Category Archives: Disease – Vegetable

Disease - Vegetable, Fact Sheet

Bacterial Spot of Tomato

Sunken, scabby bacterial spot lesions on ripening tomato fruit. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University)
Sunken, scabby bacterial spot lesions on ripening tomato fruit. (Photo courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University)

What is bacterial spot?  Bacterial spot of tomato is a potentially devastating disease that, in severe cases, can lead to unmarketable fruit and even plant death.  Bacterial spot can occur wherever tomatoes are grown, but is found most frequently in warm, wet climates, as well as in greenhouses.  The disease is often an issue in Wisconsin.

What does bacterial spot look like?  Bacterial spot can affect all above ground parts of a tomato plant, including the leaves, stems, and fruit.  Bacterial spot appears on leaves as small (less than ⅛ inch), sometimes water-soaked (i.e., wet-looking) circular areas.  Spots may initially be yellow-green but darken to brownish-red as they age.  When the disease is severe, extensive leaf yellowing and leaf loss can also occur.  On green fruit, spots are typically small, raised and blister-like, and may have a yellowish halo.  As fruit mature, the spots enlarge (reaching a maximum size of ¼ inch) and turn brown, scabby and rough.  Mature spots may be raised or sunken with raised edges.  Bacterial spot symptoms can be easily confused with symptoms of another tomato disease called bacterial speck.  For more information on this disease, see UW Plant Disease Facts D0011, Bacterial Speck of Tomato.

Where does bacterial spot come from?  Bacterial spot of tomato is caused by Xanthomonas vesicatoria, Xanthomonas euvesicatoria, Xanthomonas gardneri, and Xanthomonas perforans.  These bacterial pathogens can be introduced into a garden on contaminated seed and transplants, which may or may not show symptoms.  The pathogens enter plants through natural openings (e.g., stomates), as well as through wounds.  Disease development is favored by warm (75° to 86°F), wet weather.  Wind-driven rain can contribute to more severe disease as the pathogens are splashed and spread to healthy leaves and fruit.  Bacterial spot pathogens can survive well in tomato debris, but they survive very poorly in soil when not associated with debris.

On tomato leaves, bacterial spot leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Michelle Grabowski, University of Minnesota Extension)
On tomato leaves, bacterial spot leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Michelle Grabowski, University of Minnesota Extension)

How do I save plants with bacterial spot?  A plant with bacterial spot cannot be cured.  Remove symptomatic plants from your garden or greenhouse to prevent the spread of bacteria to healthy plants.  Burn (where allowed by local ordinance), bury or hot compost the affected plants, and DO NOT eat symptomatic fruit.  Although bacterial spot pathogens are not human pathogens, the fruit blemishes that they cause can provide entry points for human pathogens that could cause illness.

How can I prevent bacterial spot in the future?  Plant pathogen-free seed or transplants to prevent the introduction of bacterial spot pathogens on contaminated seed or seedlings.  If a clean seed source is not available or you suspect that your seed is contaminated, consider treating the seed in hot water prior to planting to eliminate the pathogens.  For details on this process, including the proper temperature and length of treatment, see UW Plant Disease Facts D0064, Hot-Water Seed Treatment for Disease Management.  To keep leaves dry and to prevent the spread of the pathogens, avoid overhead watering (e.g., with a wand or sprinkler) of established plants.  Instead use a drip-tape or soaker-hose.  Also, to prevent spread, DO NOT handle plants when they are wet (e.g., from dew), and routinely sterilize tools with either 10% bleach solution or (better due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol).  Where bacterial spot has been a recurring problem, consider using preventative applications of copper-based products registered for use on tomato, especially during warm, wet periods.  Keep in mind however, that if used excessively or for prolonged periods, copper may no longer control the disease.  Be sure to read and follow all label instructions of the product that you select to ensure that you use it in the safest and most effective manner possible.  Burn (where allowed by local ordinance), bury or hot compost tomato debris at the end of the season.  Wait at least one year before planting tomatoes in a given location again, and remove and burn, bury or hot compost any volunteer tomatoes that come up in your garden.

For more information on bacterial spot of tomato:  Contact your county Extension agent.

Disease - Vegetable, Fact Sheet

Bacterial Speck of Tomato

What is bacterial speck?  Bacterial speck is a common disease of tomato that occurs worldwide wherever tomatoes are grown.  The disease can substantially reduce yield when it severely affects leaves early in the growing season.  The disease can have an even greater impact on quality (and marketability for commercial tomato producers) when symptoms occur on tomato fruit.

Small, brown/black spots on a green tomato characteristic of bacterial speck. (Photo courtesy of S. T. Koike)
Small, brown/black spots on a green tomato characteristic of bacterial speck. (Photo courtesy of S. T. Koike)

What does bacterial speck look like?  Leaf symptoms of bacterial speck consist of small black spots (approximately ⅛ to ¼ inch in diameter) that often are more prominent on the undersides of leaves.  As the spots age, a yellow halo may develop around the edge.  Spots on fruit are very small (almost pinpoint-like) and do not penetrate very deeply into the tissue.  The spots can be raised, flat or sunken, and range in color from brown to black.  On unripe, green fruits, the spots often have darker green haloes, while on ripe fruits the spots can have subtle, yellow haloes.  Leaf symptoms of bacterial speck can be hard to distinguish from other tomato diseases.  Bacterial spot, (see UW Plant Disease Facts D0012, Bacterial Spot of Tomato) and tomato spotted wilt (a viral disease) may cause similar leaf symptoms.  Laboratory testing may be needed to determine which disease is affecting your tomatoes.

Where does bacterial speck come from?  Bacterial speck of tomato is caused by the bacterium Pseudomonas syringae pv. tomato.  The bacterium is typically brought into a garden on contaminated tomato seeds or transplants, and thrives in cool (63°F to 75°F), wet weather.  It can be moved from plant to plant via splashing water (e.g., from rain or overhead watering with a sprinkler) or on hands and gardening tools when working with contaminated and then healthy plants.  The bacterium can overwinter in dead tomato debris or on porous materials such as wooden plant stakes or trellises.

How do I save tomatoes with bacterial speck?  Once tomatoes are infected, there is no cure.  You may be tempted to cut off affected leaves as symptoms develop, but this will likely not do much to minimize or slow disease development and may actually promote spread of the pathogen.  Often, the best course of action is to allow the disease to run its course and simply salvage any unblemished fruit as they ripen over the summer.  DO NOT eat symptomatic fruit.  Although the bacterial speck pathogen is not a human pathogen, the fruit blemishes that it causes can provide entry points for human pathogens that could cause illness.

On tomato leaves, bacterial speck leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Alan Collmer, Cornell University)
On tomato leaves, bacterial speck leads to small, angular (i.e., straight-edged) spots with yellow haloes. (Photo courtesy of Alan Collmer, Cornell University)

How can I prevent bacterial speck in the future?  Start by using high quality, pathogen-free seed or transplants from a reputable seed supplier or garden center.  If you have seed that you believe is contaminated with the bacterial speck bacterium and would still like to use it (e.g., it’s a favorite variety with difficult-to-find seed), consider treating the seed in hot water prior to planting to eliminate the pathogen.  For details on this process, including the proper temperature and length of treatment, see UW Plant Disease Facts D0064, Hot-Water Seed Treatment for Disease Management.

To prevent spread of the bacterial speck pathogen from plant to plant in your garden, DO NOT use a sprinkler to water; instead use a soaker or drip hose to water at the bases of plants.  Also, only work with tomato plants when they are dry, and consider routinely disinfecting garden tools with 10% bleach or (better due to its less corrosive properties) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants that contain approximately 70% alcohol can also be used for this purpose.

If you have a problem with bacterial speck, remove contaminated tomato debris from your garden at the end of the growing season.  This material can be deep buried, burned (where allowed by local ordinance) or hot composted.  DO NOT replant tomatoes in the same area the following growing season; instead grow a nonsusceptible vegetable crop.  This approach is referred to as non-host crop rotation.  For more information on this technique, see University of Wisconsin Garden Facts XHT1210, Using Crop Rotation in the Home Vegetable Garden.

As a last resort, consider chemical treatments for bacterial speck control.  If you decide to go this route, use a product that is labeled for use on tomatoes and that contains copper as the active ingredient.  To be most effective, the first treatment must be applied before symptoms have developed.  Apply additional treatments every 10 to 14 days as long as cool, moist conditions continue.  Keep in mind however, that if used excessively or for prolonged periods, copper may no longer control the disease.  Be sure to read and follow all label instructions on the product that you select to ensure that you use it in the safest, most effective manner possible.

For more information on bacterial speck of tomato:  Contact your county Extension agent.

Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Cucumber Mosaic

What is cucumber mosaic?  Cucumber mosaic is a viral disease of worldwide distribution that affects over 1200 plant species.  Hosts include a wide range of fruits, vegetables, herbaceous and woody ornamentals, and weeds.  The disease has perhaps its biggest impact in vegetable production where it can cause significant losses in yield and vegetable quality.

Cumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)
Cucumber mosaic on pepper (left) showing yellowing and ring spots, and on broad bean (right) showing mosaic and puckering of leaf tissue. (Photos courtesy of Russ Groves)

What does cucumber mosaic look like?  Symptoms of cucumber mosaic can vary widely depending on host species, host variety, and time of infection.  Typical symptoms include stunting of entire plants, mosaic or mottling (i.e., blotchy white, yellow, and light green areas) and ring spots (i.e., ring-like areas of discolored tissue) on leaves and fruits, and a variety of growth distortions such as cupping, puckering and strapping (i.e., elongation and thinning) of leaves as well as warts on fruits.  In extreme situations, parts of an affected plant or even an entire plant may die from the disease.

Where does cucumber mosaic come from?  Cucumber mosaic is caused by Cucumber mosaic virus (CMV) which can overwinter in susceptible biennial or perennial weeds, as well as in perennial agricultural crops (e.g., alfalfa) and perennial herbaceous and woody ornamentals.  Seeds and even pollen from certain host plants can carry the virus, and thus the virus can be spread via these plant parts.  More commonly, CMV is spread by aphids [see the University of Wisconsin Garden Facts XHT1043 (Aphids) for details on these insect pests] which can pick up the virus from infected plants and transmit it to healthy plants as they feed.  Over 80 species of aphids can potentially transmit CMV.  The severity of cucumber mosaic oftentimes depends on the size and activity of aphid populations in an area, as well as on the number infected plants in an area serving as reservoirs for the virus.

Cumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)
Cucumber mosaic on hibiscus (left) showing mosaic and puckered leaves, and on bluebell (right) showing mosaic and line patterns. (Photos courtesy of Brian Hudelson)

How do I save plants with cucumber mosaic?  There is no known cure for cucumber mosaic.  Infected plants should be removed and destroyed to eliminate the plants as potential reservoirs for the virus (which can subsequently be spread to other nearby healthy plants).  Infected plants can be burned (where allowed by local ordinance), deep buried or hot composted.  Killing infected plants with herbicides can also be an effective management strategy.

How do I avoid problems with cucumber mosaic in the future?  Buy certified, virus-free seeds and plants.  Consider using CMV-resistant varieties of lettuce, spinach, cucurbits (e.g., cucumber, melon and squash) and other vegetables where available.  Seed catalogs often contain information on CMV resistance that can be useful for variety selection.  Remove weed hosts whenever possible around your garden, and mulch vegetable and ornamental gardens to inhibit weed growth.  Consider using floating row covers where possible to prevent aphids from reaching susceptible plants.  DO NOT use insecticides to control aphids because such treatments are unlikely to act fast enough to prevent aphids from transmitting CMV, and may actually stimulate aphids to move and feed more widely, thus leading to increased spread of the virus.

For more information on cucumber mosaic:  Contact your county Extension agent.

Disease - Vegetable, Fact Sheet, Uncategorized

Basil Downy Mildew

Basil Downy Mildew
Leaf curling and yellowing, as well as a gray-purple fuzz on the bottoms of leaves, is typical of basil downy mildew. (Photo courtesy of Debbie Roos)

What is basil downy mildew?  Basil downy mildew is a devastating disease that affects the leaves, branches, and stems of many types of basil (i.e., plants in the genus Ocimum), plants commonly used for cooking.  Green-leafed varieties of sweet basil are particularly susceptible to the disease, while purple-leafed varieties of basil, Thai basil, lemon basil, and spice basil are less susceptible.  Certain ornamental basils (e.g., hoary basil) appear to be highly resistant to the disease.  Basil downy mildew was first reported in the United States in 2007 and has since spread widely to wherever basil is grown, including Wisconsin.

What does basil downy mildew look like?  Symptoms of basil downy mildew typically develop first on lower leaves, but eventually an entire plant will show symptoms.  Initial symptoms include leaf yellowing (which gardeners often think is due to a nitrogen deficiency) followed by leaf browning.  Affected leaves also curl and wilt, and on the undersides of the leaves, a gray-purple fuzzy material will develop.

Where does basil downy mildew come from?  Basil downy mildew is caused by the fungus-like water mold organism, Peronospora belbahrii.  This pathogen can be easily introduced into a garden each year via contaminated seed, on infected transplants, or via wind-borne spores (technically called sporangia).  Once introduced into a garden the pathogen can spread by wind, by rain splash, or via items (e.g., hands, clothing, garden tools) that come into contact with infected plant and then are used to work with healthy plants.  The pathogen thrives in humid, warm environments and can spread rapidly, decimating an entire basil crop.

How do I save plants with basil downy mildew?  There is no known cure for basil downy mildew.  If you see basil downy mildew, harvest any asymptomatic leaves on infected plants, as well as other healthy basil plants in your garden.  Use these materials immediately (e.g., to make pesto).  Remove and bag any symptomatic plant remains and dispose of this material in your garbage.

How do I avoid problems with basil downy mildew in the future?  Avoid planting sweet basil if possible.  Instead, plant other types of basil that are more resistant to basil downy mildew.  If you decide to grow sweet basil, try growing the variety ‘Eleonora’ which has been bred for at least some resistance to the disease.  If you grow basil from seed, check to see if the seed you are buying has been steam-treated to kill the downy mildew pathogen.  Be aware however, that this information may be difficult to find, because steam treatment of basil seed is relatively new and the use of this technique is not widely advertised (at least to home gardeners).

Whatever type of basil you choose, try to grow your plants in a manner that will keep them as dry as possible, thus creating an environment that is less favorable for the downy mildew pathogen to develop and infect.  Plant basil in a sunny location, space plants as far apart as possible and orient rows in the direction of prevailing winds to promote good airflow and rapid drying of plants when they get wet.  Avoid overhead watering (e.g., with a sprinkler) that will wet leaves and spread the pathogen; instead, use a drip or soaker hose to water.

Use of fungicide treatments to control basil downy mildew is NOT recommended.  Products that currently are available to homeowners, even when applied in the best manner possible, will likely not control the disease adequately, if at all.  Using these products would be a waste of time, effort and money.

For more information on basil downy mildew:  Contact your county Extension agent.

Disease - Broad Leafed Woody Ornamental, Disease - Fruit Crop, Disease - Herbaceous Ornamental, Disease - Needled Woody Ornamental, Disease - Vegetable, Fact Sheet, Misc

Wood Mulch and Tree Health

What are the benefits of wood mulch?  Wood mulch is typically available as chipped wood, or shredded or chunked bark, and can contribute to tree health in many ways.  When high quality, composted mulches are applied two to four inches deep in a ring three to six feet in diameter (or greater) from the trunk of a tree, mulch can help preserve moisture, control weeds, limit damage to the trunk from mowers and string trimmers and moderate the soil temperature.  Use four inches of mulch when soils are light and well-drained, and two inches of mulch on heavier, clay soils.

Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.
Use of properly composted mulches can be beneficial to trees and shrubs in the landscape.

Can wood mulch harm trees?  Use of improperly composted mulches (some-times called “sour mulches”), can lead to tree nutrient deficiencies.  Sour mulches can also produce gases like methane and ammonia that can be toxic to plants.  Foliage on trees surrounded by sour mulches may initially turn yellow, then brown, die and fall off.  If your mulch smells like vinegar, ammonia or sulfur, it is likely a sour mulch and should be removed.  Replace the sour mulch with a high quality, composted mulch and consult with your local Extension agent about testing the soil for nutrient deficiencies.  Fertilize appropriately based on the results of these tests.

Improper application of mulch can also lead to problems.  Piling wood mulch up against the trunk of a tree can keep the bark underneath excessively wet.  This wetness can contribute to bark decay.  In addition, use of thick mulch layers (greater than four inches) can lead to overly wet soils that are favorable for development of root rots (see UW Plant Disease Facts D0094, Root and Crown Rot).  To avoid these problems, make sure mulch is applied at least four inches away from the trunk of a tree and that the mulch layer is the appropriate thickness for the soil type in your landscape (see above).

Does woody mulch harbor or attract insects?  Insects such as earwigs [see UW Bulletin A3640, Controlling Earwigs (available at https://learningstore.extension.wisc.edu/)], centipedes (see University of Wisconsin Garden Facts XHT1113, Centipedes), millipedes (see University of Wisconsin Garden Facts XHT1108, Millipedes) and sowbugs (see University of Wisconsin Garden Facts XHT1110, Sowbugs) can feed on decaying organic matter in mulches.  While these insects are often only nuisances, earwigs can feed on and cause damage to a variety of ornamentals, particularly to flowering plants.  If mulch is used near entrances to a home or around basement windows, these unwanted insects may get inside.  Termites ingest wood and can be attracted to wood mulch, but new termite colonies are not likely to become established due to use of wood mulches.  Typically, termites are not a problem in Wisconsin, and when colonies are found, they occur only in the southern half of the state.

Carpenter ants [see UW Bulletin A3641, Controlling Carpenter Ants (available at https://learningstore.extension.wisc.edu/)] and powderpost beetles (see University of Wisconsin Garden Facts XHT1053, Powderpost Beetles) are unlikely to utilize mulch as a food source because conditions required for their development would not be satisfied by wood mulch.  Carpenter ants do not ingest wood as a food source; instead, they chew non-living wood (in trees or landscape timbers, etc.) to excavate galleries in which they live and raise their young.  Since wood mulch is composed of small wooden pieces, it would not serve as a home.  To avoid potential insect problems, keep mulch as far away from the foundation of your home as possible and seal all holes and crevices that insects might use as entry points.  Also, periodically inspect landscape timbers and the house for termites.

Does woody mulch harbor tree pathogens?  Wood mulch may come from many sources, including trees and shrubs that have died from a wide range of diseases.  To be harmful to your trees, disease-causing organisms (pathogens) would have to survive in mulch and these organisms would have to move from the mulch either directly, or through the soil, to their new host – your tree.  There is currently very little research on this topic.

Elm trees killed by Dutch elm disease (see UW Plant Disease Facts D0045, Dutch Elm Disease), can serve as breeding areas for native and European elm bark beetles.  Bark beetles that breed in logs or firewood from these trees can pick up the fungi that cause Dutch elm disease (Ophiostoma ulmi and Ophiostoma novo-ulmi) and carry these fungi from tree to tree.  Chipping infected elm trees creates an unfavorable environment for bark beetles yet there is no scientific literature that describes the level of risk of transmitting the Dutch elm disease fungi from wood chips or bark chunks to healthy elms.

Oak trees killed by oak wilt (see UW Plant Disease Facts D0075, Oak Wilt) can be attractive to several sap-feeding beetles that can potentially pick up the oak wilt fungus (Bretziella fagacearum) and move it in the landscape.  This process is affected by moisture and temperature and would likely be disrupted by the chipping and composting process yet there is no scientific literature that describes the level of risk of transmitting the oak wilt disease fungus from wood chips or bark chunks to healthy oaks.

Research at the University of Wisconsin-Madison suggests that wood chip mulches produced from trees suffering from Verticillium wilt (see UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs) can serve as a source of the fungus (Verticillium dahliae) that causes the disease.  These studies show that Verticillium can survive for at least one year in mulch and that use of this contaminated mulch can lead to Verticillium wilt in both woody and herbaceous plants.  Therefore use of mulches produced from trees with Verticillium wilt should be avoided.

For more information on wood mulch and tree health:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

White Mold

Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.
Stem cankers, cottony mycelia and sclerotia (see arrows) of white mold on snap beans.

What is white mold?  White mold, also known as Sclerotinia stem rot, is a serious and typically lethal fungal disease that affects over 400 species of plants in many plant families.  White mold causes severe damage in commercially grown snap beans, kidney beans, lima beans and soybeans (see UW Plant Disease Facts D0099, Sclerotinia Stem Rot), as well as commercially grown sunflowers.  White mold also can be a serious problem in home vegetable gardens on tomatoes, potatoes, cucurbits (e.g., cucumber, pumpkin and squash), carrots, lettuce, celery, brassicas (e.g., cabbage, broccoli, and cauliflower), basil, and rosemary.  White mold is a common disease on many annual and perennial ornamentals as well.

What does white mold look like?  Symptoms of white mold vary depending on the plant infected.  White mold can lead to crown and stem cankers (i.e., localized infected areas), root rots, wilts, damping-off of seedlings, and blossom and fruit rots.  Plants affected by white mold can wilt rapidly due to stem-girdling cankers at or near the soil line.  Brown spots can appear on flower petals and buds.  Cottony masses of fungal threads (called mycelia) may appear on stems or on nearby soil.  Hard, irregularly-shaped masses (called sclerotia) develop within or on the surface of infected plants.  The sclerotia are white at first and then turn dark brown or black as they mature, resembling mouse or rat droppings.

Where does white mold come from?  White mold is caused by several species of the fungus Sclerotinia, most commonly Sclerotinia sclerotiorum.  White mold fungi overwinter as sclerotia in dead plant material or in infested soil.  Sclerotia can survive in soil for at least five years.  During periods of cool, wet or humid weather, sclerotia germinate to form either fungal threads (called hyphae) or tiny, mushroom-like spore-producing structures (called apothecia).  Apothecia can release millions of spores over a period of several days.  Spores are dispersed primarily by wind, but also by rainsplash and insects.  Both spores and hyphae can infect plant tissue, with infection often occurring through dead or declining plant parts (e.g., flowers, leaves), and then spreading to healthy tissue.  Infections may also occur through healthy plant parts that are growing near or in contact with the soil.  In some plants, seeds can be contaminated by sclerotia or mycelia of white mold fungi.

White mold can be destructive on ornamentals such as Liatris.
White mold can be destructive on ornamentals such as Liatris.

How do I save plants with white mold?  White mold is difficult to manage once infections have occurred.  Prune four to five inches below obviously disease tissue or remove entire plants if they are severely affected.  DO NOT compost these materials because of the risk of spreading white mold fungi via long-lived sclerotia.  Burn these materials instead.

How do I avoid problems with white mold in the future?  Check seed for sclerotia and remove these structures if you find them.  Make sure your soils are well-drained, and avoid overwatering as well as frequent, light watering.  DO NOT overhead water (e.g., with a sprinkler).  Instead water deeply early in the day with a drip or soaker hose.  Promote good air circulation in your garden so that plants more rapidly dry when they do get wet (e.g., from natural rain).  Better air circulation can be achieved by choosing cultivars or varieties of plants that have a more open growth form, by spacing plants farther apart, by avoiding excessive use of nitrogen fertilizers (which can promote excessive production of lush, thick foliage), and by keeping weeds under control.  Weed control also eliminates potential alternate hosts that white mold fungi can infect.  As leaves and flowers or your plants naturally wither and die, remove them, as well as any other plant debris that may harbor white mold fungi.  Finally, in beds where severe cases of white mold have occurred, consider removing and replacing infested soil.

For more information on white mold:  Contact your county Extension agent.

Disease - Vegetable, Fact Sheet

Verticillium Wilt of Vegetables

What is Verticillium wilt?  Verticillium wilt is a typically fatal disease that affects a wide range of vegetable crops in Wisconsin.  Vegetables commonly and severely affected by Verticillium wilt include tomato, potato, pepper, eggplant, cucumber, muskmelon, pumpkin and watermelon.  Other vegetables such as asparagus, bean, beet, broccoli, Brussels sprouts, cabbage, cauliflower, celery, chive, collards, garlic, kale, kohlrabi, leek, lettuce, mustard, onion, parsley, peas, radish, rutabaga, spinach and turnip are susceptible to Verticillium wilt, but show less severe symptoms.  Some vegetables, such as sweet corn are immune to the disease.  Many woody and herbaceous ornamentals are also susceptible to Verticillium wilt (see also UW Plant Disease Facts D0121 Verticillium Wilt of Trees and Shrubs).

A blotchy yellow leaf color can be an initial symptom of Verticillium wilt on eggplant. (photo courtesy of Amanda Gevens)
A blotchy yellow leaf color can be an initial symptom of Verticillium wilt on eggplant. (photo courtesy of Amanda Gevens)

What does Verticillium wilt look like?  Initial symptoms of Verticillium wilt include sudden yellowing of foliage, typically first appearing on one side of a plant.  These leaves then wilt and die.  Oftentimes, symptoms mimic the natural aging process of leaves.  Eventually however, the entire plant becomes affected and dies.  By peeling away the outer layers of affected branches or vines, you may observe brown streaking in the vascular (water-conducting) tissue.  Vascular browning is common in tomato and potato plants.

Where does Verticillium wilt come from?  Verticillium wilt is caused primarily by two fungi, Verticillium dahliae and Verticillium albo-atrum.  These fungi are commonly found in Wisconsin soils.  V. dahliae and V. albo-atrum have wide host ranges including many common weeds, such as ragweed, cocklebur, and velvetleaf.  Susceptible plants and their debris can serve as an overwintering site for these fungi.  In potato, root lesion nematodes (Pratylenchus spp.) can contribute to the development of Verticillium wilt.  However, the role of these nematodes in Verticillium wilt development in other vegetable crops is not well understood.

How do I save a plant with Verticillium wilt?  Vegetables infected with Verticillium cannot be cured and will eventually die.  However, you can extend the life of infected plants by fertilizing and watering them properly.  Fertilization should be based on a soil nutrient test and should be tailored to the specific vegetables being grown.  Most vegetables require approximately one inch of water per week.  If natural rains are not sufficient, then supplemental water should be applied using a soaker or drip hose.

Verticillium wilt eventually leads to wilting and plant death. (photo courtesy of Amanda Gevens)
Verticillium wilt eventually leads to wilting and plant death. (photo courtesy of Amanda Gevens)

How do I avoid problems with Verticillium wilt in the future?  Do not repeatedly grow crops that are highly susceptible to Verticillium wilt in the same area of your garden each year.  Try to rotate highly susceptible crops with other less susceptible crops.  See University of Wisconsin Garden Facts XHT1210, Using Crop Rotation in the Home Vegetable Garden, for details on this process.  Also, try to plant vegetable varieties that are resistant to Verticillium wilt.  In particular, many modern tomato varieties are resistant to the disease.  Watch for varieties with the letter “V” after the variety name.  If Verticillium wilt resistant varieties of your favorite vegetables are not available, select varieties that mature early.  Early maturing varieties are more likely to produce usable vegetables before they eventually succumb to the disease than late maturing varieties.

In addition to using resistant varieties, try to eliminate sources of Verticillium in your garden.  Keep broadleaf weeds under control, as these plants are often susceptible to Verticillium wilt and can serve as a reservoir for the fungus.  DO NOT use mulches that may have been produced from trees (e.g., maple and ash trees) that are susceptible to Verticillium wilt, and immediately collect and discard leaves that have fallen from symptomatic trees.  Also, remove and discard debris from susceptible vegetable plants.  Burn (where allowed by local ordinance) or landfill infected plants and infested plant debris.  DO NOT bury or compost these materials.  Finally, decontaminate any gardening items (e.g., shovels, hoes, plant stakes, pots, working surfaces, etc.) that may have come into contact with Verticillium to prevent spread of the fungus.  Rinse these items with water to remove clinging soil, then treat them for at least 30 seconds with a 10% bleach solution or (preferable for metal tools due its less corrosive properties) 70% alcohol (e.g., rubbing alcohol, certain spray disinfectants).  If you use bleach, be sure to thoroughly rinse and oil metal tools after gardening to prevent rusting.

For more information on Verticillium wilt:  See UW Plant Disease Facts D0121, Verticillium Wilt of Trees and Shrubs (available at https://pddc.wisc.edu); or contact your county Extension agent.

Disease - Vegetable, Fact Sheet

Septoria Leaf Spot

What is Septoria leaf spot?  Septoria leaf spot is one of two common fungal diseases that can devastate tomatoes in both commercial settings and home gardens.  The second common tomato blight, early blight, is detailed in UW Plant Disease Facts D0046, Early Blight.

Septoria leaf spot. Note whitish spots with dark borders characteristic of the disease.
Septoria leaf spot. Note whitish spots with dark borders characteristic of the disease.

What does Septoria leaf spot look like?  Symptoms of Septoria leaf spot first appear at the base of affected plants, where small (approximately ¼ inch diameter) spots appear on leaves and stems.  These spots typically have a whitish center and a dark border.  Eventually multiple spots on a single leaf will merge, leading to extensive destruction of leaf tissue.  Septoria leaf spot can lead to total defoliation of lower leaves and even the death of an infected plant.

Where does Septoria leaf spot come from?  Septoria leaf spot is caused by the fungus Septoria lycopersici, which survives in plant debris or on infected plants.  Septoria leaf spot symptoms typically begin as plant canopies start to close.  Denser foliage leads to high humidity and longer periods of leaf wetness that favor the disease.

How do I save a plant with Septoria leaf spot?  Once symptoms of Septoria leaf spot appear, control is difficult.  Thinning of whole plants or removal of selected branches from individual plants may slow the disease by increasing airflow and thus reducing humidity and the length of time that leaves remain wet.  Fungicides labeled for use on vegetables and containing copper or chlorothalonil may also provide control of Septoria leaf spot if they are carefully applied very early in the course of the disease (before symptoms develop is best) and on a regular basis throughout the rest of the growing season.  If you decide to use fungicides for disease control, be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the product in the safest and most effective manner possible.

How do I avoid problems with Septoria leaf spot in the future?  Septoria leaf spot is best controlled using preventative measures.  Destroy infested plants by burning (where allowed by local ordinance) or burying them.  Rotate vegetables to different parts of your garden each year to avoid areas where infested debris (and thus spores of Septoria lycopersici) may be present.  See University of Wisconsin Garden Facts XHT1210, Using Crop Rotation in the Home Vegetable Garden, for details on this technique.  Use Septoria leaf spot-resistant tomato varieties whenever possible.  Increase spacing between plants to increase airflow and decrease humidity and foliage drying time.  Mulch your garden with approximately one inch of a high quality mulch, but DO NOT overmulch as this can lead to wet soils that can contribute to increased humidity.  Finally, where the disease has been a chronic problem, use of preventative applications of a copper or chlorothalonil-containing fungicide labeled for use on vegetables may be warranted.

For more information on Septoria leaf spot:  Contact your county Extension agent.

Disease - Vegetable, Fact Sheet

Powdery Mildew – Vegetable

Vegetables such as squash and pumpkin are very susceptible to powdery mildew.
Vegetables such as squash and pumpkin are very susceptible to powdery mildew.

What is powdery mildew?  Powdery mildews are diseases that occur on the above-ground parts (especially the leaves) of many agricultural crops (including vegetables), as well as deciduous trees and shrubs, herbaceous ornamental plants, and indoor houseplants.  Conifers are not affected by this disease.

What does powdery mildew look like?  The name of these diseases is descriptive.  The upper and lower surfaces of leaves, as well as stems of infected plants, have a white, powdery appearance.  They look as though someone has sprinkled them with talcum powder or powdered sugar.

Where does powdery mildew come from?  Powdery mildews are caused by many closely related fungi that survive in plant debris or on infected plants.  These fungi are fairly host specific.  The powdery mildew fungus that infects one type of plant (e.g., pumpkins) is not the same powdery mildew fungus that infects another (e.g., phlox).  However, if you see powdery mildew on one plant, then weather conditions (high humidity) are favorable for development of the disease on a wide range of plants.

How do I save a plant with powdery mildew?  DO NOT panic!  For many plants, powdery mildews are cosmetic, non-lethal diseases.  For other plants (e.g., cucumber, squash, pumpkin), powdery mildew can cause severe leaf loss.

Peas are another vegetable that can have severe powdery mildew problems.
Peas are another vegetable that can have severe powdery mildew problems.

When a highly valued plant has had severe leaf loss due to powdery mildew for several years, you may want to consider using a fungicide for control.  Fungicides containing chlorothalonil, copper, mancozeb, myclobutanil, triadimefon, sulfur or thiophanate-methyl are registered for powdery mildew control.  A combination of baking soda (1½ tablespoons) and a light weight (i.e., paraffin-based) horticultural oil (3 tablespoons) in water (1 gallon) has also been shown to be effective.  When treating vegetables, be sure to select a product that is labeled for use on edible plants.  Most products should be applied every seven to 14 days from emergence until humid weather subsides.  DO NOT use myclobutanil, triadimefon, or thiophanate-methyl as the sole active ingredient for all treatments.  If you decide to use one of these active ingredients, alternate its use with at least one of the other listed active ingredients to help minimize problems with fungicide-resistant strains of powdery mildew fungi.  DO NOT alternate myclobutanil and triadimefon as these active ingredients are chemically related.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the product(s) in the safest and most effective manner possible.  Also consider pretesting any product that you decide to use on a small number of leaves or plants before treating a larger area to make sure there are no toxic effects, particularly when treating during warmer weather.

How do I avoid problems with powdery mildew in the future?  Consider buying plant varieties that are powdery mildew resistant.  This will not guarantee that your plants will be powdery mildew free every year, but should result in less severe disease when it occurs.  Reduce the humidity around your plants by spacing them further apart to increase air flow.  Be sure not to over-water as this can lead to higher air humidity as well.  Finally, at the end of the growing season, remove and destroy any infected plant debris as this can serve as a source of spores for the next growing season.  You can burn (where allowed by local ordinance), bury or hot compost this material.

For more information on powdery mildew:  Contact your county Extension agent.

Disease - Herbaceous Ornamental, Disease - Vegetable, Fact Sheet

Root-Knot Nematode

Root-knot nematodes (Meloidogyne spp.) are small, soilborne, worm-like organisms that infect many agricultural and horticultural plants.  Root-knot nematodes are found worldwide, and are named for the swellings (called “galls” or “knots”) that they cause on plant roots.  Economically-important species of Meloidogyne include M. arenaria, M. hapla, M. incognita, and M. javanica.  Of these, M. hapla (commonly known as Northern root-knot nematode) is most likely to be found in Wisconsin soils.

Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.
Root-knot nematodes cause swollen, distorted roots that can interfere with movement of water and nutrients within a plant.

Appearance:  Root-knot nematodes are about 1/10 the size of a pinhead and are typically embedded inside roots.  They are impossible to see with the naked eye.  Juvenile root-knot nematodes (both males and females), as well as adult males, are vermiform (i.e., worm-shaped) and live in the soil.  Adult females are spherical in shape and live inside roots.  Both males and females possess a thin, tube-like structure called a stylet that they use for penetrating root tissue.

Symptoms and Effects:  When root-knot nematodes enter roots, they release chemicals that cause nearby root cells to enlarge.  This leads to the formation of swollen, distorted areas in roots known as galls or knots.  The number and size of galls varies depending on plant species and cultivar, and the number of root-knot nematodes in the soil.  On some hosts (e.g., grasses) root swelling can be very difficult to detect.  Nematode feeding interferes with proper root function (e.g., water and nutrient movement).  Thus, infected plants may be stunted and wilted, may exhibit discolorations (e.g., yellowing) typical of plants with nutrient deficiencies, and ultimately (in field or vegetable crops) may have lower yields.  Because root-knot nematodes tend not to be uniformly distributed in the soil, symptomatic plants often occur in patches and are typically surrounded by plants of normal height and appearance.  Environmental factors such as slope, soil type, or soil moisture can cause similar patchy patterns, so identification of a root-knot nematode problem requires examination of symptomatic plants at a lab qualified to perform nematode diagnostics.

Life Cycle:  Root-knot nematodes (i.e., M. hapla) are native to Wisconsin and can be spread whenever contaminated soil or infected plants are moved.  Root-knot nematodes survive the winter as eggs in the soil.  Like insects, root-knot nematodes have several juvenile stages and the nematodes molt (i.e., shed their outer layers) as they grow.  The second juvenile stage of root-knot nematode is the most important, because at this stage the nematode seeks out and infects plant roots.  Once it has entered a root, a root-knot nematode molts three more times before becoming an adult.  A male root-knot nematode is able to move about freely and can leave a root.  A female root-knot nematode remains and feeds in a given location within a root.  Eventually, a female enlarges to the point where a portion of her body extends to the root surface and this allows her to lay her eggs in the soil.  In some hosts, eggs can also be found within the galls.

Control:  If you are having a root-knot nematode problem in your garden, consider crop rotation and the use of cover crops as management tools.  See University of Wisconsin Garden Facts XHT1210, Crop Rotation in the Home Vegetable Garden and XHT1209, Using Cover Crops and Green Manures in the Home Vegetable Garden for details.  If used properly, these techniques can be effective in reducing the number of root-knot nematodes in the soil.  M. hapla, the root-knot nematode species most common in Wisconsin, does not infect corn, wheat, oats or rye, so use of these crops in a rotation or as cover crops often provides great benefit.

Cover crops of French marigolds (Tagetes patula) also have been shown to reduce the number of root-knot nematodes in soil.  This common garden ornamental releases a chemical (alpha-terthienyl) that is highly toxic to root-knot nematodes and prevents their eggs from hatching.  As an added bonus, root-knot nematodes are not able to develop properly in marigold roots.  When using crop rotation or cover crops, proper broadleaf weed control is critical because weeds can provide a place for root-knot nematodes to survive and reproduce.

Finally, consider amending the soil in your garden with organic matter such as compost or leaf mulch.  Such amendments tend to increase the diversity of microorganisms in the soil and can encourage the growth of certain soilborne fungi that ensnare and feed on root knot nematodes, and parasitize their eggs.

For more information on root-knot nematode:  Contact your county Extension agent.