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Garden Word of the Day
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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.  Potatoes infected with zebra chip disease (USDA) CC BY 2.0 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 psyllids carry zebra chip bacteria (Whitney Cranshaw, Colorado State University, bugwood.org) CC BY 3.0 US 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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.jpg) Potato tuberworm moth (Ilia Ustyantsev) CC BY-SA 2.0 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) CC BY 3.0 US 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 infected with scab (Clemson University - USDA Cooperative Extension Slide Series, bugwood.org) CC BY 3.0 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. Pitted lesions look like moon craters, while erumpent lesions are raised corky areas. Russet lesions (not to be confused with russet potatoes) are just extra corky tissue. In most cases, these lesions start out circular, but can spread out into larger, irregularly shaped areas. Potato scab lesions look a lot like another potato disease, called powdery scab. Powdery scab is a fungal disease caused by Spongospora subterranea. 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 (Whitney Cranshaw, Colorado State University, bugwood.org) CC BY 3.0 US 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.] 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 larvae. Image from page 27 of "Montana insect pests for ... being the ... report of the State Entomologist of Montana" (1925) Public Domain 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. 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 (Andy Murray) CC BY-SA 2.0 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, peaches, 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) CC BY 3.0 US 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 (USDA) 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. 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) Life cycle of Phytophthora infestans on potato (M. Piepenbring) CC BY-SA 3.0 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.  Lesion on underside of leaf caused by potato blight (Howard F. Schwartz) CC BY 3.0 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) CC BY 3.0 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! In light of the 2016 $1 million potato photo sale, I thought I would share the amazing story of potatoes.  Potatoes growing in large container (Kate Russell) Food storage and geophytes Potatoes are tubers. Tubers are are type of geophyte. Geophytes are plant organs used to store food and water. They are also used in asexual reproduction. There are several types of geophytes: bulbs, corms, and everything else. That “everything else” is what we call tubers. Potatoes and yams are stem tubers. Stem tubers can emerge from modified stems. These stems can start out as stolons or rhizomes. Stolons are stems that grow at or just below the soil surface as “runners”. These “stems” are converted into adventitious roots at the nodes and what would have been a bud above ground becomes a spud below. Rhizomes are 'runners' that connect a parent plant to its offspring. A modified stem The “eyes” seen on a potato are actually stem nodes. Within each potato, you will find the same plant cells you would find above ground: vascular bundles, pith (spongy tissue), and cortex (outer tissue). Now here’s the funny part. While our standard spud grows from stem tubers, sweet potatoes grow from root tubers. The internal cell structure is very different. Root tubers have no nodes. That is why sweet potatoes have a more elongated form. At one end, you will find crown meristem tissue, which grows into stems and leaves. At the other end, called the distal end, the tuber produces roots. But enough of that, let's start growing some potatoes! Commercial potato farming Growing potatoes is surprisingly easy and I urge you to give it a try. In his book, The Botany of Desire, Michael Pollan describes how one potato farmer would not feed what they had grown commercially to their family, due to all the fungicides, herbicides and pesticides that are applied to commercial crops. That was a little misleading, since those chemicals are heavily regulated and rarely last long enough to be found on the food we eat. The real reason behind growing an untreated patch of potatoes was so they could enjoy the potatoes whenever they wanted them, rather than having to wait for the chemicals to dissipate. Personally, I don't use chemicals. How to grow potatoes While potatoes can can certainly be grown from spuds bought at the grocery store, this is a bad idea. Foods bought in the store are safe for human consumption, but they are not guaranteed to be free from common garden pests and diseases. You are far better off buying certified “seed potatoes”. The easiest way to grow potatoes is in a barrel, raised bed, or in a tower. If potatoes are planted in the ground, you will be finding rouge spuds for many years. Also, digging them up from the ground is, let’s face it, work. Growing potatoes in containers makes harvesting significantly easier and they make nice summer patio plants! To begin, fill the bottom of the container(s) with 4" of loose, moistened soil. Cut seed potatoes into 2 inch chunks, making sure that each chuck has several eyes (small seed potatoes can be planted whole). Place the chunks 6" apart and cover them with 3" of moistened soil and repeat the process until the container is filled. Water lightly and be sure to place planters where they will get lots of sun. To build a tower (which works nicely for strawberries and herbs, too), simply take a section of chicken wire or hardware cloth and create a cylinder. Landscape cloth can also be used, but it may fall over. You can also grow potatoes indoors, near a window, if it gets enough light. Potatoes need loose, well-drained soil and frequent, light watering. Never let potato plants sit in water, they will rot. Potatoes use a lot of nitrogen and potassium, and they prefer acidic soil (as low as 4.8 pH). At first, it will look as though nothing is happening. As a gardener, you know the value of patience. With time, water and sunlight, tubers will send out roots and stems that will pull nutrients from the soil and create carbohydrates out of sunshine. (Don’t you just love photosynthesis?) Before long, the container will be filled with lush, green growth. Aside from occasional watering and feeding (aged compost works great!), that’s all you have to do until it completes the season’s life cycle. Harvesting potatoes Eventually, the lush above ground growth will start to die off. When it starts looking ragged, dump the contents of the container out on a tarp and remove the mature potatoes. Now comes the really cool part: mix the remaining soil with some aged compost and do it all again with the immature spuds! I have been growing potatoes from the same batch of seed potatoes since 2011. To me, homegrown potatoes taste far better than store bought spuds. Pests and diseases of potato We’ve all heard about the Great Potato Famine. Over one million people died and another two million abandoned Ireland, all because of potato blight. Potato blight causes the tuber to rot in the ground. Other potato diseases include charcoal rot, corky ringspot, cucumber mosaic, curly top, fusarium wilt, leafroll, pink rot, sclerotium stem rot, cankers, verticillium wilt, and white mold. Many of these diseases can be prevented with good drainage and proper spacing between plants. Common potato pests include aphids, beet leafhoppers, cutworms, flea beetles, potato psyllids, potato tuberworms, silverleaf whiteflies, Colorado potato beetles, Jerusalem crickets, and wireworms. But don’t let these threats stop you from creating your own potato patch.  Potatoes grown in a raised bed with various mustards and a couple of cabbages (Kate Russell) Start your own potato patch today!
Tubers are ‘taters, but there’s more to them than that. Tubers are modified underground stems (geophytes) that store water, sugar, and other nutrients, to get plants through winter and drought, to help them start back up in spring, and to regenerate themselves, asexually. Some people say that underground runners (stolons) are also tubers, but not everyone agrees.  Young potato plant with baby tubers (Kate Russell) Stem tubers
Potatoes and yams are stem tubers. These structures are modified stems from thickened stolons (runners) or rhizomes (underground stems). The ‘eyes’ you see on a potato are buds. New plants can grow from these buds, as well as from the distal (far) end of the tuber. Root tubers Root tubers, or tuberous roots, on the other hand, are modified lateral roots which store nutrients and sugars in the same way as stem tubers. Sweet potatoes, dahlias, and cassava (tapioca) are examples of root tubers. These tubers do not have ‘eyes’ that can generate new plants. Instead, growth occurs at the far (distal) end, as well as from the crown area where foliage is already growing. Tuberous roots are biennial:
What sort of tubers are you growing? |
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