresses from a certain prairie-crossing children’s series, but calico in the plant world is something else entirely.  Alfalfa mosaic virus (UCANR) Calico is a viral disease that can infect alfalfa, lentils, potatoes, tomatoes, peas, tobacco, and 600 or so other plants. There are several strains of this virus, most of which are species dependent.
The calico virus prefers warm, sunny days and sap with a slightly alkaline pH of 7–7.5. Research has shown that plants infected with calico causes reduced levels of important plant nutrients such as copper, iron, manganese, and zinc. Symptoms of calico Calico, also known as Lucerne mosaic, or alfalfa mosaic virus (AMV), is easy to spot. Clearly visible in dark green sea of potato plants, you will see a bright yellow patch, or yellow blotching. Infected leaves may look shiny, compared to their healthy neighbors. You may also see wilting or severe stunting. Closer inspection will show dead stems and tubers, or dry, corky areas inside your potato harvest. If your potato plant looks more like a pale yellow Christmas tree, it is probably potato psyllid feeding. How calico is spread Calico is spread by several species of aphid, but potato aphids and green peach aphids are the usual culprits. Infection is normally spread when aphids move from alfalfa, clover, or wheat to potato plants. Infected seeds and pollen can also carry this viral disease, as can parasitic dodder. Infected plants should be removed and tossed in the trash, not the compost pile. To avoid AMV in your potato patch, plant only certified disease-free tubers, keep your potatoes away from clover and alfalfa, and sanitize your tools regularly. Blackened roots, failure to thrive, yellowing leaves, and irregular stunting may all be signs of root rot, but not all root rots are black root rot.  Black root rot of pansy on the right (MSU Extension) Root rot might refer to the cabbage family’s black rot, asparagus’ Fusarium crown and foot rot, or phytophthora root and crown rot, which attacks a wide variety of plants and trees. And then there is black root rot. To tell the difference, you would need a microscope. But, knowing what to watch for can reduce your losses. Black root rot is caused by a fungus (Thielaviopsis basicola). Yes, I know. It’s a strange word. [It is pronounced THEE-lay-vee-OP-sis.] But being able to pronounce the Latin isn’t as important as being able to recognize this plant disease before it spreads. Before looking for symptoms, however, you need to know which plants are susceptible to this fungal disease.  Black root rot of carrots (Public Domain) Black root rot host plants Black root rot is a serious problem for commercial growers of ground covers, cotton, rice, many herbaceous perennials, snapdragons, tobacco, and our holiday poinsettias. Those lovely spring vinca, pansy, and viola plants can all carry this disease to your garden, even though they might look healthy in the store. This is why quarantining new plants is so important. In addition to those nursery crops, black root rot can appear on several of your garden plants, including beans and peas, carrots, citrus, cucurbits, horseradish, lentils, melons, peanuts, soybeans, strawberries and other berries, potatoes and tomatoes. In many cases, you won’t see damage to roots until after harvest.
 Elongated red-purple lesions due black root rot on melon seedling (PSU Extension)  Black root rot of soybeans (U of Ark) Conditions that favor black root rot
Black root rot occurs most often in cool, moist conditions. It is most likely when temperatures are between 55° and 61°F. Black root rot can be spread by fungus gnats and shore flies, and it is more commonly found in alkaline soil, such as we have here in the Bay Area. Fungal spores can also be spread via splashing rain or irrigation water, or on infected flats, containers, and garden tools. Soggy soil, poor drainage, and too much fertilizer all contribute to the likelihood of these soil-borne fungi taking hold of your plants. Preventing black root rot Once infection has become well established, the plant is a goner, so prevention is your only course of action. [Always throw diseased plants in the trash bin.] In severe cases, soil solarization may be needed to prevent infecting the next plants installed in that location. In commercial nurseries, chemical fungicides are used as preventive measures only. The best way to avoid black root rot is to provide plants with good drainage, avoid overwatering and excessive use of fertilizer, and control fungus gnat populations with yellow sticky paper. Acidifying the soil can help somewhat, but soil pH is very difficult to change without ongoing treatments. Crop rotation can also interrupt this disease cycle. Remember, mulching with arborist wood chips is one of the best ways to improve soil structure and drainage, reducing the chance of black root rot finding its way to your garden. What looks like a light dusting of snow may actually be life-threatening pests, called adelgids.  Pine needle adelgids (UCANR) Like their cousins, the aphids, adelgids pierce vascular bundles to suck out nutrient rich fluids. While mature, healthy trees can withstand a mild adelgid infestation, saplings, young trees, and unhealthy trees can be killed by this tiny, soft-bodied pest. Adelgid species Scientists are still trying to nail down adelgid classification. There are 50 known species, all of which are native to the northern hemisphere, though several invasive species have made their way into the southern hemisphere. The most commonly found adelgids in California include the invasive balsam wooly adelgid (from Europe), the Cooley spruce gall adelgid (Adelges cooleyi Gillette), and pine adelgids. Adelgids are commonly found on stone pine and other conifer species, such as pine and spruce. Depending on the host plant, the pests are commonly known as “pine aphids” or “spruce aphids”, respectively, even though they are not actually aphids. [Thanks to my friend, Chuck, I now know that adelgids are also found on apple trees. Thanks, Chuck!]  Cooley spruce gall adelgid adult (UCANR) Aphids vs. adelgids Aphids are significantly larger than adelgids, and they have two structures that adelgids do not: cornicles, and a tail-like cauda. Cornicles are tubes found sticking out of the 5th or 6th abdominal segments. These tubes are used to excrete a defensive chemical wax. Contrary to popular belief, cornicles are not used in honeydew distribution. Adelgids are covered with a dense wooly wax, so it is easy to mistake them for wooly aphids. This white fluff may be found on twigs, needles, bark, or cones.
Adelgid lifecycle Unlike aphids, which reproduce using both eggs and live birth, adelgids only lay eggs. Adelgids generally live for two years and each female can lay from one to several hundred eggs, depending on the species. Adelgid nymphs are called sistentes, which comes from a Latin that means ‘to stand’. When these sistentes overwinter, they are called neosistens. Some adelgid species require six generations to complete their lifecycle, moving between different tree species. Much like the Monarch butterfly, these insect pests do not live long enough to complete migration as individuals. Generally, it is only the immature stage that causes damage.  Twig tip dieback due to adelgid feeding (Dan Foster) Damage caused by adelgids Heavy infestations can cause yellowing, drooping, and dieback of twig tips. As they feed, adelgids release toxins that interfere with the tree’s ability to produce conductive sapwood. Eventually, the tree suffers severe water-stress and dies. These infestations can appear as swollen twigs, galls, or twig dieback. Adelgid galls look like tiny pineapples and can be green, red, or purple. The initial damage is usually seen on the underside of buds, before infestation and damage spread to the entire bud.  Adelgid galls (Dan Foster) Adelgid controls
These pests are easily dislodged with a stream of water from your garden hose, but that only works you see them, which means you have to go outside and look. Beneficial predators, such as lady beetles, green lacewings, and some fly larvae. Horticultural oils can slo be used, but they will discolor spruce tree needles. Infested twigs can be pruned out while they are still green (before adelgids have emerged) and thrown in the trash. Also avoid applying excess nitrogen, which can stimulate vulnerable new growth. The National Park Service estimates that adelgids are responsible for the death of 90% of the mature fir trees found in the Great Smokey Mountains National Park, since this pest’s arrival in 1962. If you have conifers on your property, it is a good idea to inspect them periodically for signs of adelgid infestation.  Black tunnels and feeding damage from potato tuberworms (UCANR) Also known as the potato tuber moth or tobacco splitworm, potato tuberworms (Phthorimaea operculella) love to feed on members of the nightshade family, such as eggplants, tomatoes, peppers, and tobacco, but they prefer potatoes. The potato tuber moth is unique in the moth world in that her ovipositor (egg-laying organ) has sensors that can pick up chemical signals given off by potato plants. [If you are really into this sort of thing, the chemical signal is an amino acid called L-glutamic acid. But don’t worry, there won’t be a quiz.] She doesn’t necessarily have to be on the potato plant to lay her eggs, either, but you can be sure she will be close. These moths are usually seen an hour or two after sunset.
Potato tuberworm description Potato tuberworms are the larval form of a small grayish-brown moth. The adult moth has a 1/2-inch wingspan and dark grey or black markings. At rest, both sets of fringed wings are held close to the body, giving them a slender appearance. Females moths have a distinctive “X” pattern on their forewings when at rest. Eggs are very tiny, oval, and yellowish white. The larvae, or caterpillars, are just under 1/2 an inch in length, and their color can vary, depending on what they are eating, from white or grey, to tan, pink, or yellowish. Larvae have a brown head and prothoracic shield. [A prothoracic shield is the segment just behind the head.] Cocoons are 1/2 an inch long and pale colored.  Potato tuberworm larva (David Jones) Potato tuberworm lifecycle Each female potato tuber moth will lay over 200 eggs in her short lifetime. Those eggs are normally laid next to a leaf vein, near a bud, or under a stem, though they can also be found in the soil near a host plant. In five days, those eggs will hatch. For the next two weeks, the larvae will eat as much as they can. The way they decide where to feed may surprise you. This is not a simple case of taking bites out of whatever is at hand. Nay, nay! Our newly hatched potato tuberworm larva will spend the first 5 to 15 minutes of its life walking around on its home plant. As it walks, it attaches a silk thread to the plant every few steps, turning this way and that way, taking an occasional bite as it meanders. If it has hatched on an unacceptable plant, the larva will walk faster and ultimately leave the plant altogether, until it can find an acceptable host plant.
Damage caused by potato tuberworms
Young potato tuberworm larvae might burrow through leaves and stems, causing stunting and reduced crop size. As feeding and tunneling continue, the tuberworms head for their favorite food: your potatoes. Webbing and frass (bug poop) deposits can be seen at entry holes, normally found at the eyes of a potato. While other pesky tunneling insects, such as wireworms and leaf miners, tend to keep their tunnels neat and tidy, potato tuberworms are slobs. Those dark tunnels are filled with excrement. Controlling potato tuberworms Row covers can be used to prevent adult moths from laying eggs on your potato plants. The deeper your potatoes are growing, the more difficult it is for tuberworms to get to them, so selecting a deep growing variety is helpful if you know tuberworms are around. Also, avoid furrow irrigation, which can cause cracks in the soil. These cracks are used as elevators to lower soil levels by tuberworms. Research has shown that insecticides do not prevent potato tuberworm infestations when erosion or soil cracks are present, or when potatoes are left in the ground longer than is necessary. Finally, harvest potatoes as soon as they are ready. Infested potatoes should be thrown in the trash and not added to the compost pile. Heavy infestations can be treated with spinosad. Scabby potatoes? Yuck! What causes this condition, and how can it be prevented?  Potato scab (UMass Extension) First classified as a fungal disease, we now know that potato scab is a bacterial disease caused by Streptomyces scabies. There are other strains of Streptomyces that cause other potato diseases. S. scabies is found in the soil pretty much any place potatoes are grown. This bacterium can infect young seedlings of any plant, but it is most commonly associated with root and tuber crops, especially potatoes. Potato skins Delicious twice-baked and cut into wedges, served with sour cream and butter, potato skins are actually the cork, or periderm, layer normally found underneath bark. This layer normally provides protection from pests and disease. You may see tiny nicks of color in a potato’s skin. These are called lenticels and are used for respiration. This is also where the S. scabies bacterium gets in and starts infecting a potato. Symptoms of potato scab After entering a potato through a lenticel or wound site, S. scabies start setting up house. As they feed and reproduce, these bacteria release toxins into the surrounding plant tissue. The first sign of potato scab is nothing more than reddish-brown spots on the potato skin. These spots expand as the potato grows, becoming corky and necrotic. Then, the bacteria start reproducing (sporulating) in earnest, producing different types of lesions, depending on host resistance, time of infection, the aggressiveness of the bacterial strain, and other environmental conditions. There are three basic types of lesions caused by potato scab: russet, erumpent, and pitted.
How to prevent potato scab
Being a seed and soil borne bacteria, potato scab is best prevented by manipulating soil moisture, soil texture, and soil pH, and planting healthy stock You won’t get rid of the bacteria completely, but you can significantly reduce their numbers with these tips:
You can still eat potatoes infected with potato scab, but you should probably cut out the lesions and toss them in the trash.  Potato psyllid adults and nymphs (USU Extension) Potato psyllids (Bactericera cockerelli) are disease-carrying, life-sucking plant lice. These invasive pests also feed on tomatoes and other members of the nightshade family, along with several other garden plants. Potato psyllid description Potato psyllids are tiny. When I say tiny, I mean that an adult potato psyllid could stretch out comfortably across the edge of an American nickel, without dangling. If you get close enough, preferably with a hand lens or magnifying glass, you would see that they look like miniature cicadas. Potato psyllid adults are black, with a white band across the first abdominal segment and an inverted “V” on the final segment. They have clear wings that are held roof-like over the body when not flying or jumping. [They jump a lot.] Potato psyllid lifecycle Potato psyllids start out as eggs. Each female lays approximately 200 eggs, each of which hatches in 6 to 10 days. Those eggs look like microscopic footballs held to the underside of leaves with short stalks. [Do not mistake those short-stalked eggs to the longer stalked, beneficial lacewing eggs.]  Potato psyllid eggs (UCANR)  Lacewing eggs on apple leaf (NCSU Extension) After those eggs hatch into green, fringed nymphs, they look more like whiteflies or soft scale insects. Then, they go through five developmental stages, also known as molts or instars. Under ideal conditions, all that growing can be completed in less than two weeks.  Potato psyllid nymphs (UCANR) Damage caused by potato psyllids If sucking nutrient rich plant fluids wasn’t problem enough, potato psyllids cause other problems, too. For one thing, as nymphs feed, they release a toxin that can kill young transplants. This toxin also causes upward curling of leaflets closest to the stem on the upper portions of the plant. This condition is known as “psyllid yellows” or “vein greening”. The characteristic yellowing usually starts along leaf margins and then moves inward, turning purple in some cases. As this condition worsens, nodes [bumps where leaves emerge] become enlarged and closer together, rosetted clusters of leaves emerge from axillary (or lateral) buds, and aerial tubers begin to form. Aerial tubers grow at the end of aboveground stems, as opposed to underground stems, the way proper potatoes grow. When this pest feeds on tomato plants, it can cause no fruit production or overproduction of poor quality fruits.  Psyllid yellows caused by potato psyllid feeding (USU Extension) Eventually, the once green, bushy potato plant looks more like a pitiful yellow Christmas tree. [If chlorosis is spotty and leaf rosetting is not present, the problem is more likely to be calico virus.] If potato psyllids are removed from the plant, the condition will stop progressing.
Potato psyllids are also carriers of another condition, known as zebra chip. Zebra chip is a bacterial disease that causes potatoes to store sugar, rather than starch. That might sound like a great idea for a new dessert food, but the presence of sugars cause ugly brown lines across the length of the potato. When cooked, these brown lines turn black, hence the name. This condition reduces crop size by 20 to 50%. Healthy appearing potatoes from plants affected by zebra chip are more likely to sprout while in storage. Managing potato psyllids You can’t control potato psyllids if you don’t know where they are. The first step to managing potato psyllids is to use yellow sticky traps. You can buy these at any garden center, or you can make your own with some yellow paperboard and sticky barrier goo. You should also inspect the undersides of leaves, looking for nymphs. While you’re at it, you should probably check the underside of any nearby bean or pepper plants, as these may also become infested. In commercially grown potato fields, where potato psyllid is known to occur, a type of systemic neonicotinoid neurotoxin, called imidacloprid, is applied. [While not yet noted in California, resistance to imidacloprid has been documented in Texas.] Organic growers, like myself, use spinosad. Because potato psyllids are not native to California, our local team of predators, which include lady beetles, lacewing larvae, and minute pirate bugs, have not been very effective at controlling this pest. Not yet, anyway. Aphids on potatoes? Well, why not? They’re on everything else! Potatoes are susceptible to two different types of aphids: green peach aphids and potato aphids. Today, we will learn about potato aphids.  Pink and green forms of potato aphids (UCANR) Originally from North America, these pests are now found everywhere potatoes are grown. And potatoes are not their only food of choice. Your cabbages, tomatoes, eggplant, and peppers are also at risk, along with many other food crops. Potato aphid description Potato aphids (Macrosiphum euphorbiae) can be either green or pink, with a dark dorsal stripe, and they tend to be larger, with longer legs, than most other aphid species. When feeding on tomatoes, potato aphids become distinctly red. They have the same long-legged, soft, pear-shaped wingless body of other aphids. As populations boom, or food becomes otherwise scarce, some aphids will develop wings with which to fly to new feeding grounds.  Various life stages of potato aphids (Whitney Cranshaw, CSU, bugwood.org) Potato aphid lifecycle
Potato aphids, like other aphids, are phenomenally prolific. A single female aphid can produce 600 billion descendants in a single season. Aphids reproduce both sexually and asexually. When females produce offspring without male intervention (parthenogenesis), the offspring are born live and significantly smaller than their co-authored siblings. When reproduction involves a male counterpart, offspring are laid as eggs that overwinter in nearby weeds, or on other host plants. Adult aphids molt four times, leaving behind telltale white skins. Damage caused by potato aphids Aphid feeding is usually first seen as deformed leaves. As aphids feed, they damage plant tissue and disrupt the balance of growth hormones. This can reduce or eliminate crop size, and it can kill young plants. These sap sucking pests tend to cluster together, piercing plant tissue and sucking out nutrient rich fluids. They also poop out sugary honeydew, which attracts protective, disease-carrying ants, and creates habitat for sooty mold. Potato aphid feeding can certainly weaken plants, but the real problem is that these aphids carry and transmit a number of viral diseases, such as cucumber mosaic, lettuce mosaic, bearded iris mosaic, narcissus yellow stripe virus, tulip breaking virus, potato virus Y, beet mild yellowing virus, beet yellows virus, alfalfa mosaic, and potato leafroll disease. Plants infected with potato leafroll disease will produce potatoes with a network of browning phloem tissue, called net necrosis, that is very unappetizing. Once a potato plant is infected with leafroll, it and three plants in all directions should be removed to prevent further spread of the disease. Controlling potato aphids The battle against aphids in the garden never ends. It starts by monitoring plants regularly for signs of infestation. Potato aphids tend to prefer the lower portions of plants, the undersides of leaves, and around new buds. You can dislodge aphids with a powerful stream of water from the garden hose, but it is practically impossible to get every single aphid off your potato plants in this way, and it only takes one aphid to start the whole process over again. Insecticidal soaps can be used with better results, but you have to make sure you wet every surface of the plant. Personally, I wipe them off whenever I see them. I like to think it slows them down a little, if nothing else. The next step in controlling potato aphids is to remove nearby plants that might harbor these pests. This means keeping weeds away from potato patch. Malva, penny cress, and various mustards, in particular, can act as early season host plants for this pest. Luckily, lady beetles, lacewings, syrphid or hoverfly larvae, and parasitic wasps will all help control potato aphid populations. That’s assuming you haven’t used broad spectrum pesticides and wiped out your helpers. What's eating your potatoes? From 1845 through 1852, over one million residents of Ireland starved to death, and another two million were forced to emigrate elsewhere, all because of potato blight. Before you lose your crop to potato blight, let’s learn more about this tiny water mold.  Potato infected with late blight (Public Domain) Water mold In the world of scientific classification, water molds are a type of mostly land dwelling organisms called oomycetes. Oomycetes fall between fungi and algae. These pathogens attack stems, roots, and tubers, and frequently kill host plants. Common water mold diseases include phytophthora tentaculata, crown rot, damping off disease, sudden oak death, and potato blight. Potato blight, also known as late blight, is caused by a specific oomycete called Phytophthora infestans. The word phytophthora means ‘plant killer’, and rightfully so. The Great Potato Famine The pathogen responsible for potato blight was first identified in 1843, in New York and Philadelphia. Wind then spread the spores throughout neighboring regions. Since potatoes weren’t found in North America until the 1500s, and then not grown regularly until the 1700s, potato blight wasn’t seen as a serious threat to anyone. Then, when seed potatoes were sent to Belgium in 1845, all hell broke loose for potato farmers across Europe. Ireland was hit the hardest in what became known as the Great Famine, or the Great Starvation. Since monoculture of a single potato species was common practice at the time, it wasn’t difficult for this disease to take hold.  Late blight on potato stems and petioles (UCANR) Potato blight lifecycle The potato blight pathogen prefers cool, moist environments, which Ireland has in abundance. Spores are produced 54°F to 65°F, while lesions develop when temperatures are between 64°F and 75°F. And it takes surprisingly little moisture to create a water mold habitat. Morning dew on a leaf is all it takes, though more water is preferable. These pathogens can also attack other members of the nightshade family, such as tomatoes, though another disease, called early blight (Alternaria solani) is often the culprit on tomatoes. .png) Lifecycle of Phytophthora infestans on potato (M. Piepenbring) Water mold reproduction is odd. [Remember, oomycetes fall somewhere between algae and fungi.] Water mold reproduction starts with an asexual phase during which branching structures, called hypha, grow, followed by spore development. Then, the receptacle where spores develop, called sporangia, begin to germinate, much the way pollen granules germinate in fertilization. Then, our tiny water mold grows more hypha, and the process continues. Sexual reproduction occurs when two mating types meet. Symptoms of potato blight Potato blight symptoms start out as small, dark green, irregularly shaped, water-soaked spots on leaves, stems, petioles, and tubers. These spots have a yellowish halo. These lesions expand rapidly when moisture is present, turning purplish brown. Grayish white fuzz can also be seen on the underside of leaves as spores develop.
A special group of genetically modified potatoes has been developed with a resistance to potato blight. These cisgenic potatoes appear unable to catch the disease. If you prefer not growing genetically modified plants, there are other ways to prevent potato blight from taking hold.  Potatoes exposed to Phytophthora infestans. Normal potatoes have blight but the cisgenic potatoes are healthy. (Ronald Hutton) How to prevent potato blight
Fixed copper sprays are the best preventative measures against potato blight. In fact, during WWII, when copper was being used to make artillery shells, farmers faced new threats from potato blight because they were unable to spray their fields. Potato blight can find its way into your potato bed through contaminated potatoes, visitors and materials which have come from areas infested with the pathogen, and by rain or irrigation water splashing from contaminated plants to healthy plants. These are excellent reasons for quarantining new plants and avoiding the use of grocery store produce as a plant source. [Just because a plant is healthy enough to eat now does not mean it isn’t carrying diseases that may stay in your soil for years.] Excess moisture should be avoided in areas susceptible to potato blight. This means allowing the soil to dry out between waterings, pruning for good air flow, and adding organic material to the soil to improve drainage. A healthy potato bed is a thing of beauty. Let’s keep it that way! While it might be fun to imagine tiny worms wearing hardhats and utility belts, there's nothing cute about carpenterworms.  Carpenterworm larva (MSU Extension) Carpenterworms (Prionoxystus robiniae) are the larval form of a common moth, and they love to burrow into apricot and pear trees. They can also be found in many ornamental trees, such as maple, oak, birch, cottonwood, ash, and willow. Once these pests are inside your trees, they can be difficult to evict. As you can see, this is a robust caterpillar. They can be 1/2 an inch in diameter and 2 to 3 inches long. They have a dark, brownish head and a yellowish white body that is covered with fine hairs. They have sharp, hooked legs on the middle section (thorax) and distinct fleshy legs on the abdomen Damage caused by carpenterworms These wood-boring insects live in galleries, feeding on sapwood. Knowing the signs of infestation can help you get a handle on this pest before the damage becomes irreversible. The galleries created by carpenterworm feeding tend to be vertical, except for the entrance. These entrances are often found in branch crotches and in bark crevices. Tunnels are 1/2 an inch in diameter and 6 to 10 inches long. This tunneling creates points of entry for many other pests and diseases. Adult female carpenterworms seem to prefer areas that are already infested for egg laying, which can result in multiple galleries in the same area of the same tree. All that feeding and tunneling can weaken branches, making them more likely to break in strong winds, or when supporting heavy crops. Branches can also become girdled by carpenterworm feeding and tunneling, and die. Carpenterworm description Adult carpenterworms are large, mottled grey moths that can have a 3-inch wingspan. Their coloration blends with tree bark and lichen. This camouflage makes them difficult to see. If you are able to catch one and spread out its wings, you wing be able to see if you have a male, with orange hind wings, or a female, with off-white hind wings.  Male carpenterworm adult (UCANR)
Carpenterworm lifecycle Because adult female carpenter moths cannot fly very far, they tend to lay their eggs near the gallery where they were feeding. Three to six eggs are laid in the crevices near an existing gallery entrance. Upon hatching, the larvae immediate start boring into the sapwood, leaving small, rectangular entrance holes. As they feed, the larvae will occasionally push sawdust and frass (bug poop) out of the ever-widening entry. The larvae will feed on the sapwood and hardwood until they reach maturity, molting 8 to 31 times over the next 2 to 4 years. Finally, mature pupae wriggle their pudgy selves to the entry hole and create a protective pupal case, which will block the hole until adult moths emerge. In California, this usually occurs May through July. As soon as adults start flying, they mate and the cycle continues.  Carpenterworm pupal case (UCANR) Signs of carpenterworm infestation The first sign of carpenterworm infestation is stained areas on the trunk. These stains are a combination of sap, sawdust, and frass. You may also see pupal cases sticking 2/3rds of the way out of the tree. Since the stained areas and branch dieback may also be caused by clearwing moths/currant borers, flatheaded borers, bark beetles, and longhorned borers, it is important to identify the pest before trying to control the problem.  Walnut infested by carpenterworms (UCANR) Carpenterworm controls
Healthy trees are better able to protect themselves, so start by planting trees in the right location, at the proper depth, with regular fertilization and irrigation. Because these caterpillars are already protected by the tree, insecticides do not work. There are a couple of specific nematodes, Steinernema feltiae or S. carpocapsae, that have been very successful at controlling carpenterworm larvae. Before you place your order, however, make sure that these are exactly the type of beneficial nematodes you are buying. Any other variety will be ineffective against carpenterworms. And be sure to follow the package directions exactly, or you will have wasted your money. Small infestations can sometimes be controlled by poking long, sharp, flexible wires into the galleries and skewering the caterpillars. This is tricky because you really can’t see if you killed them or not. The only way to really know is to clear all the frass and other debris away from the area and mark the spot with some paint. Then, check the area every week for signs of frass. If frass and sawdust are seen, you missed and the caterpillar is still alive and busy feeding and burrowing. Heavy infestations are dangerous and should be left to a professional arborist. This is because tree branches that are compromised this badly are very likely to fall on you. Since none of us are exempt from the laws of physics, and heavy branches can paralyze or kill you, stay away from them, and call an expert. This winter, take a few minutes each week to inspect your trees for signs of frass and sawdust, or pupal cases, and cut those cute little, hardhat wearing pests off at the knees. Glass snails, such as this whimsical piece by GlassBorisov, are a delightful way to add art and color to houseplants and your garden.  Glass snail by GlassBorisov The same is generally not true of real snails. Yesterday morning, after our first rain of the year, I noticed a snail trail. Snail trails are pretty normal in most gardens, but this one ended with a flat-bodied snail I’d never seen before.  Glass-snail in hand (Kate Russell) Introducing, the glass-snail family (Oxychilidae). Glass-snails get their name because their shells are translucent. Looking closely at my discovery, I could see the snail body through the shell! Glass-snails are land snails that breathe air. Unlike marine snails, which breath using a single gill, land snails have evolved a single, simple lung. Most glass-snails are omnivores. They eat everything: live plants, dead plants, dead animals, insects, poop, other slugs and snails (and their eggs), sowbugs, and earthworms. Sorting out glass snails Specific characteristics are used when comparing different snail species, including height, width, number of whorls, and the umbilicus. The umbilicus is the snail’s bellybutton. It is the tiny opening at the center of the whorls on the underside of the snail’s shell. Only three glass snails are found in California, at this time: cellar snails, garlic snails, and Drapernaud’s snail. [I'm not sure if my guest is a cellar or Drapernaud's glass-snail, but I'll keep you posted.]  Cellar glass-snail (UCANR) Cellar glass-snails Cellar glass-snails (Oxychilus cellarius) have shiny, translucent yellowish-brown shells are just under 1/2 an inch wide, 1/6 of an inch tall, with 5-1/2 to 6 whorls. The umbilicus is very narrow. The snail itself is bluish-grey, with small brown freckles and a groove that runs along each side of the foot  Garlic snail shells (H. Zell) Garlic snail The garlic glass-snail (Oxychilus alliarius) gets its Latin name, a twist on the onion family (Allium), because they emit a garlic odor when disturbed. Originally from the Netherlands, Great Britain, Ireland, Poland, and the Czech Republic, the garlic snail has spread its range to include Columbia, Latvia, and California. Garlic snails are reddish or greenish brown, and the snail is blackish blue. The shell is 1/4 of an inch in diameter, a little more than 1/8 of an inch high, with 4 or 4-1/2 slightly convex whorls. The umbilicus is 1/6 of the overall diameter, and the whorls are coiled more narrowly than cellar glass-snails.  Drapernaud’s glass-snail (Michal Maňas) Drapernaud’s glass-snail
Drapernaud’s glass-snail (Oxychilus draparnaudi) is larger than the other glass snails, being slightly more than 1/2 an inch in diameter, and the shell is a waxy yellowish-brown on top and somewhat lighter underneath. The body is a dark blue and grey color. Drapernaud’s glass-snail is carnivorous. How to control snails Unless you are enjoying artistic versions of this common pest, managing snails is an ongoing task. It comes as no surprise that these snails’ peak breeding season in the Bay Area is autumn, just before our rainy season begins. The first step in snail management is to inspect and quarantine new plants. A single snail can lay over 400 eggs. Putting new plants into isolation for a couple of days, with a beer trap nearby, can prevent years of frustration. Once infestation occurs, try to reduce hiding places, such as boards, stones, and other debris. Regularly applying slug and snail bait, and using beer traps, can take a big bite out of the snail population, before they start taking bites out of your plants. Going outside with a flashlight at night, you can catch them feeding - handpick them and feed them to your chickens or dispose of them in the trash. Did you know that snails have a powerful sense of smell? Now you know. Mealybugs have been around for a long time. There is a relatively new, invasive mealybug that may be attacking your grapes.  Grape cluster infested with vine mealybugs (UCANR) Traditionally, California grape growers have had to watch for grape mealybugs, obscure mealybugs, and long-tailed mealybugs. These species generally do not cause significant problems, as long as their populations do not get out of hand. They are easy to recognize because of the clusters of grey, soft-bodied females gathering on the underside of leaves and in nooks and crannies. The invasive vine mealybug is another problem altogether. Vine mealybugs (Planococcus ficus) are native to the Mediterranean areas of North and South Africa and Europe. Vine mealybugs were first seen in California in the mid-1990s and had spread to 17 California counties by 2011. Vine mealybugs are now considered a significant pest of grapes, figs, avocado, apple, bananas, mango, citrus, date palm, and several ornamental plants.  Vine mealybug feeding on grape leaf (UCANR) Vine mealybugs are difficult to see because they spend most of their lives protected under the bark, on roots, and around developing buds. Only during spring, when they become active again, can you sometimes see them moving away from the roots and trunk and into the leaf canopy. By summer, vine mealybugs may be found under the bark of first- and second-year canes, among fruit clusters, and under leaves. Sometimes, ants can be seen providing the mealybugs with transportation to their summer feeding grounds. Vine mealybug description Vine mealybug females are 1/8 of an inch long, pink, oval-shaped, and covered with a white, mealy wax that also covers filaments (spines) along the sides and posterior end. These filaments are shorter than those seen on other mealybugs, and there are no long tail filaments. Like their cousins, vine mealybugs have a segmented body. Males are tiny, winged, and you’ll probably never see them, unless you have a 30x microscope. They are 0.7 inches long, amber colored, with beaded antennae, one pair of wings, and 4 tail filaments that may stick together. It is important to know which mealybugs you are dealing with. If you see mealybugs, try to collect some and place them in a sealed plastic bag, or in a container of alcohol, and take them to your local County Extension Office for identification. This also helps authorities better understand the spread of this invasive pest.  Male and female vine mealybugs (UCANR) Vine mealybug lifecycle In summer, females lay 300 to 700 eggs in the leaves above the fruit in little pouches, called ovisacs. First instar nymphs, called crawlers, are orange and very tiny. During winter, only nymphs are present. They can be found hiding under the bark around the graft union, below the base of spurs, and around pruning wounds. There can be 3 to 7 generations a year.  Vine mealybug ovisacs and crawlers (UCANR) Damage caused by vine mealybugs Vine mealybugs are phloem sap suckers that produce significantly more honeydew than native mealybugs. This honeydew attracts protective, disease-carrying ants and creates a growth medium for sooty mold on fruit clusters. These invasive pests can also carry grapevine leafroll viruses and corky bark disease. Vine mealybugs reproduce at a much faster rate than their native cousins. How to control vine mealybugs Being an invasive pest, vine mealybugs do not have as a many natural predators as their native cousins. Because vine mealybugs are such a serious threat to California grape growers, parasites of these particular mealybugs have been released in the state. This has helped somewhat, but eradication appears to be impossible at this point. Since these beneficial insects are unavailable to the home grower, the best things you can do to protect your vines is to inspect them regularly, especially during spring, monitor and control ant traffic with sticky barriers, and to quarantine new vines and other plants before installing them. Also, sanitize your tools regularly. Vine mealybugs also feed on burclover, malva, black nightshade, sowthistle and lambsquarters, so controlling these weeds can also help prevent infestation.  Vine mealybug infestation at the base of a cane (UCANR)
Protecting your grapevine from vine mealybugs is an important step toward providing your family with fresh, delicious, organic, homegrown grapes.
You’ve proba-bly never heard of proba bugs. They are another relatively new pest on the California scene. And they love artichokes. Proba bugs (Proba californica) have been around for some time, but they used to prefer coyote brush. Coyote brush is a common native plant found along highways in agricultural areas of California. At some point (around 1997) a proba bug decided to give artichokes a try. From that moment on, proba bugs have become an increasing threat to artichoke plants. So, what do they look like?  Proba bug description Adult proba bugs are plain brown and only 0.2 inches long. [That means you could line up 3-1/2 proba bugs across the top of a dime.] Nymphs start out looking like pale yellowish green aphids, except that they move a lot faster than aphids, due to their long legs. During the next to developmental stages (instars) they are reddish-brown, and then they develop light and dark bands on they abdominal area during the final two instars. [I couldn't find any usable photos of proba bug nymphs - sorry!] Proba bug lifecycle Proba bugs are active year round (just a lot slower in winter). As temperatures begin to rise, usually in March, they begin feeding and breeding in earnest. Eggs are laid on artichoke petioles (leaf stems) and hatch within 20 to 30 days. Nymphs go through five instars before reaching adulthood.  Damaged foliage, stunted buds, and twisted bud stalks of artichoke caused by Proba californica (UCANR) Damage caused by proba bugs
The damage caused by proba bugs is similar to that of lygus bugs, only proba bugs are more aggressive in their feeding habits. Adults and nymphs feed on young artichoke leaves and at the base of developing buds. They feed by piercing the tissue and injecting a toxin that kills plant cells. As the surrounding leaf tissue continues to grow, these punctured areas turn into brown dead spots that dry and fall off, leaving a shot hole appearance. Feeding on the base of flower buds causes the bud [the part we eat] to turn black. Not very appetizing. This phytotoxin also causes stunting and deformed flower buds. Severely affected leaves will be smaller than normal and chlorotic. Controlling proba bugs Until relatively recently, commercial artichoke fields were treated with organophosphates and chlorinated hydrocarbon insecticides. Use of these neurotoxins is being phased out, so proba bugs are becoming more of a problem. Infested fields can lose 20 to 30% of the harvest to proba bugs. Farmers are now removing the coyote brush near their fields and tilling the crop residue under, in a practice called stumping, to help combat this pest. Natural predators, such as big-eyed bugs, damsel bugs, minute pirate bugs, and spiders all feed on the nymph stage of proba bugs, so avoid using broad spectrum pesticides. You can help protect your artichoke plant by cutting the plant off at ground level, once flower production is done for the year, and monitoring for signs of infestation in March and April. It is unusual for a new disease or pathogen to be discovered. It is even more rare when a new disease is found to be caused by a common pest. This is Fusarium dieback, and it can kill your trees.  Gumming caused by Fusarium dieback (UCANR) Fusarium dieback is a fungal disease carried by invasive borers. As borers burrow into trees, they carry three different fungal pathogens with them. These fungi form colonies within a tree’s vascular system, blocking the flow of water and nutrients. Trees infected with Fusarium dieback must be destroyed and disposed of by professional arborists. Symptoms of Fusarium dieback Since this disease affects many different types of trees, and is caused by different fungi, it is no wonder that there are different symptoms. Infected avocado trees, for example, will exhibit sawdust-like frass (bug poop), gumming, and sugar volcanoes. Sugar volcanoes are white discharges of sugary sap. On other host trees, you may see dark, greasy looking areas on the bark, withered leaves and stem tips, and the presence of white mycelium under the bark. Mycelia are the vegetative growths of fungi.  Sugar volcano on avocado infected with Fusarium dieback (UCANR) As the infestation progresses, perfectly round, tiny borer entry and exit holes may become visible. These holes are only 0.03 inches in diameter, so you have to look very closely, usually just below areas showing symptoms of disease. Eventually, you will start seeing branches die. If you cut into an infected branch, you will see that the wood is discolored, brown or black. If you scrape the bark away from entry or exit holes, you will also see discoloration. After cutting, be sure to disinfect your tools with a 5% bleach solution or bathroom disinfectant, to avoid spreading the disease. Once Fusarium dieback has infected a tree, the wounds and weakened condition of the tree make it susceptible to many other fungal infections and other diseases. Disease background Originally found in Israel, Fusarium dieback was first seen in Southern California in 2003. The carrier was believed to be the tea shot hole borer, a common pest of tea plants in Sri Lanka. DNA testing, however, showed that this was an entirely new species, now named the polyphagous shot hole borer. By 2010, this borer, and the disease it carries, had become a serious threat to SoCal’s box elder trees, palm trees, black locust, and our beloved avocado trees. In 2015, a second variety of carrier, the Kuroshio shot hole borer, joined the party and started infecting the popular California palms (Washingtonia filifera). It is estimated that this disease now threatens 25% of all the trees lining Southern California’s streets.  Female polyphagous shot hole borer beetles on snow and sand (UCANR) Beetle description To the naked eye, the two species of borer responsible for Fusarium dieback look identical. Female beetles are tiny and black, only 0.07 to 0.1 inches long. Males are brown and even smaller, at only 0.06 inches long. Female beetles can fly and will leave their birthplace to find other host trees to use as nurseries, carrying the disease with them when they go. Males do not fly and generally stay in the tree of their birth. These beetles are most active during summer and fall. Fusarium dieback hosts Shot hole borers can be found feeding on and breeding in over 200 species of woody plants. That’s a lot of potential hosts. To date, the disease has been found in more than 130 different host species. While this disease prefers palms and ornamentals, such as maple, birch, and tulip trees, it is becoming a serious threat to avocados and California live oaks. California bay laurel, carob, chestnut, elderberries, figs, olive, peaches, persimmons, pineapple guava, pistachios, and pomegranate are also vulnerable to Fusarium dieback. How infection occurs This disease starts when female beetles bore into tree trunks and branches, creating galleries of tunnels. Within the tunnels, chambers are built for eggs. Female beetles have developed a symbiotic relationship with three different fungi, which they carry around in their mouths, much the way we carry around bacteria in our gut. [The fungal pathogens of Fusarium dieback are Fusarium euwallaceae, Graphium euwallaceae and Paracremonium pembeum, if you enjoy the Latin.]  Fusarium dieback infection darkens borer galleries - note the borer larva at the tip of the knife (UCANR) The fungi that set up housekeeping within the tree end up being food for the newly hatched beetle larvae. By eating the fungi, the larvae then become carriers of the disease. These fungal colonies develop very rapidly, once they are inside a tree, and there is no known treatment at this time. Complicating matters even more, not all infected trees will show signs of infection. Some infected trees simply serve as breeding grounds, without showing any signs of disease, and we don’t yet know why.
Preventing disease Healthy trees are far better able to protect themselves against borers. This means selecting plants appropriate to your microclimate, irrigating and fertilizing them properly, and providing healthy soil. Also, monitor your trees regularly. Currently, Fusarium dieback is limited to Israel and Southern California. You can see a map of the disease, as it spreads through California, here. [It’s a big file, so it make take some time loading.] While Fusarium dieback has only come as far north as San Luis Obispo, to date, that can change overnight. Research is underway, to try and identify an effective lure that can be used to trap the borers. If you suspect Fusarium dieback on a tree, please contact your local County Extension Office. Together, we may be able to slow or stop the spread of this disease. |
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