Many plant diseases are caused by viruses.
If you can get beyond their disease-carrying behavior, however, viruses are amazing things.
Most viruses are made up of half a DNA strand, called RNA, and are protected by a coat made out of protein. A small handful of plant viruses contain full DNA strands. There is another group, called viroids, which contain an RNA strand but do not have a protein coat. In a recent article, The Scientist reported that new research shows different segments of a virus’ genetic information are used to infect separate cells, creating a domino effect of plant disease. I swear, the more I learn, the more amazing the world gets! But I digress…
The science of viruses
Viruses enter a plant cell and use their RNA strand to reprogram the cell’s genetic instructions. This causes the cell to start producing more of the virus’ RNA. These new strands then infect neighboring cells, and so on.
There are several families of viruses that cause plant disease. We won’t go into that now. What’s important to know is that the common names of most viruses start with the plant most likely to be infected, followed by the most characteristic symptom. For example, bean yellow mosaic is commonly seen in beans and a yellow mosaic pattern is the most common symptom.
Symptoms of viral infection
While there are more viral diseases than I can count, many of them share similar symptoms. The most common symptoms of viral disease in plants include:
Bronzing and leaf rolling may also be seen.
Viruses are generally spread to plants through insect feeding. Common disease-carrying insects include:
Dagger nematodes and some fungi and single-celled organisms also carry viruses. Viruses can also be moved around the garden on pollen, clothing, tools, and plant debris. Many viruses overwinter in seeds, flowers, perennial weeds, and crop root systems, where they can lie dormant for years.
California’s viral diseases
There are dozens of viral plant diseases found in California. The most commonly seen include:
You can find lists of viral diseases common to other areas by contacting your local County Extension Office.
Controlling viral diseases in the garden
Healthy plants are better able to ward of viral infections. This means proper feeding, irrigation, and pruning. It also means selecting resistant plants that are suitable to your microclimate, buying only certified disease-free plants and seeds, planting at the proper depth, and avoiding mechanical injuries from rubbing branches and weed wackers, among other things.
Use an integrated pest management (IPM) program to control viral diseases. This means encouraging natural predators and parasites of viral diseases, sanitizing garden tools regularly, and using cover crops and crop rotation to interrupt disease triangles. Yellow sticky sheets can be used to trap many disease-carrying pests. Pesticides and insecticides used to kill disease carriers are not effective. Reflective mulches have been used successfully to confuse some disease-carrying pests. Diseased plants should be removed and thrown in the trash to prevent healthy plants from becoming infected.
Viral diseases of plants are on the rise, largely due to monoculture, mass production, climate change, global shipping and other human activities. You can reduce the likelihood of viral diseases affecting your plants by placing new plants in quarantine and knowing what to look for.
Now you know.
Brown soft scale (Coccus hesperidum) may be found on pomegranate and citrus trees, as well as bay laurel.
These pests can suck the life out of your lemon, orange, kumquat, lime, and pomegranate trees, killing twigs, and reducing your harvest. Like other sap-sucking insects, brown soft scales excrete sugary honeydew, which creates habitat for sooty mold, and attracts disease-carrying ants.
Brown soft scale description
These pests live under yellowish, mottled shells. They may look like nothing more than little bumps on leaves and twigs, Brown soft scale look similar to citricola scale insects.
Brown soft scale lifecycle
Brown soft scale females give live birth, or lay eggs which hatch almost immediately. These young crawlers move around freely on leaves and twigs, feeding as they go. They continue moving around until they are about half grown, molting twice.
Controlling brown soft scale
Sticky barriers around the trunks of susceptible trees can cut off protection to brown soft scale pests by ants. Also, avoiding the use of broad spectrum pesticides allows natural predators to feed on these pests. If you see scale shells with holes in the top of them, you will know they have been parasitized by beneficial insects. Heavy infestations can be treated with dormant oil in winter, but this is rarely necessary.
Psyllids are jumping plant lice that suck plant juices. There are over 160 psyllid species in California, 140 of which are native to the area.
Most native psyllid species do not pose a serious threat to your garden. Local predators tend to keep those populations in check most of the time. Invasive psyllid species are something else altogether.
Psyllids look like tiny cicadas or winged aphids, with tubular mouthparts. They have very strong legs and short antennae. Psyllids can be 1/12 to 1/5” long. Adults hold their wings in a roofline position. Nymphs are flattened and look a lot like soft scale insects. Psyllid nymphs commonly produce waxy filaments or covers, called lerps. Lerps are made from wax and honeydew.
Regardless of the species, psyllids start out as tiny eggs that hatch and go through five developmental stages, or instars, before reaching adulthood. Adult psyllids can fly, but most prefer to jump. If you see what you think is a psyllid run or fly away, it is probably a psocid [SO-sid]. Psocids are beneficial insects that feed on fungi. They differ from psyllids in that they have a narrow “neck” and chewing mouthparts.
Psyllid host plants
As a species, psyllids have strong preferences for particular host plants. While some psyllids will prefer your sweet peppers and chili peppers, other varieties will go after your peaches and nectarines, while others will only feed on olive or pear trees, and yet other psyllid species will only feed on potato and tomato plants.
The invasive Asian citrus psyllid carries huanglongbing, a deadly citrus disease. Orange, lemon, lime, kumquat, and grapefruit trees infected with huanglongbing must be destroyed by a professional. Sad, and expensive. These pests, when present, are most active April through June in the San Francisco Bay Area.
Psyllid species most likely to threaten your garden include:
There are dozens of psyllid species that infest ornamental trees and shrubs, as well. These include the recent invasions of Ficus leaf-rolling psyllids and spotted gum psyllids. On the other hand, some psyllid species are being used to our advantage. The Australian melaleuca psyllid, for example, has been purposefully introduced to Florida to help control paperbark trees, an invasive weed tree.
Damage caused by psyllids
One of the biggest problems associated with psyllids is their poop. After they have robbed your plants of valuable nutrients, weakening the plant, they add insult to injury by excreting a large portion of the sap they stole and depositing on leaves. Known as honeydew, the excrement of sap-suckers is filled with sugar and other nutrients. Honeydew ends up being food for fungal sooty mold and disease-carrying ants.
Psyllid feeding can also spread viral diseases, such as calico, bacterial diseases, such as zebra chip, galls, leaf and bud discoloration and deformation, and premature leaf drop. Leaf distortions often look similar to peach leaf curl. Pear psyllids inject fruit with toxins that blacken leaves and fruit skins. Psyllid feeding also creates points of entry for other pests and diseases.
How to control psyllids in the garden
Once psyllids appear in your garden, insecticidal soaps and yellow sticky sheets can be used to help control them. Parasitic wasps and pirate bugs can put a serious dent in psyllid populations, so avoid using broad spectrum insecticides. Severely infested plants should be removed and destroyed or thrown in the trash. Usually, simply monitoring plants regularly can make controlling these and other pests much easier.
To prevent invasive psyllids from finding your garden, only buy pest-free plants from reputable nurseries, place new plants in quarantine, and do not bring plant products that may be infested into your state, community, or yard.
Because of the risks posed by invasive psyllids, any unrecognized psyllids should be taken to your agricultural commissioner or local County Extension Office for identification.
You walk past a tree and notice leaves rolled up into neat tubes. What causes this and is it a problem?
There are several leafroller species found in California:
A quarantine is in place for the light brown apple moth. See if you live in an affected area by clicking on the CA Dept. of Food & Agriculture’s Boundary Index Map. If you live outside of California, you can contact your local Cooperative Extension Office for more information
Leafrollers start out as clusters of flat, irregularly shaped eggs often found on twigs and leaves. These egg masses are coated with a dark gray or brown glue that later bleaches to white, giving them an appearance similar to fish scales. If you look closely at an egg mass in spring, you can see tiny pinholes where larvae have hatched.
After hatching, larvae pull leaves into a cylinder for protection as they feed. Most larvae feed through summer and then overwinter as pupae, though some species continue feeding throughout the year, causing considerable damage.
When disturbed, leafroller caterpillars tend to wriggle wildly and then rappel to the ground on a single silken thread.
April is the time to start checking apple, apricot, avocado, cherry, peach, pear, plum and prune trees, and blueberries, for signs of the dreaded Pacific flathead borer.
Like other borers, these pests chew tunnels in wood, weakening a tree’s structure, and robbing it of important nutrients found in the inner cambium layer. Newly planted trees and trees weakened by drought, water-stress, scale insects, carpenterworms, or diseases, such as Phytophthora or Armillaria, are particularly susceptible. These weakened areas are then more likely to be attacked by other pests, such as shot hole borers.
Pacific flathead borer feeding can also girdle a young tree, killing it. The only symptom you may see is a dark colored depression in the bark, or tiny cracks where you might see frass (bug poop), usually on the side receiving the most sunlight.
Pacific flathead borer identification
Pacific flathead borers (Chrysobothris mali) are flattened, wedge-shaped, dark bronze beetles that can be 0.5 to 0.75” long. You may see copper-colored spots on the wing covers.
Eggs are very tiny, only 0.04” in diameter, flattened, oval, and white. Larvae grow to 0.75” in length and are white, with an amber colored head. Larvae are flattened, with a widened area just behind the head, tapering towards the rear end. Pupae are also whitish and flattened, getting darker as they mature.
Pacific flathead borer lifecycle
These pests overwinter in a prepupal stage. As temperatures begin to rise, they pupate. From April through July, adult beetles emerge, usually beginning around the same time apple trees are blooming. Then females mate and begin laying eggs in the bark, favoring areas weakened by sunburn or mechanical injury from tree supports, weedwackers, and out of control lawn mowers. When the eggs hatch, larvae burrow directly into the bark and begin feeding on the nutrient-rich cambium layer, robbing your trees of the nutrients they need to grow and produce fruit. As the larvae mature, they will either create a pupal chamber in the xylem, or burrow under the bark, where they will stay until the following spring.
Pacific flathead borer controls
Healthy trees are better able to resist and recover from Pacific flathead borer attack. This means selecting trees suitable to your microclimate, planting them at the proper depth, and feeding, irrigating, training, and pruning them properly.
Since eggs are laid in weakened bark, protect trees from mechanical injury, and be sure to whitewash exposed bark before sun damage can occur.
Birds, especially woodpeckers, will find and remove Pacific flathead borers, and carpenter ants eat both larvae and pupae. Insecticides are commonly used in commercial orchards to kill new larvae, but once the larvae are inside the tree, there is nothing you can do besides pruning out infested wood and burning it.
Now you know.
Ashy stem blight, also known as charcoal rot, is a fungal disease of cucurbits. This means that your melons, squash, and cucumbers are susceptible. It can also affect common beans, blackeyed peas, lima beans, chickpeas, corn, fenugreek, soybeans, sorghum, and sunflowers.
Ashy stem blight is a soil borne fungus (Macrophomina phaseoli) that loves hot days (> 85°F) and cool nights. This pathogen can stick around for up to 12 years. It is common in California and often infects plants within 2 weeks after being planted, but symptoms generally do not appear until much later in the growing season, as temperatures begin to rise - after you’ve invested weeks of irrigation, feeding, and weeding. So, learning how to recognize and prevent this disease can help ensure a better harvest.
Symptoms of ashy stem blight
The first sign of ashy stem blight are black, water-soaked lesions or cankers along the stem at the soil line, stunting, and chlorosis (yellowing) of the upper, or crown leaves. If you look closely at the lesions, you may be able to see concentric rings. Infected pods may ripen prematurely. As the fungi population grows within the plant, you may see an amber gum oozing from the infected plant. Eventually, the stem turns dry and brown. If lesions girdle the plant, it will die. If you dig up an infected plant, you will see blackened roots and a lack of feeder roots.
Preventing ashy stem blight
Ashy stem blight is known as a “stress pathogen”. This means it preys on stressed plants. Stresses, such as a heavy fruit load, high temperatures, drought, and water-stress can make plants more susceptible to infection. Keeping your plants healthy can help them protect themselves.
While furrow-irrigated plants rarely have severe cases of ashy stem blight, you may be surprised to learn that the disease is common with drip-irrigated systems. It is believed that this particular set-up increases salt levels near the soil surface, creating salt stress.
Monitor plants regularly for signs of infection. Once infection occurs, affected plants should be removed and thrown in the trash, and a 3-year crop rotation with non-susceptible crops should be put into place. There are no effective chemical treatments for this disease.
Now you know.
We’ve all heard that beans cause gas, but did you know beans rust? Well, not rust like the undercarriage of a New England truck, but rust just the same.
Bean rust, like other plant rusts, is a fungal disease. Rust is found worldwide and it can wipe out your bean crop if it takes hold early enough in the growing season.
California’s cool, wet springs are just the conditions needed for rust to thrive. Add overhead watering or a decent breeze and the stage is set for an epidemic. Fungi are so efficient that, under ideal conditions, the disease cycle can be repeated every 10 to 14 days!
There are several strains of bean rust. Two of the most common are Uromyces appendiculatus and Uromyces phaseoli typica, but you don't need to know the Latin to recognize bean rust in the garden.
Bean rust symptoms
Similar to other rusts, bean rust prefers moist places and moderate temperatures (65 to 85°F). While it can occur on any aboveground portion of a plant, bean rust is most commonly found on the underside of leaves. Pods can also be affected. At first, it just looks like tiny white or yellow bumps. Then those bumps break open and turn into bright orange, reddish, or yellowish flecks. Those flecks are pustules that are made up of more fungal spores than any of us cares to count. [Okay, some scientists love counting things like that.] A yellow outer ring is sometimes visible. Leaves may begin to curl downward and plants may develop a scorched appearance. These symptoms are easy to see and make identifying the condition simple. Getting rid of it is something else all together.
Bean rust control
The fungi that cause bean rust can be spread by ants, aphids, and gardeners. It can stick to tools, fingers, and clothing. As with many other plant diseases, prevention is far easier than eradication. Use these tips to prevent and control bean rust in your garden:
Keep in mind that rust pustules are easily dislodged and can land somewhere else, or on the soil, where they can be bounced back up into your plants by rain, wind, and overly exuberant irrigation. And be sure to disinfect your tools after removing rust-infected leaves, to avoid spreading the fungus to healthy plants.
With a name like Halo, you might expect little cherubs in today's post, but that is not the case. The bacteria responsible for halo blight are no angels.
In common blight, those lesions have wide, lemon-colored borders, and they continue to grow. Bacterial brown spot lesions have narrow light green borders and the centers tend to dry out and look tattered. Halo blight lesions tend to stay small and they have prominent light green halos, hence the name.
Leaves are not the only place damage occurs. Pods can also become infected, making them inedible. Pods infected with common blight have lesions with red or rust colored borders, while symptoms of the other two diseases are difficult to distinguish from each other, both being the same water-soaked lesions seen on leaves.
Managing halo blight
As with other diseases, prevention is the easier way to go. Since moisture is needed for halo blight to develop and spread, avoid overhead watering and save the sprinklers for your lawn. Furrow irrigation will get water to the roots without creating a potential disease site. This is especially important when temperatures are between 68 and 74°F (20 to 23 °C). Unlike many other blights, halo blight bacteria prefer these slightly cooler temperatures.
Also, be sure to start with certified disease-free seed, and place new plants into quarantine before exposing the rest fo your garden to whatever they may be carrying. Speaking of carrying disease, the bacteria responsible for halo blight can also travel on rain splashes, wind, pet fur, shoes and clothing. If you have been exposed to halo blight, you might want to change your clothing and swap shoes to avoid spreading the disease throughout your garden. Finally, when your bean plants have completed their life cycle, cut them off at ground level, leaving soil microorganisms in place, and add plant debris to the compost pile. Leaving plants to break down in the garden can provide potential overwintering sites.
Fixed copper or Bordeaux mixture may be used to prevent halo blight. Plants infected with halo blight should be removed and destroyed, followed by a 2 to 4-year crop rotation program.
Bean common mosaic is a viral disease caused by several different virus strains. Close cousin to bean yellow mosaic and clover yellow vein virus, bean plants can be unfortunate enough to be infected with all three at the same time.
What’s really fascinating about this disease is that there are two different sets of symptoms that may occur.
Symptoms of bean common mosaic
Bean plants infected with these viruses may come down with bean common mosaic or bean common mosaic necrosis.
Bean common mosaic is very common in California. It appears as mosaic patterns of light and dark green on the leaves. Puckering, blistering, rolling, and downward cupping are also common symptoms. Plants infected while young will also be stunted.
Bean common mosaic necrosis has not been seen in California since its first sighting in 1996, but you’ll want to be on the lookout, just in case. Symptoms include small, reddish-brown spots on the leaves. Nearby leaf veins become brown or black and this necrosis (death) then spreads to the phloem and throughout the plant, ultimately killing it. It you take a cross-section of an infected stem or pod, you will see reddish streaking in vascular tissue. These symptoms look very similar to black root rot and Fusarium wilt, but neither of these conditions cause streaking in the pods.
In both cases, leaves may also be smaller than normal, and blossoms and pods may be deformed. Which set of symptoms your plants will exhibit depends on which virus is involved, and whether or not your plants have dominant or recessive genes, or if one gene is present at all. Plants with the dominant gene are resistant to common mosaic, but hypersensitive to common mosaic necrosis. Some bean varieties, which lack the gene altogether, have symptoms which could fall under either category. Symptoms of bean common mosaic are most likely to appear when temperatures are between 68 and 77°F.
Bean common mosaic transmission
The bean common mosaic virus overwinters in infected seeds and weeds. The virus is most often transmitted on pollen and by aphids. It can also move from plant to plant on clothing, tools, and plant debris.
The best way to avoid introducing this virus into your garden is to:
Resistant bean varieties
According to UCANR, these varieties are resistant to one or more strains of this virus:
Once these viruses are in your soil, they are difficult to get rid of - it is far better to start with clean seeds and do what you can about those pesky aphids.
You don’t have to grow tobacco to have reason to worry about tobacco mosaic virus.
Tomatoes are highly susceptible to this disease that can be carried on tools, clothing, cigarettes, and, yes, even the saliva and other bodily excretions of smokers. This tenacious virus can stay alive even after its host is dead, and it can withstand extreme temperatures.
Tobacco mosaic virus host plants
In addition to tomatoes and tobacco, the tobacco mosaic virus (TMV) has been found on over 350 different plant species, including cucumbers, many flowers and ornamental plants, and all members of the nightshade family, such as eggplant, potatoes, groundcherries, tomatillos, and peppers. While they may not show symptoms, grapes and apple trees can also become infected.
Symptoms of tomato mosaic virus
Tobacco mosaic virus starts out as nothing more than paler than normal green between the veins of young leaves.
This lightened area quickly becomes mottled, leaving a green, white, or yellow mosaic pattern. Bumpy wrinkles may also appear, in a behavior known as rugosity, and leaves may appear distorted or stringy, or exhibit cupping.
Leaf veins may also turn yellow, and yellow streaking on the leaves may also occur. Many of these symptoms can be mistaken for signs of chemical overspray, but the mosaic pattern is usually distinct enough to rule this out. While this disease does not kill plants, it can stunt them severely. Infected leaves soon die, leaving dead patches in the plant and reducing production by up to 20%. Fruit that is produced is often discolored and deformed.
How is tobacco mosaic virus spread?
Unlike many other diseases, which are spread by sap-sucking insects, such as thrips and aphids, tobacco mosaic virus is mostly spread by direct contact. Tobacco mosaic virus has also been spread by chewing insects, such as grasshoppers and caterpillars, and by bumblebees, as they pollinate flowers.
Preventing tobacco mosaic virus
Plants infected with tomato mosaic virus must be removed and destroyed. According to the Michigan State University Extension, you can prevent the virus from moving onto uninfected plants by spraying them, just before transplanting, with a 20% nonfat dry milk solution. This spray can also be used on containers, walkways and other surfaces. The milk solution coats the virus, rendering it inactive. The milk treatment only works while the milk is wet.
These other tips can also help reduce the likelihood tobacco mosaic virus in your garden:
*Check plant labels for the letters V, F, N, T or A. These symbols indicate a resistant to verticillium wilt, fusarium wilt, nematodes, tobacco mosaic virus or alternaria stem canker, respectively.
According to a study conducted by the National Institutes of Health, all brands of cigarettes studied tested positive for tobacco mosaic virus, while only 53% of those viruses were viable. Also, 45% of the saliva samples taken from smokers tested positive for tobacco mosaic virus. So, smokers and users of other tobacco products, please wash your hands before entering your (or someone else’s) garden, and always throw your butts in the trash. Thank you.
While it may sound like a short, reckless redhead, tomato bushy stunt is a viral disease of tomatoes. The strange thing is, we don’t yet know where it comes from or how it gets to our tomato plants.
Most diseases are spread by vectors, often sap-sucking or leaf chewing insects. When it comes to tomato bushy virus, we don’t know how it spreads, though many experts believe it may be spread through irrigation water. Contaminated seed, sewage, and tools may also be guilty.
The virus is thought to enter plants through damaged roots. And tomatoes are not the only plants at risk.
Tomato bushy stunt host plants
First identified in tomatoes in 1935, this is not an economically significant disease, but it can cause problems in your home garden if it gets established. Apples, artichokes, cherries, grapes, hops, sweet peppers, chili peppers, eggplant, and tulips can also become infected, and the virus can cause severe leaf dieback of many lettuce varieties.
Tomato bushy stunt symptoms
Plants infected with the tomato bushy stunt virus have smaller than normal leaves which are cupped and curled downward. New leaves are crinkled and twisted, with dead tips. Infected plants produce more lateral shoots, creating the bushier, though often stunted appearance. Lower leaves may have a purplish tinge and tend to be chlorotic. Tomato bushy stunt causes a significant reduction in fruit production. The fruit that is produced, well, let's just say it doesn't look very appetizing.
Preventing tomato bushy stunt
Since damaged roots create a point of entry for this and other diseases, avoid digging once plants are established. Instead, feed plants by top dressing and banding, and disinfect tools regularly.
Once the virus is present in the soil, it is suggested that long crop rotations, of 4 or more years, are suggested. Infected plants should be removed and tossed in the trash.
Tomato yellow leaf curl is a devastating viral disease of tomatoes that made its way to California in 2007. Mostly limited to greenhouse environments, this disease can wipe out all of your tomato plants, so you need to know what it looks like.
This disease is spread by whiteflies and leafhoppers, and it is not limited to tomatoes. Other members of the nightshade family, such as peppers, can also be infected, as well as beans and many as yet to be identified weeds. Most often, the disease is spread through contaminated plants.
Symptoms of tomato yellow leaf curl
Infected tomato plants tend to grow unusually upright, while being stunted. This occurs because the virus shortens the internodes. Internodes are the spaces between the nodes where leaves emerge. Shortening the internodes makes the plant look bushier, but not healthier. Tomato leaf curl virus also causes up to 100% flower drop, which means no harvest. The most obvious symptom of tomato yellow leaf curl is the leaves. Infected leaves are smaller than normal, crinkled, and curled upwards. They also tend to turn yellow at the edges and between the veins. Unfortunately, many other viruses have similar symptoms. If you believe you have a plant infected with tomato yellow leaf curl, contact your local County Extension Office right away.
Whiteflies and disease transmission
Specific varieties of whiteflies (Bemisia) are responsible for transmitting this disease. If you look closely, you can see that some whiteflies hold their wings tent-wise, over their bodies, while others hold their wings flat. In the same way, some nymphs will have smooth edges, while others have a fringe of filaments. To see this level of detail, it is relatively easy to take a piece of clear packing tape and wrap it around your hand the same way you might to remove lint or pet hair from a pair of paints and capture the whiteflies on the tape. Then you can use a magnifying glass or hand lens for a closer look. It is the whiteflies that hold themselves under a tent and whose nymphs have smooth edges that carry the tomato yellow leaf curl virus. Leafhoppers are also believed to carry and transmit the disease.
Preventing the spread of tomato yellow leaf curl
Tomato yellow leaf curl has the potential to temporarily eliminate tomato growing in certain areas, including your garden. This disease is the reason why tomatoes are generally not grown in tropical and semi-tropical regions. Cold winters tend to kill of the whitefly vectors, but the San Francisco Bay’s winters are not necessarily cold enough to provide that protection. Instead, you can use these tips to protect your tomato plants, and tomatoes, in general:
Now you know.
Tomato ringspot is a viral disease that can kill far more than your dreams of summer sweet salsa.
This virus infects an astounding number of other plants and it is fatal.
In addition to tomatoes, the tomato ringspot virus also infects stone fruits, apples, grapes, cucumbers, cowpeas and other beans, strawberries, currants, soybeans, and caneberries, including those luscious raspberries and blackberries. Adding insult to injury, this disease can also infect begonias, geraniums, iris, hydrangeas, and many other popular garden flowers. Sadly, tomato ringspot is an incurable, highly contagious disease. Infected plants (and their neighbors) must be removed and destroyed to prevent further spread.
The virus responsible for tomato ringspot can be carried through the air, on pollen, or by dagger nematodes in the soil. As these nematodes feed on roots, the virus is transferred to healthy plants. Dandelion seeds can also carry this disease.
Symptoms of tomato ringspot
Plants infected with tomato ringspot may simply not thrive, slowly declining over time, or they may exhibit yellow ring spots, general yellowing (chlorosis) or mottling, or they may show no signs at all, acting as a way station for the disease without being impacted directly. Caneberries may turn dry and crumbly, similar to dryberry mite infestations. Stone fruits may develop prunus stem pitting or yellow bud mosaic as a result of infection:
Yellow bud mosaic causes lower branches to lose leaves, moving upward into the canopy as the virus spreads. Leaf veins on either side of the midrib may turn white, and leaflike growths, called enations, may grow along the midrib on the underside of the leaf. This is usually seen on infected almond, cherry, nectarine, peach, and plum trees.
Prunus stem pitting causes later then normal leafing out. Leaves look pale and tend to wilt in summer, turning red or purple earlier in the season than is normal. Fruit size, quantity, and quality are significantly reduced. All of this is because the virus interferes with the flow of water and nutrients through the graft union, effectively starving the tree. These symptoms look very much like root damage caused by rats and voles, girdling roots, and fungal diseases of roots. The difference being that the tomato ringspot disease causes the bark, both above and below the soil line, to thicken and become spongy. This weakened area often allows the tree to topple over. Before that happens, you will also see pitting in the sapwood of the trunk. Usually the tree dies before pitting is seen in any branches.
How to control tomato ringspot
In a word - you can’t. The disease is incurable and infected plants put nearby plants at risk. The only thing you can do is remove the infected plants, and those nearby, and burn them or toss them in the trash. Just because symptoms disappear does not mean the infection is gone. Plants that no longer show symptoms are still carrying the disease, which can then spread throughout the garden. Once plants are removed, the affected area should be allowed to go fallow for at least 8 months, to starve out any dagger nematodes that may be lurking underground, and remove any potential disease-carrying weeds.
To prevent tomato ringspot, there are a few steps you can take:
Yes, removing plants from the garden or landscape is disappointing, but having to remove even more plants because of an initial delay could be both costly and heartbreaking.
And you thought this post was going to be all about tomatoes…
So did I!
Sugar volcanoes are a tree’s response to boring, disease-carrying insects.
Now, when I say boring, I do not mean dull witted conversationalists. These pests are invasive shot hole borers that chew holes through bark to get at the sugary, nutrient rich sap found in the cambium layer.
Borers and disease
Borer entry and exit holes, while very tiny (0.03” in diameter) compromise a tree’s outer layer of defense. As the beetles bore into the wood, they carry with them three different species of fungal spores. These fungal spores enter the tree, bringing Fusarium dieback. Fusarium dieback is a fungal disease in which fungi block a tree’s vascular system, halting the flow of water and nutrients. Trees infected with Fusarium dieback must be destroyed and disposed of by a professional arborist.
Avocados and exudates
Peach, citrus, and pecan are just a few of the more than 200 tree species impacted by this problem, as are grapevines, but the sugar volcano is specific to avocados and box elders.
Exudates are secretions. When an avocado tree is first attacked by shot hole borers and Fusarium dieback, it will respond by pushing a sugary exudate out of the borers’ entry and exit holes. This sugary secretion often includes frass and sawdust. As it dries, this sugary exudate forms a white, crusty ring or cone-shaped patch on the affected branch.
And these crusty white patches, dear readers, are sugar volcanoes.
Now you know.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 (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).
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 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.
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.
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.
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.
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.
You can grow a surprising amount of food in your own yard. Ask me how!
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