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Dark brown spots on your tomatoes? It’s probably early blight.  “Bullseye" pattern leaf lesion of early blight (USDA Cooperative Extension) CC BY-SA 3.0 Cool spring temperatures and too much rain or other moisture create the perfect storm for this fungal disease. The early blight fungus (Alternaria solani) is a disease of the nightshade family, which means your potatoes, eggplants, bell peppers, and chili peppers are equally susceptible, as are other plants in the nightshade family, such as petunias and blue potato bushes. If similar lesions occur later in the season, it is probably late blight, also known as the dreaded potato blight. Early blight was once a disease found only on the east side of the Rocky Mountains. Sadly, that is no longer the case. The early blight pathogen now occurs everywhere host plants have been grown and can result in up to 30% of your potatoes and 79% of your tomato crop ending up in the trash bin. Early blight symptoms Small black or brown spots, usually 1/4 to 1/2 inch in diameter, may appear on fruit, leaves, and stems. These spots may have a concentric ring pattern or bullseye. Fruit spots are dry, sunken areas, most commonly seen near the calyx (flower). Spots on leaves feel leathery. Symptoms are seen on older leaves first. Stem lesions do not have the same circular bullseye pattern.  Early blight stem lesion (USDA) CC BY-SA 3.0 This pathogen can also cause collar rot and damping off, which usually kills seedlings. As the disease progresses, leaf loss can significantly reduce fruit production. Honestly, the fruit that grows doesn’t look particularly appetizing. Infected potatoes either rot in the ground or storage.
Early blight lifecycle Fungal spores overwinter in the soil and on infected fruit and plant debris. Rain and overhead irrigation splash spores onto plants, where they begin reproducing. To reduce the chance of early blight in your garden:
Fixed copper or sulfur sprays can provide fungicidal benefits in heavy infestations. Also, healthy plants are less likely to become infected, so feed and water your plants properly, give them enough space to reach full size, and help them avoid physical injuries, which provide entry points for early blight fungal spores. Are your strawberry plants looking pitiful? Do they fail to produce big, luscious fruits? It may be virus decline.
Causes of virus decline Virus decline is a combination of viruses. It starts with either strawberry pallidosis-associated virus (SPaV) or Beet pseudo yellows virus (BPYV), spread by whiteflies. Then aphids come along with any of these other viruses: Strawberry latent ringspot and Fragaria chiloensis latent viruses may also play a role. Research is currently underway. Virus decline symptoms Because virus decline is a combination of infections, plants may look fine one day and terrible the next. Initial infection of SPaV or BPYV may cause no symptoms. Add the second virus, and everything starts to fall apart. Plants infected with virus decline will exhibit stunting, sometimes severely. Leaves turn purple or red, though new growth that forms near the center of the plant may remain green. There is little or no fruit production, and the roots are brittle and reduced. These plants will never be productive. Control of virus decline The only way to know if your strawberry plants have virus decline is to send a sample in for lab testing. County Agriculture Offices may offer this service free of charge. Either way, declining plants are best removed and replaced, preferably in a new location. One way to reduce the likelihood of virus decline in your strawberry patch is to keep other plants that might host these viruses at a safe distance. The SPaV virus favors specific weeds, including nettle, creeping sibbaldia, mock strawberry, sharp leaf groundcherry, and cheeseweed. BPYV prefers melons, squashes, lettuces, cucumbers, endive, and spinach. Cheeseweed, dandelions, marigolds, and zinnia may also harbor BPYV. Use these tips to reduce the possibility of virus decline in your strawberries:
These steps are no guarantee, but they will significantly reduce the chance of your plants becoming infected. If you can get beyond their disease-carrying behavior, viruses are amazing things.  Pepper mild mottle virus (Pamela Roberts) CC BY 2.5 What are viruses? First off, they are obligate, which means they cannot reproduce on their own. Instead, viruses need living plant tissue to survive and reproduce. The jury is still out on whether or not viruses are alive at all. Most viruses consist of half a DNA strand, called RNA, protected by a coat made out of protein. A small handful of plant viruses contain full DNA strands. Another group, called viroids, contains an RNA strand but does not have a protein coat. In a recent article, The Scientist reported that new research shows different segments of a virus’ genetic information infect separate cells, creating a domino effect of plant disease.  Leaf curl virus (Philarium) CC BY-SA 3.0 The science of viruses Viruses enter a plant cell and use their RNA strand to reprogram that cell’s genetic instructions, causing the cell to start producing more of the virus’ RNA. These new strands then infect neighboring cells, and so on. Several families of viruses cause plant disease. And you can use their names to determine which plant is most likely to become infected and the most characteristic symptom. For example, bean yellow mosaic commonly occurs in beans, and a yellow mosaic pattern is the most common symptom. Symptoms of viral infection Many viral diseases share similar symptoms. The most common symptoms include the following:
Viral vectors Viruses often infect plants through insect feeding. Common disease-carrying insects include aphids, artichoke plume moths, leafhoppers, mealybugs, psyllids, thrips, and whiteflies. Dagger nematodes and some fungi and single-celled organisms also carry viruses. Viruses also move 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. Common viral diseases There are an estimated 1,000 viral plant diseases. But you may only face a handful of those problems, and many are avoidable. The most commonly seen viral diseases include the following:
You can find lists of viral diseases common to your area by contacting your local County Extension Office.
Controlling viral diseases in the garden Healthy plants are better able to ward off viral infections. Proper feeding, irrigation, and pruning help keep plants healthy. Select resistant plants suitable to your microclimate, buy certified disease-free plants and seeds, plant at the proper depth, and avoid mechanical injuries from rubbing branches and weedwackers, among other things. Integrated pest management (IPM) programs can help control viral diseases. An IPM lures natural predators and parasites with insectary plants and water. Sanitizing garden tools can slow the spread of disease. Cover crops and crop rotation can interrupt any potential disease triangles. Yellow sticky sheets can trap many disease-carrying pests. Pesticides and insecticides used to kill disease carriers are generally not effective. Reflective mulches can confuse some disease-carrying insects, but they can burn sensitive plants during the peak of summer. 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, mainly 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 putting new plants into quarantine and knowing what to look for. Now you know. Ashy stem blight, or charcoal rot, is a fungal disease of cucurbits. Your melons, squash, and cucumbers are all susceptible. It can also affect common beans, blackeyed peas, lima beans, chickpeas, corn, fenugreek, soybeans, sorghum, and sunflowers.  Ashy stem blight on mungbean (Photchana Trakunsukharati, Dept. of Agriculture, Thailand) CC BY 3.0 AU.jpg The soil-borne fungus (Macrophomina phaseoli) responsible for this disease loves hot days (> 85°F) and cool nights. This pathogen can stick around for up to 12 years. It often infects plants within two weeks after being planted. Symptoms generally do not appear until much later in the growing season, as temperatures 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 signs 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 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. Lesions may girdle the main stem and kill the plant. If you dig up an infected plant, you will see blackened roots and a lack of feeder roots. Preventing ashy stem blight This fungus is a stress pathogen. It preys on stressed plants. 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 due to increased salt levels near the soil surface, creating salt stress. Monitor plants regularly for signs of infection. Once infection occurs, remove infected plants and throw them in the trash. A 3-year crop rotation with non-susceptible crops may break the disease cycle. 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.  Bean rust (USDA) CC BY 3.0 US 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 blight, you might expect to read about little cherubs, but that’s not the case. The bacteria responsible for halo blight are no angels. Halo blight is a global disease of legumes, affecting kidney beans, lima beans, snap beans, scarlet runners, and other bean varieties.
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 direct water to the roots without creating a potential disease site. Halo blight is most likely when temperatures are between 68 and 74°F (20 to 23 °C). Unlike many other blight diseases, halo blight bacteria prefer these slightly cooler temperatures. Use these tips to prevent halo blight in your garden:
Fixed copper or Bordeaux mixture treatments may prevent halo blight. Bean common mosaic is a viral disease caused by several different virus strains. A close cousin to bean yellow mosaic and clover yellow vein virus, bean plants can be unfortunate enough to be infected with all three simultaneously. What’s fascinating about this disease is that two different sets of symptoms may occur.  Bean common mosaic necrosis virus (USDA) Public Domain Symptoms of bean common mosaic Bean plants infected with these viruses may come down with bean common mosaic or bean common mosaic necrosis:
In both cases, leaves may be smaller, and blossoms and pods may be deformed. The symptoms your plants will exhibit depend on the virus involved, whether or not your plants have dominant or recessive genes, or if a particular gene is present. 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. It is most often transmitted by aphids or spread on infected pollen. It can also move from plant to plant on clothing, tools, and vegetative material.
Resistant bean varieties
These bean varieties are resistant to one or more strains of this virus:
Once these viruses are in your soil, it is hard to get rid of them. 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 the tobacco mosaic virus (TMV). Tomatoes are highly susceptible to tobacco mosaic, and the virus can travel on tools, clothing, cigarettes, and, yes, even on saliva and other bodily excretions of cigarette smokers. The tobacco mosaic viruses remain viable after their host dies and can withstand the extreme temperatures of tobacco curing.  Tobacco mosaic virus symptoms on orchid leaves (USDA Forest Service) Public Domain Tobacco mosaic hosts
In addition to tomatoes and tobacco, the tobacco mosaic virus infects over 350 plant species, including cucumbers and all plants in the nightshade family, as well as many flowers and ornamental plants. While they may not show symptoms, grape vines and apple trees can also become infected. Symptoms of tobacco mosaic Tobacco mosaic starts as nothing more than pale 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. Leaf cupping may also occur. Leaf veins may also turn yellow, and you may see yellow streaking on the leaves. 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%. The fruit that does grow is often discolored and deformed. How is the tobacco mosaic virus spread? Unlike many other diseases, which are spread by sap-sucking insects, such as thrips and aphids, the tobacco mosaic virus spreads primarily by direct contact. Chewing insects, such as grasshoppers and caterpillars, may infect plants. And bumblebees may transfer the infection as they pollinate flowers. Preventing tobacco mosaic virus Plants infected with tomato mosaic must be removed and destroyed. According to the Michigan State University Extension, you can prevent the virus from moving onto uninfected plants by spraying, just before transplanting, with a 20% nonfat dry milk solution. The milk solution coats the virus, rendering it inactive. The milk treatment only works while wet. You can spray containers, walkways, and other surfaces, as well. These other tips can also help reduce the likelihood of tobacco mosaic in your garden:
*Check plant labels for the letters V, F, N, T, or A. These symbols indicate resistance to verticillium wilt, fusarium wilt, nematodes, tobacco mosaic virus, and Alternaria stem canker. According to a study by the National Institutes of Health, all the cigarette brands studied tested positive for tobacco mosaic, 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. But no one knows where it comes from or how it gets to our tomato plants.  Unripe tomatoes infected with tomato bushy stunt virus (WikiGardener) CC BY-SA 3.0 Most diseases are spread by vectors, often sap-sucking or leaf-chewing insects. It is still a mystery how these viruses move around. Some experts believe they may travel in irrigation water. Contaminated seed, sewage, and tools may also be guilty. And we think the virus enters plants through damaged roots, but that has yet to be proven. We do know that 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 can cause problems in your home garden if it gets established. Apples, artichokes, cherries, grapes, hops, sweet peppers, chili peppers, and eggplant can also come down with tomato bushy stunt. The virus can cause severe leaf dieback in many lettuce varieties. Tomato bushy stunt symptoms Plants infected with the tomato bushy stunt virus have smaller, cupped leaves that curl downward. New leaves are crinkled and twisted, with dead tips. Infected plants produce more lateral shoots, creating a bushier, stunted plant. 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 does reach maturity, 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 around established plants. Instead, feed plants by top dressing and banding, and disinfect tools regularly. Once the virus is present in the soil, crop rotations of four or more years may break this disease triangle. Remove infected plants and toss them 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'll want to know what it looks like.  Tomato yellow leaf curl (Dr. Michael Melzer) Public Domain Whiteflies and leafhoppers carry this disease, and it is not limited to tomatoes. Other members of the nightshade family, such as peppers, can also be infected, as can beans and many as yet to be identified weeds. Most often, the disease spreads through contaminated plants.
Symptoms of tomato yellow leaf curl Infected tomato plants tend to grow unusually upright while being stunted. 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. Infected leaves are smaller, 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. Whiteflies and disease transmission Specific varieties of whiteflies (Bemisia) are responsible for transmitting this disease. Look closely to see that some whiteflies hold their wings tent-wise, over their bodies, like butterflies, while others hold theirs flat, like moths. In the same way, some nymphs will have smooth edges, while others have a fringe of filaments. You can capture whiteflies for these close inspections the same way you use clear packing tape wrapped around your hand to remove lint or pet hair from a pair of pants. Once they are stuck, you can use a magnifying glass or hand lens for a closer look. The whiteflies that hold themselves under a tent and whose nymphs have smooth edges that carry the tomato yellow leaf curl virus. Leafhoppers may also 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 why tomatoes are generally not grown in tropical and semitropical regions. Cold winters tend to kill off the whitefly vectors, but not always. You can use these tips to protect your tomato plants:
Tomato ringspot is a viral disease that can kill far more than your dreams of sweet summer salsa. This virus infects an astounding number of other plants and is fatal.  Tomatoes infected with tomato ringspot virus (sorhocam.com) Public Domain In addition to tomatoes, the tomato ringspot virus infects stone fruits, apples, grapes, cucumbers, cowpeas, beans, strawberries, currants, soybeans, and cane fruit, including those luscious raspberries and blackberries. This disease also infects begonias, geraniums, iris, hydrangeas, and many other popular garden flowers. Dandelion seeds can also carry this disease. Sadly, tomato ringspot is an incurable, highly contagious disease. Infected plants, and their neighbors, should be removed completely to prevent further spread.  Tomato ringspot virus on currants (USDA) Public Domain The virus responsible for tomato ringspot can be carried through the air, on pollen, or by dagger nematodes in the soil. As they feed on roots, they transfer the virus to healthy plants.
Symptoms of tomato ringspot Plants infected with tomato ringspot may fail to thrive but have no symptoms, acting as a way station for the disease without being impacted directly. They may slowly decline. You may see chlorosis or mottling. Cane fruits may turn dry and crumbly, similar to dryberry mite infestations. Stone fruits may develop prunus stem pitting or yellow bud mosaic. 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. Prunus stem pitting causes late leafing out. Leaves look pale and tend to wilt in summer, turning red or purple early in the season. Fruit size, quantity, and quality are all reduced due to the virus blocking the flow of water and nutrients through the graft union, effectively starving the tree. These symptoms can be mistaken for root damage caused by rats and voles, girdling roots, and fungal diseases. The difference is that the tomato ringspot disease causes the bark, 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 occurs 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. All you can do is remove the infected plants and those nearby and toss them in the trash. Just because symptoms disappear does not mean the infection is gone. Plants that no longer show symptoms can still spread disease throughout the garden. After removing infected plants, the affected area should be allowed to go fallow for at least eight months to starve out any dagger nematodes lurking underground and remove any potential disease-carrying weeds. These tips can help prevent tomato ringspot:
While removing plants from the garden or landscape is disappointing, removing more plants because of an initial delay is worse. You need to know about hollow stem if you grow members of the cabbage family or cucumbers. Hollow stem is a physiological condition that damages the main supporting stem that holds up your cabbage, broccoli, or cauliflower. At first, small cracks develop in the center of the main stem. As the stem grows, the cracks enlarge, creating cavities prone to pest infestations and diseases, especially rotting. The damage is conspicuous when you cut open the stem of a plant with hollow stem.  Cucumber with hollow stem (David B. Langston, University of Georgia, Bugwood.org) CC BY-SA 3.0 Causes of hollow stem
Too much nitrogen and hot temperatures are bad news for cole crops. The nitrogen causes rapid growth, but the heat puts too much strain on these cool-weather crops. As cracks develop within the stem, the pith is lost, and the stem weakens. Hollow stem and boron For many years, botanists and gardeners blamed boron deficiencies for hollow stem. We now know that this is not true. Boron deficiencies darken the edges of areas damaged by hollow stem, but the boron deficiency does not cause the disorder. If you suspect your soil is deficient in boron, send a sample to a lab for an inexpensive soil test. Preventing hollow stem While there’s nothing you can do about a sudden heat wave, there are many things you can do to prevent hollow stem from happening in the first place:
Keep your cucumbers and brassicas healthy by allowing them to grow slowly and steadily. Gumming does not mean your tree has lost its dentures. Instead, it is responding to injury.  Canker-induced gummosis of Tibetan cherry (Rosser1954) Public Domain Unlike people and animals, plants do not have an active immune system. Instead, injured or infected areas are walled off to prevent further injury or the spread of infection or infestation. Gumming refers to how a specialized sap, or botanical gum, oozes out of an injury site or canker to provide protection.
Gumming is particularly common among stone fruits, such as nectarine and almond. It also occurs in mango and citrus. Causes of gumming Environmental stress, mechanical injury, insect attacks, and disease can all trigger a tree to start gumming. Gumming creates a protective barrier and may push any invaders out. You can use specific details surrounding gummosis to identify the problem. For example, gummosis caused by insect infestation or mechanical injury often exhibits bits of bark or sawdust mixed in with the gum. Look for other damage around the gumming site: Do leaves look sick or chewed upon? Has the bark’s integrity been breached? Do you see discoloration under the bark near the gumming?
How to manage gumming You can help your fruit and nut trees stay healthy by avoiding mechanical injuries, monitoring for pests and diseases, and regular feeding and irrigation. If you suspect disease has taken hold, scrape some of the bark from the area surrounding the gum. If you see discoloration or streaking, it is probably a disease that needs further attention. Removing affected branches can sometimes halt the progression of some diseases. Be sure to sanitize cutting tools between each cut with a household cleaner. If the gum emerges from circular holes and contains insect larvae, the tree has the problem in hand. All you need to do is monitor the situation. Who knew apples could get the measles? Unlike the human variety, which is caused by a virus, apple measles is a symptom of manganese toxicity.  Apple exhibiting measles from manganese toxicity University of Georgia Plant Pathology , University of Georgia, Bugwood.org CC BY-SA 3.0
Treating apple measles
Since apple measles is a symptom of too much manganese in the soil, you need to alter the soil chemistry to help your apple tree. Apple measles occurs most commonly in acidic soil, so increasing the soil pH with lime or other alkaline soil amendment will help reduce future damage. Unfortunately, it is very difficult for trees to recover from apple measles, so get your soil tested every 3 to 5 years, so you know what they are dealing with - before the damage is done. Zebra chip may sound like a fun new black-and-white striped snack, but it’s not. It 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 alone. 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 and tubers.  Potato psyllids carry zebra chip bacteria (Whitney Cranshaw, CSU, 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. Zebra chip symptoms are only visible after you cut into a potato. The zebra chip bacteria cause potatoes to store sugar instead of starch. That might sound like an idea for a new dessert food, but the presence of sugars causes 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 may not sprout at all, or they produce weak, infected plants. Controlling zebra chip The only way to control zebra chip is by managing potato psyllids. Yellow sticky sheets trap potato psyllids with little effort on your part. Spinosad can also be used to reduce potato psyllid populations. These treatments won’t eliminate the psyllids, but they will help. Inspect potato, bean, and pepper plants regularly for signs of psyllids. 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 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..  Cherry blossoms infected with blossom brown rot (Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org) CC BY 3.0 US Blossom brown rot looks similar to brown rot (Monilinia fructicola), affecting flowers from the 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 flower blossoms. What should be a colorful, flower-laden tree buzzing with pollinators looks more like sticks with 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 see tan to gray spore masses. Blossom brown rot life cycle 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 and blossom losses. Spores are airborne and spread by irrigation, splashing raindrops, and insects. How to control blossom brown rot This fungus thrives in rainy weather with temperatures in the 70s(F). High humidity can also encourage spore development. Growth is almost directly related to local humidity and temperature levels, which can be tough 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 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. The following species of almond tree 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 any mummies as soon as they are seen and disposing of them in the garbage. Pruning and training for better airflow can also reduce the time needed for blossoms 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 a fatal disease spread by insects. 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. Curly dwarf, also known as artichoke curly dwarf, is caused by the artichoke curly dwarf virus (ACDV). While we know very little about this particular virus, we do know that it is almost always 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, indicate that your plant has become infected with curly dwarf. Leaves may also have dark, dead areas. Preventing curly dwarf As of this writing, we do not know which insects spread curly dwarf. We do know that it is transmissible. Remove infected plants right away. This virus can also spread when divided infected plants 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. Scalybutt is a disease of citrus tree bark. Also known as exocortis, scalybutt is a virus-like disease caused by a particle, not a virus, called the Citrus exocortis viroid (CEVd).
Viroids are the smallest known infectious pathogens, made of a single, naked RNA strand. Other diseases caused by viroids include potato tuber spindle disease, avocado sunblotch, and peach latent mosaic. Scalybutt used to be a serious threat to citrus trees, especially those grown with Trifoliate rootstocks, but strict regulations and agricultural inspections have reduced the likelihood of scalybutt affecting your citrus trees. That is if your trees are relatively young. Older trees (>40 years) may still be infected. This is important because you don’t want to spread exocortis viroids to uninfected trees. Symptoms of scalybutt If you see drying, cracking, and lifting bark, it may be scalybutt. Damaged bark may also peel away from the tree trunk in thin strips in a behavior called shelling. Of course, these are the same symptoms of sunburn damage, so how would a gardener know the difference? For one thing, you may also see gum droplets under the loose bark or stunting. Stunting occurs because nutrients have difficulty moving through damaged or exposed vascular bundles. Sunburn damage generally does not cause stunting or gummosis. Dealing with scalybutt You can’t cure scalybutt, and it is highly contagious. But it probably won’t kill your tree. It will reduce harvests and make the tree susceptible to other pests and diseases. 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, as I do, or you may have fond memories of calico dresses from a favorite prairie-crossing children’s series, but calico in the plant world is something else entirely.  Alfalfa mosaic virus on potato leaves (Howard F. Schwartz, Colorado State University, bugwood.org) CC BY 3.0 Calico is a viral disease that can infect lentils, potatoes, tomatoes, peas, tobacco, and 600 or more other plants. There are several strains of this virus, most of which are species-dependent.The calico virus prefers warm, sunny days and soil with a slightly alkaline pH of 7–7.5. Research has shown that plants infected with calico have difficulty absorbing critical 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. Visible in a 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. The inside of your potato harvest may look dry or corky. If your potato plant looks more like a pale yellow Christmas tree, it is probably potato psyllid feeding. How calico reaches your garden Several aphid species can carry calico. Potato aphids and green peach aphids are the usual culprits. Infected seeds and pollen can also transmit this viral disease, as can alfalfa, clover, wheat, and 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. Galls are like warts or tumors in the plant world.  Oak apples are a type of gall (Maksim) CC BY-SA 3.0 Not really. Galls are neither warts nor tumors, but that’s how many of them appear. The word gall comes from the Latin galla, for oak-apple. Oak apples are not fruits. They are a plant’s reaction to the presence of a foreign substance.  Lime nail galls (Rosser1954) Public Domain The study of plant galls is called cecidology [see-SID-ology]. Most commonly associated with baseball-sized knobs seen on oak trees, galls come in all sizes and can occur on many different plants. Gall description Galls are swellings that occur in response to invasion. That invasion may be in the form of bacteria, fungi, insect larvae, eriophyid mites, nematodes, other pests, and even other plants. Mistletoe is one example of a gall-forming plant. Unlike fungal cankers, which involve plant tissue death, galls, fungal or otherwise, are cases of extra tissue growth. Galls are nearly always woody knobs that may occur anywhere on a plant. Galls may be simple, with a single chamber (unilocular), or highly complex, with multiple pockets (plurilocular). Galls can also look like a sphere, a saucer, pineapples, pinecones, pouches, pods, or fantastic, tiny red spikes. It just depends on the host plant and the cause of the gall.  Root galls found on roses Where they occur and how they look inside can tell you a lot about what caused them.
 Maple leaf galls (Jmeeter) CC BY-SA 3.0 If you cut a gall open, you will see distinctly arranged vascular tissues, depending on the cause of the gall, and an enlarged cambium layer. These distortions interfere with the flow of water and nutrients, leading to wilting and stunting. Or, you may see a large, open area, perfect for use as a larval nursery, with no noticeable impact on the host plant. Insect, mite, and nematode galls When insects invade a plant, they build galls. These galls can act as food or shelter for insects. They are not the same as the plant-produced domatia (tiny apartments) found in some thorns for beneficial insects. Insects inject chemicals (pseudo plant hormones) into host plants, triggering gall formation. Often, eggs are laid in these galls, providing developing larvae with food and protection. Gall wasps, sawflies, gall flies, scale insects, some aphid species, weevils, psyllids, and gall midges can all cause insect galls, but it is nearly always gall wasps or gall midges. Nematodes are microscopic round soil worms that can cause small galls on roots. Root knot nematodes are one such pest. These galls are made up entirely of plant tissue, unlike fungal and bacterial galls, which incorporate fungal or bacterial tissues, respectively. Insect galls may also house interlopers, technically called inquilines. Fungal galls When a fungus infects a plant, it grows alongside plant cells, creating swollen areas that can develop into galls. Several varieties of rust can cause galls to form. When these galls form on conifers, as in the case of cedar apple rust, they look like glutinous fingers called telial horns.  Dried cedar-apple rust gall Fungal galls on other types of leaves tend to look more spherical.  Eucalyptus leaf gall (fir0002) GFDL 1.2 Bacterial and viral galls Bacterial and viral galls develop because the bacteria or virus reprograms plant cells into producing more bacteria, viruses, or other supportive cells. Galls at or just below the soil level are nearly always crown gall. Crown gall is a bacterial disease that can occur on blackberries, sunflowers, grapes, and roses, along with almond, apple, apricot, cherry, and pear trees. Galls on roots may mean clubroot, a disease caused by parasites known as Phytomyxea. On the other hand, root galls may also indicate the presence of beneficial, nitrogen fixing Rhizobium bacteria. Galls have long been used in leather tanning, to make ink, and as astringents. Most galls contain high levels of tannic acid and resin. There are even a few edible galls, corn smut being the most notable. Sometimes, what looks like a gall is herbicide overspray. New and undifferentiated plant cells are most vulnerable to gall formation. Spring is a good time to monitor plants for signs of galls. Once gall development begins, the tissues have been reprogrammed and cannot return to normal. Gall management In a word, you can’t. Insect and mite galls rarely harm plants, and you can’t control these pests completely, anyway. Once they are inside the plant, there is nothing you can spray or apply that will reach them. Anyway, the gall is already in place. Fungal and bacterial galls may be prevented or reduced with fungicide treatments if you can time it perfectly. Or not.  Leaf galls on rhododendron (Xemenendura) CC BY-SA 3.0 If you are galled by galls, remove them with a sharp knife. Otherwise, recognize that galls are just another amazing aspect of playing with plants.
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. 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 several varieties 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 of carrots (Public Domain) Black root rot is a fungal disease. This fungus has a strange name, Thielaviopsis basicola. [THEE-lay-vee-OP-sis.] But being able to pronounce the Latin isn’t as important as recognizing 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 host plants Black root rot is a frequent problem for commercial growers of ground covers, cotton, rice, 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, which is why quarantining new plants is so important. In addition to those nursery crops, black root rot can appear on several garden plants, including beans and peas, carrots, citrus, cucurbits, horseradish, lentils, melons, peanuts, soybeans, berries, potatoes and tomatoes. Many times, you won’t see damage to roots until after harvest. Identifying black root rot Black root rot first appears as irregular growth patterns followed by uneven stunting. At this point, dig up a sample plant, wash its roots, and look for signs of infection. If the roots or stele exhibit elongated, reddish-purple lesions, it is probably black root rot. [The stele is the center, darker portion of a root.] Over time, those lesions will turn black. Infected roots may also be enlarged, with rough, black cracks. Infected roots are usually severely stunted. Conditions that favor black root rot Black root rot occurs most often in moist soil at 55° and 61°F. Fungus gnats, shore flies, splashing rain or irrigation water, infected flats, containers, and garden tools can spread black root rot. Soggy soil, poor drainage, and too much fertilizer all increase the likelihood of these soil-borne fungi taking hold of your plants. This condition occurs more often in alkaline soil. Preventing black root rot Commercial nurseries use chemical fungicides as preventive measures, but once the infection has occurred, the plant is a goner. [Always throw diseased plants in the trash bin.] In severe cases, soil solarization may be necessary. The best way to avoid black root rot is to provide plants with good drainage, avoid overwatering and excessive fertilizer, and control fungus gnat populations with yellow sticky paper. Acidifying the soil can help somewhat, but soil pH is 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. Before placing a bare-root tree or containerized plant in the ground, check for girdling roots. Girdling roots work very much as your great-grandmother’s elastic undergarment did. They cut off circulation and restrict movement. Girdling roots can kill your tree.  Girdling roots (cultivar413) CC BY 2.0 What is girdling? To ‘girdle’ means to surround something. When girdling happens aboveground, it refers to the removal of a ring of bark. This bark contains the cambium layer, which houses the vascular bundles that transport water, oxygen, and nutrients. Tree supports can result in girdling if left in place too long or installed improperly. Girdling roots create the same problem by encircling the trunk and neighboring roots, effectively cutting off the supply of water, oxygen, and nutrients. Wherever girdling occurs, whatever lies beyond the girdle will, in most cases, die. Healthy trees Perennial roots of healthy trees reach out horizontally, anchoring the tree in place and providing a pathway for resources to enter the tree. Those elements move through the xylem of transport roots after being collected by feeder roots. Unless they can’t, that is.
Roots emerge just below the root collar, where the trunk flares out. Trees planted too deep look more like pencils sticking out of the ground.If the root collar is buried below the soil level, as often occurs with modern, mass-produced trees, girdling, pest infestations, and fungal diseases are more likely. Planting trees too deep makes it harder for roots to access the water, nutrients, and oxygen they need.
Symptoms of girdling roots In some cases, girdling roots can be seen simply by looking at the base of the trunk. If the problem is occurring underground, these symptoms may provide a clue about what is happening underground:
In severe cases, the bole indentation can create structurally weak areas prone to twisting or breaking. Of course, a tree will struggle on, growing but not thriving. Within 2 to 10 years, the tree dies for no apparent reason. How to prevent girdling roots Inspect the root system before planting a tree or shrub. Loosen and straighten any circling roots. Remove any that are too woody to straighten. Next, ensure the planting hole is of the proper depth and diameter. Do not “spin” roots into a too-small hole. Dig the hole wide enough to allow the roots to spread out horizontally. Wider holes are better. Deeper holes are not. Dig the planting hole as deep as needed to keep the roots slightly below the soil surface. Planting it any deeper than that is setting the stage for failure. Loosen compacted soil, or the new roots will circle in this new ‘container’. Building foundations and big rocks can cause girdling roots. Mulching the area with arborist wood chips is an excellent way to (slowly) reduce compaction while keeping down weeds, stabilizing soil temperatures, and retaining moisture. Responding to girdling roots Giving a tree with girdling roots more water and fertilizer will not fix the problem. The offending roots must be exposed and removed. If that sounds like a lot of work, it is. First, remove enough soil to expose the choking roots. Make the first cut six to twelve inches from the trunk, usually with a chisel or a saw. This first cut prevents damage to the tree trunk. Very often, there is a lot of tension held in big roots. Suddenly releasing this tension in the wrong way can be catastrophic. For example, an apple tree with a 20” diameter trunk can weigh nearly 2-1/2 tons, so tension is probably occurring somewhere. The final cut is where the root meets the trunk. You can leave the severed roots where they are. Eventually, they will decompose. And this allows helpful microorganisms to migrate and continue their good works. Substantial roots must be removed in stages by a professional arborist. Cutting them off all at once eliminates their supply of water and nutrients, putting the tree at risk. Tree roots provide the lifeblood of the tree. If those roots start circling, you need to take corrective measures for the tree to survive. Yeast may help bread rise, beer froth, and wine ferment, but what does it do in the garden? Is yeast a plant? An animal? Actually, yeast is a one-celled, sugar-eating fungus that urinates alcohol and farts carbon dioxide. It is those farts that make bread rise, and the other excretion that puts alcohol into beer and wine.  Beer and bread produced with yeast (Jon Sullivan) Public Domain Yeasts have been around for hundreds of millions of years. In many ways, yeasts and other fungi are a lot like plants, but they are different in many ways, too. Yeast vs. plant cells Yeast cells are smaller than plant cells. While plant cells can become leaf, stem, fruit, or root cells, yeast cells remain the same one-celled creature. Like plant cells, yeast cells have a cell wall, but it is not made out of cellulose. Both yeast and plant cells have membranes, but they are made out of different materials. Yeast cells do not have chlorophyll-producing chloroplasts, either, which is why they do not use sunlight to make food. Inside a plant cell, you can find food stored as a starch, whereas yeasts store food as sugar. Other common fungi include mushrooms and molds. The yeast you buy in the store to make bread, beer, or wine is called Saccharomyces cerevisiae. How do yeasts grow? Yeasts do not require sunlight to grow. Yeasts feed on dead, decaying matter, making them saprophytes. They are also parasites, which makes them heterotrophs. Instead of using the sun’s energy to generate food, fungi absorb carbon, in the form of sugars, organic acids, and other easy-to-digest carbon-based edibles from their hosts. This is why you will often see fungi growing on the skins of apples, grapes, or peaches. Yeasts grow best when the environment has a neutral or slightly acidic pH.
Yeasts reproduce in a variety of ways, depending on the species and environmental conditions. The most common method of yeast reproduction is an asexual, vegetative method called budding. In budding, a clone offspring is produced as an attachment to the parent cell. When the clone, also known as a bleb or daughter cell, reaches maturity, it separates from the parent cell, leaving behind scar tissue. In some cases, these buds can link themselves together into chains, called false hyphae. Other yeasts reproduce using mitosis. In mitosis, the genetic information is duplicated and the nucleus of the cell is split in half, with each half being a twin to the other. Fission and meiosis are also used by some yeast species, but we digress Harmful yeasts in the garden Yeasts love to eat sugar. As such, they can spoil fruits and vegetables before you ever get a chance to enjoy them. Yeasts can grow on almonds, pineapples, lettuces, and pretty much anything else you decide to grow. Peach leaf curl is also caused by a yeast. That being said, yeasts actually perform many beneficial services to your garden.  Peach leaf curl on nectarine Beneficial yeasts in the garden
Yeasts are not all bad. When it comes to protecting your strawberries, cherries, and cane fruits from the invasive spotted wing drosophila, torula yeast is used to lure this pest to its death. Also, recent research has demonstrated that adding brewers yeast to the garden may help plants counteract the effects of toxins in the soil. In another amazing study, it was found that a yeast found on the bodies of pollinating bees gets knocked onto flowers, as bees move around. Those yeasts then feed on the sugars in the flowers’ nectar. Breaking down those sugars generates heat, which keeps the flowers and the surrounding air space (used by the bees) warmer. This helps both parties survive winter. Yeasts also play a role in soil aggregate formation, improving soil structure. While other yeasts are believed to be part of the sulfur and nitrogen cycles, and make insoluble phosphates available to plants. Yeasts are food for many bacteria, insects, and other soil predators. If nothing else, adding yeast to the compost pile will speed decomposition, assuming moisture and temperature levels are appropriate. Did you know that some yeast species produce toxins that kill off other yeast species? Now you know. 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. Other strains of Streptomyces cause other potato diseases. S. scabies is found in the soil everywhere potatoes are grown. These bacteria can infect young seedlings of any plant but are most commonly associated with root and tuber crops, especially potatoes.
Potato skins Delicious twice-baked and cut into wedges and served with sour cream and butter, potato skins are the cork, or periderm, layer found underneath the bark of other plants. This layer protects plants from pests and disease. You may see tiny nicks of color in a potato’s skin. These are called lenticels used in respiration. And where the S. scabies bacteria gets in and starts infecting a potato. Symptoms of potato scab After entering a potato through a lenticel or wound site, S. scabies gets to work. 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 lesions that can vary in size and color 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:
These lesions start circular but can spread into larger, irregularly shaped areas. Potato scab lesions look much like powdery scab. Powdery scab is a fungal disease caused by Spongospora subterranea. How to prevent potato scab Prevent potato scab by manipulating soil moisture, texture, and pH. And by 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. 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 Water molds are 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, 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 Botanists identified the potato blight pathogen in 1843. Initially, it was limited to New York and Philadelphia. But local winds spread spores into neighboring regions. Potatoes were not grown in North America until the 1500s. And they did not become a popular crop until the 1700s. At first, potato blight wasn’t considered a serious threat to anyone. Then, seed potatoes sent to Belgium in 1845 spread this devastating disease across Europe. Since monoculture of a single potato species was common practice, it wasn’t difficult for this disease to take hold. It spread like wildfire. Ireland suffered the most in what became known as the Great Famine, or the Great Starvation. Potato blight lifecycle The potato blight pathogen prefers cool temperatures and moist environments, which Ireland has in abundance. Spores are produced from 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 plants in the nightshade family, such as tomatoes. Early blight (Alternaria solani) is more 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.] It 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 as small, dark green, irregularly shaped, water-soaked spots on leaves, stems, petioles, and tubers. These spots have a yellowish halo. These lesions rapidly expand 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 Scientists have crafted a group of genetically modified potatoes resistant to potato blight. These cisgenic potatoes appear unable to catch the disease. If you prefer not to grow 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 collected to make artillery shells, farmers faced new threats from potato blight because they could not spray their fields. Potato blight can find its way into your potato bed through contaminated potatoes, visitors, materials, and rain or irrigation water splashing from infected plants to healthy plants. These are excellent reasons for quarantining new plants and not planting grocery store produce. [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.] Allow the soil to dry out between waterings, pruning for good airflow, and add organic material to your potato bed to improve drainage. A healthy potato bed is a thing of beauty. Let’s keep it that way! |
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