Category Archives: Disease – Vegetable

Common Corn Smut

What is common corn smut? Common corn smut is a fungal disease that affects field, pop, and sweet corn, as well as the corn relative teosinte (Zea mexicana). Common corn smut is generally not economically significant except in sweet corn where relatively low levels of disease make the crop aesthetically unappealing for fresh market sale and difficult to process for freezing or canning. Interestingly, the early stages of common corn smut are eaten as a delicacy in Mexico where the disease is referred to as huitlacoche (see University of Wisconsin Garden Facts XHT1230, “Huitlacoche”)

Common corn smut leads to tumor-like galls on corn filled with a sooty, black powder.
Common corn smut leads to tumor-like galls on corn filled with a sooty, black powder.

What does common corn smut look like? Common corn smut leads to tumor-like swellings (i.e., galls) on corn ears, kernels, tassels, husks, leaves, stalks, buds, and less frequently on aerial roots. Some galls (particularly those on leaves) are small and hard. More typically, however, galls are fleshy and smooth, silvery-white to green, and can be four to five inches in diameter. As fleshy galls mature, their outer surfaces become papery and brittle, and their inner tissues become powdery and black. Galls eventually rupture, releasing the powder (i.e., the spores of the causal fungus).

Where does common corn smut come from? Common corn smut is caused by the fungus Ustilago maydis, which can survive for several years as spores in soil and corn residue. Spores are spread by wind or through water splashing up onto young plants. Spores can also be spread through the manure of animals that have eaten infected corn. U. maydis most typically infects corn ears via the silks. The fungus can also enter plants through wounds caused by insect feeding, hail, or injuries from machinery. Infection is favored by warm weather (79-93°F) and moderate rainfall. Corn grown in low fertility soils or soils with excessive nitrogen also have a greater likelihood of infection.

How do I save plants with common corn smut? Once galls have formed, treatment is not available. Remove and burn (where allowed by local ordinance) or bury smut galls before they burst to prevent spores from spreading and overwintering. In order for eradication to be effective, however, you will need to coordinate efforts with your neighbors. Fungicides are currently not an effective control against common smut.

How do I avoid problems with common corn smut in the future? Plant resistant corn varieties whenever possible. Check with your favorite sweet corn seed supplier for available varieties. Also, avoid injuring plants (e.g., when weeding) to reduce possible entry points for the common corn smut fungus. Maintain well-balanced soil fertility (specifically nitrogen) based on a soil nutrient test. Use crop rotation (see University of Wisconsin Garden Facts XHT1210 “Using Crop Rotation in the Home Vegetable Garden” for details) to allow time for corn smut spores to naturally die off in the soil.

For more information on common corn smut: Contact your county Extension agent.

Blossom End Rot

What is blossom end rot? Blossom end rot is a physiological disorder of tomato in which the tissue of the blossom end of the fruit (the portion of the fruit opposite the stem) breaks down and rots, thus reducing yield. Pepper, eggplant and squash (e.g., zucchini) fruits can also be affected.

Blossom end rot of tomato.
Blossom end rot of tomato.

What does blossom end rot look like? Blossom end rot often occurs on the first fruits formed on plants. Initially, water-soaked spots (resembling small bruises) appear, most often on the blossom ends of fruits. On peppers these spots can resemble sunscald and can form on the sides of the fruits near the blossom end. Spots enlarge, becoming dark brown to black, sunken and leathery. Half the fruit may eventually be affected. Sometimes, when a fruit is cut, the exterior will be sound, but the interior will be discolored and shrunken. Often, bacteria and fungi invade the discolored areas, leading to tissue decay.

What causes blossom end rot? Blossom end rot is caused by a lack of calcium in the fruit. This lack of calcium may be due to low calcium levels in the soil. More often, there is plenty of calcium in the soil, but its availability for uptake and transport to fruits is impaired. Drought stress, alternating soil moisture extremes, and damage to a plant’s roots all can inhibit calcium uptake, as can waterlogged or cold soils, and high concentrations of ammonium (NH4+), potassium (K+), and magnesium (Mg++) cations in soil. Movement of calcium within plants depends on active transpiration (i.e., loss of water through above-ground plant parts). Because leaves transpire more than fruits, calcium moves more easily into leaves where it remains. Calcium is not later redistributed from leaves to fruits. This preferential distribution of calcium to leaves can be made worse by over-fertilizing with nitrogen which promotes excessive production of leaves. In addition, high relative humidity, OR low relative humidity in combination with hot, windy weather can limit transpiration, thus preventing calcium from reaching fruits.

How can I control blossom end rot? Avoid conditions of too much or too little water. Irrigate evenly and mulch the soil to retain moisture during dry periods. Avoid practices that would damage roots (e.g., cultivating too near plants thereby cutting roots). Use nitrate (NO3-) rather than ammonium (NH4+) forms of nitrogen fertilizer. DO NOT over-fertilize. Have your soil tested periodically to determine if there is sufficient calcium in the soil. If not, add calcium (e.g., lime, bonemeal, eggshells). Check the soil pH on a regular basis, particularly if you use lime as a calcium source. A pH of about 6.5 is ideal for growing most vegetables. Finally, grow vegetable cultivars that are tolerant of calcium deficiencies and less likely to show blossom end rot symptoms.

For more information on blossom end rot: See UW-Extension bulletin A3798 or contact your county Extension agent.

Black Walnut Toxicity

What is black walnut toxicity? Black walnut trees (Juglans nigra) produce a toxic substance (called juglone) that prevents many plants from growing under or near them. Related trees like butternut (J. cinerea) and shagbark hickory (Carya ovata) also produce juglone, but in lower concentrations than black walnut. Juglone occurs in all parts of black walnut trees, but especially in buds, nut hulls and roots. The toxic effects of a mature black walnut tree can extend 50 to 80 feet from the trunk of the tree, with the greatest toxicity occurring within the tree’s dripline. In this area, plants susceptible to juglone may wilt or die; plants tolerant to juglone will grow normally. Vegetables such as tomato, potato, eggplant and pepper, and ornamentals such as lilac, peony, rhododendron and azalea are particularly sensitive to juglone. What do the effects of black walnut toxicity look like? Plants sensitive to juglone may be stunted, have yellow or brown, twisted leaves, exhibit wilting of some or all plant parts, and die over time. Often, the vascular (i.e., water-conducting) tissue of affected plants will be discolored. Symptoms may occur rapidly, even within a few days after sensitive species are transplanted into a walnut tree’s root zone. Alternatively, some plants may survive for years near a young walnut tree, but will wilt and die as the tree increases in size. Black walnut toxicity can be confused with wilts caused by bacterial and fungal pathogens (e.g., see University of Wisconsin Garden Facts X1008), herbicide injury (see University of Wisconsin Garden Facts X1004), or drought. How do I avoid problems with black walnut toxicity? There is no cure for a plant affected by walnut toxicity. Removing a walnut tree may not be practical as the tree could be the focal point in a landscape. In addition, even if a walnut tree is removed, juglones will not immediately be eliminated, because it is next to impossible to remove all root pieces from the soil and remaining pieces may continue to exude toxins for several years as they decay. When establishing a garden around a walnut tree, try to plant species that are tolerant to juglone (see table on the reverse side). If you are growing sensitive species near a walnut tree, transplant them elsewhere in your garden. If you must grow sensitive plants near a black walnut, keep beds free of walnut leaves and hulls and remove walnut seedlings as they appear. Growing shallow rooted woody and herbaceous plants, and improving drainage can also diminish the effects of juglone. Alternatively, consider building raised beds with wood, stone, or concrete barriers that limit root growth through and under the beds. When disposing of bark and wood from a walnut tree, do not use these materials for mulch. The information in the following table is intended to provide guidance in selecting plants to grow near walnut trees. Inclusion of plants in this table is based on observation, not on formal testing. In addition, the plant lists in this table are by no means exhaustive. Oftentimes the juglone sensitivity or tolerance of specific plants has never been observed or documented. Finally, sources often disagree on whether particular plants (e.g., columbine, lily, narcissus, tulip) are juglone sensitive or tolerant. Some varieties may be susceptible while others are tolerant. Most plant species with conflicting information regarding their sensitivity or tolerance to juglone have not been included in the table.


Sensitive to Juglone

Tolerant of Juglone
Vegetables asparagus, cabbage, eggplant, pepper, potato, rhubarb, tomato beans, beet, carrot, corn, melon, onion, parsnip, squash
Flowers autumn crocus, chrysanthemum, forget-me-not, petunia, peonies aster, astilbe, bee balm, begonia, black-eyed Susan, bluebell, calendula, crocus, daylily, ferns, grape hyacinth, some hosta varieties, hollyhock, impatiens, Jack-in-the-pulpit, Jacob’s ladder, marigold, morning glory, pansy, phlox, Siberian iris, squill, sweet woodruff, trillium, zinnia
Trees alder, apple and crabapple, basswood, pine, spruce, silver maple, white birch black locust, catalpa, Eastern redbud, hackberry, Canadian hemlock, hickory, most maples, oaks, pagoda dogwood, poplar, red cedar
Shrubs and Vines azalea, blackberry (and most berries other than black raspberry), cotoneaster, hydrangea, lilac, mountain laurel, potentilla, privet, rhododendron, yew arborvitae, bittersweet, black raspberry, clematis, currant, forsythia, euonymus, greenbrier, most honeysuckle, pachysandra, rose-of-Sharon, sumac, most viburnum, Virginia creeper, wild grape, wild rose, willow, witch hazel
Field Crops and Grasses alfalfa, tobacco fescue, Kentucky bluegrass, orchard grass, soybean, timothy, wheat, white clover

For more information on black walnut toxicity: See UW-Extension Publication A3182 or contact your County Extension agent.

Black Rot of Crucifers

What is black rot? Black rot is a potentially lethal bacterial disease that affects cruciferous vegetables such as broccoli, Brussels sprouts, cabbage, cauliflower, kale, rutabaga and turnip, as well as cruciferous weeds such as shepherd’s purse and wild mustard. Black rot occurs worldwide wherever cruciferous plants are grown and makes cruciferous vegetables unfit for the marketplace or the table.

Black rot causes V-shaped yellow and brown/ dead areas in affected leaves.  (Photo courtesy of Amanda Gevens)
Black rot causes V-shaped yellow and brown/ dead areas in affected leaves. (Photo courtesy of Amanda Gevens)

What does black rot look like? Black rot symptoms may not develop for more than a month after cruciferous vegetables start to grow. Initial symptoms are irregular, dull, yellow blotches that appear on the edges of leaves. As the disease progresses, these blotches expand into V-shaped areas with the wide part of the “V” at the edge of the leaf and the point of the “V” toward the attachment point of the leaf to the plant. The V-shaped areas are initially yellow, but eventually become brown and necrotic (i.e., dead) in the center with a yellow border or halo. Veins in affected areas are brown or black, forming to a net-like pattern (often most visible when leaves are held up to the light). Later, interior stem tissue (specifically the water conducting tissue) will also turn brown or black. At this point, affected plants tend to show symptoms of wilting. Black rot can also predispose vegetables to other rot diseases such as bacterial soft rot (see University of Wisconsin Garden Facts XHT1224 “Bacterial Soft Rot”).

Where does black rot come from? Black rot of crucifers is caused by Xanthomonas campestris pv. campestris (Xcc). This bacterium is most often introduced into a garden on or in seeds and transplants of susceptible vegetables. By some estimates, a single infected seed in 10,000 can lead to a severe outbreak of the disease if environmental conditions are favorable. Favorable conditions include warm temperatures (approximately 80°F) and high humidity. Once introduced into a garden, Xcc can survive in residues from susceptible vegetables or on weed hosts. Xcc can subsequently enter susceptible plants through roots, through natural openings in leaves or through wounds made by tools, rough handling, or insect feeding. Cruciferous plants grown near infected plants and healthy plants handled with the same tools as diseased plants are at highest risk of becoming infected.

How do I save a plant with black rot? There are no curative treatments available to combat black rot once the disease has occurred. However, when disease severity is low, copper-containing fungicides that are labeled for use on cruciferous vegetables may help limit additional disease development even though this disease is caused by a bacterium. See University of Wisconsin Garden Facts XHT1211, “Home Vegetable Garden Fungicides” for specific products. 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. At harvest, vegetables with low levels of black rot may be salvageable. Remove symptomatic leaves (or other plant parts) and store the remaining parts of the vegetables in a cool, but not overly wet environment.

How do I avoid problems with black rot in the future? Prevent introduction of Xcc into your garden by using certified disease-free crucifer seeds and transplants. If certified disease-free seed is not available, use hot water seed treatments to eliminate Xcc. Treat seeds of Brussels sprouts, collards, and cabbage for 35 minutes in water that is 122oF. Treat seeds of broccoli, cauliflower, kale, kohlrabi, rutabaga, and turnips for 20 minutes in water that is 122°F. DO NOT plant cruciferous vegetables in the same area of your garden every year; rotate (i.e., move) these vegetables to different locations within your garden. For more information on rotation see University of Wisconsin Garden Facts XHT1210, “Using Crop Rotation in the Home Vegetable Garden”. Once your cruciferous vegetables are growing, be sure to fertilize them appropriately. In particular, inadequate nitrogen can predispose plants to black rot. Also, be gentle with cruciferous vegetables to prevent any wounds that might serve as entry points for Xcc. DO NOT use a sprinkler to water your vegetables as this can splash Xcc from plant to plant. Instead use a soaker or drip hose that applies water directly to the soil. Avoid working with plants when they are wet to help limit spread of Xcc. If severe black rot develops, promptly remove symptomatic plants as well as all cruciferous plants within a three to five foot radius. Dispose of these plants by burning (where allowed by local ordinance), burying or composting them. If you decide to compost, make sure your compost pile heats to a high enough temperature and that any infested material decomposes for at least one year before it is reincorporated into your garden. For more information on how to properly compost, contact your local county Extension office. Finally, decontaminate any pots, tools, or other gardening items that have come into contact with Xcc-infected plants or Xcc-infested debris by treating them for at least 30 seconds with 10% bleach or 70% alcohol (preferable for metal tools because of its less corrosive properties). Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol.

For more information on black rot: Contact your county Extension agent.

Bacterial Wilt of Cucurbits

What is bacterial wilt? Bacterial wilt is a common and destructive disease that affects cucurbits (i.e., plants in the cucumber family), including economically important crops such as melon (Cucumis melo), cucumber (Cucumis sativus) and, to a lesser extent, squash and pumpkin (Cucurbita spp.). This disease is distributed throughout the United States; and can be found anywhere that cucurbits are grown.

Sudden wilting and eventual death of melon, cucumber and squash plants can be due to bacterial wilt. (Photo courtesy of ISU-PIDC.)
Sudden wilting and eventual death of melon, cucumber and squash plants can be due to bacterial wilt. (Photo courtesy of ISU-PIDC.)

What does bacterial wilt look like? The most distinctive symptom exhibited by a plant with bacterial wilt is wilting and ultimately death. These symptoms are a consequence of the blockage of water movement inside of the plant. Symptoms appear first on leaves of a single vine (runner). Leaves may develop chlorotic (i.e. yellow) and necrotic (i.e. dead) areas as the disease progresses. Symptoms typically develop rapidly along individual runners and eventually the plant’s crown is affected, resulting in the entire plant dying. To determine if a symptomatic plant has bacterial wilt, cut a wilted vine near the base of the plant. Next cut a section from this vine and look for sticky threads to form between the two vine sections as you slowly pull them apart. The presence of these sticky threads is diagnostic. This technique works best for cucumbers and melon, but less well for squash and pumpkins.

Where does bacterial wilt come from? Bacterial wilt of cucurbits is caused by the bacterium Erwinia tracheiphila. This bacterium is moved from plant to plant by two insects: the striped cucumber beetle (Acalymma vittatum) and the spotted cucumber beetle (Diabrotica undecimpunctata) (see University of Wisconsin Garden Facts XHT1092 “Cucumber Beetles” for details). The bacterium primarily overwinters in the guts of adult beetles, and is released when beetles feed on healthy cucurbit plants and excrete contaminated frass (i.e., feces) onto fresh feeding wounds. E. tracheiphila has been found in association with wild cucurbits and other plants such as goldenrods (Solidago nemoralis and S. altissima), Johnson grass (Sorghum halepense) and even corn (Zea mays). However, most of these plants never show wilt symptoms, and none are considered an important reservoir for the bacterium.

How do I save a plant with bacterial wilt? Bacterial wilt cannot be controlled once a plant is infected. In particular, chemical sprays are not effective for control once plants show symptoms. If you find bacterial wilt in your garden, immediately remove infected plants, and dispose of them by burning (where allowed by law) or burying them. DO NOT compost infected plants. Prompt removal and disposal of infected plants is important not only because they serve as a source of E. tracheiphila, but because they actually attract more cucumber beetles, thus increasing the risk of spread of the bacterium to other, healthy plants.

How do I avoid problems with bacterial wilt in the future? Management of bacterial wilt relies on control of the cucumber beetle to prevent infection. Target non-chemical and chemical control methods to protect plants at the beginning of the growing season when plants are more attractive to cucumber beetles. Use mechanical barriers, such as row covers, to protect plants from cucumber beetle feeding. Also, inspect cucurbits on a regular basis for cucumber beetles and their damage (two to three times per week early in the season, and weekly thereafter). When cucumber beetle numbers are high (more than 20 per plant), spray your plants with an appropriate insecticide (see UW Garden Facts XHT1092 for recommended insecticides). Be sure to read and follow all label instructions of the insecticide(s) that you select to ensure that you use the insecticide(s) in the safest and most effective manner possible. Commercial cucurbit varieties resistant to bacterial wilt are not currently available. However, some cucurbits such as watermelons and pickling cucumbers tend to be less attractive to cucumber beetles, and thus tend to have fewer problems with bacterial wilt. These crops can be used as alternatives to more susceptible cucurbit species.

For more information on bacterial wilt of cucurbits: Contact your county Extension agent.

Bacterial Soft Rot

What is bacterial soft rot? Bacterial soft rots are a group of diseases that cause more crop loss worldwide than any other bacterial disease. Bacterial soft rots damage succulent plant parts such as fruits, tubers, stems and bulbs of plants in nearly every plant family. Soft rot bacteria degrade pectate molecules that bind plant cells together, causing plant structure to eventually fall apart. Woody tissues are not susceptible. Soft rots commonly affect vegetables such as potato, carrot, tomato, cucurbits (e.g., cucumbers, melons, squash, pumpkins), and cruciferous crops (e.g., cabbage, cauliflower, bok choy). These diseases can occur on crops in the field, as well as on harvested crops in storage. Rot can occur over a wide temperature range, with the worst decay between 70 and 80°F, particularly when oxygen is limited.

Bacterial soft rots cause the collapse of plant parts such as potato tubers. (Photo courtesy of the UW-Madison/Extension Plant Disease Diagnostics Clinic)
Bacterial soft rots cause the collapse of plant parts such as potato tubers. (Photo courtesy of the UW-Madison/Extension Plant Disease Diagnostics Clinic)

What does bacterial soft rot look like? Initially, bacterial soft rots cause water-soaked spots. These spots enlarge over time and become sunken and soft. Interior tissues beneath the spots become mushy and discolored, with the discoloration ranging anywhere from cream to black. Seepage from affected areas is common. Soft rots are known for a strong, disagreeable odor that accompanies the breakdown of plant tissue.

Where does bacterial soft rot come from? Soft rots are caused by several bacteria, most commonly Pectobacterium carotovorum (previously called Erwinia carotovora), Dickeya dadantii (previously called Erwinia chrysanthemi), and certain species of Pseudomonas, Bacillus and Clostridium. These bacteria can enter plants through wounds caused by tools, insects, severe weather such as hail, or through natural openings. The bacteria can be spread from plant to plant by insects, on contaminated tools, or by movement of infested plant debris, soil, or contaminated water. Bacterial soft rots tend to be more of a problem during wet weather and can be more severe when plants lack sufficient calcium.

How do I save a plant with bacterial soft rot? Once soft rot bacteria have infected plant tissue, there are no treatments. Immediately remove and discard infected plants or plant parts. DO NOT bury or compost this material.

How do I avoid problems with bacterial soft rot in the future? Avoiding wet conditions is key for managing soft rot. Plant vegetables in well- drained soils and control watering times and amounts, making sure plants are watered adequately (but not excessively) and uniformly. DO NOT crowd plants; wider spacing will promote more rapid drying of plants and soil. Make sure that soil fertility (particularly soil calcium) is optimal for the vegetables that you are growing based on a soil nutrient test. Add calcium (e.g., bone meal) at planting as needed. Use soft rot-resistant vegetables in rotation with susceptible vegetables. Corn, snap beans and beets are vegetables that are not considered susceptible to soft rot. When growing broccoli, avoid varieties with flat/concave heads that trap moisture and promote soft rot. Instead select varieties with domed heads where water readily drains away. Avoid damaging vegetables when weeding and during harvest. Minimize any handling of soft-rotted plants, but if you must handle such plants (e.g., to remove them from the garden), wash your hands afterwards with soap and water. Decontaminate garden tools before and after use by treating them for at least 30 seconds with 10% bleach or preferably (because of its less corrosive properties), 70% alcohol. Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol. Also, keep insects that can wound vegetables such as cabbage maggot under control (see University of Wisconsin Garden Facts XHT1030 “Cabbage Maggot” for details). Harvest only during dry conditions. Closely inspect vegetables from infected gardens that will go into long-term storage and be sure not to store any diseased vegetables. Cure vegetables where appropriate prior to storage (see for additional curing information). Store vegetables in a cool, dry, well-aerated place to suppress bacterial growth. At the end of the growing season, remove any infested plant debris remaining in your garden, and destroy the material by burning or landfilling it. If soft rot is a serious, recurring problem in an area in your garden, DO NOT grow susceptible crops in that area for a minimum of three years.

For more information on bacterial soft rot: Contact your county Extension agent.

Aster Yellows

What is aster yellows? Aster yellows is a chronic, systemic disease that affects over 300 species in at least 38 families of broad-leaf, herbaceous plants. Members of the aster family (Asteraceae), such as asters, marigolds, Coreopsis and purple coneflower are commonly affected by this disease. Vegetable crops such as carrots and potatoes are also susceptible. Aster yellows occurs throughout North America.

Coneflowers with aster yellows (right) often have deformed, discolored flowers.
Coneflowers with aster yellows (right) often have deformed, discolored flowers.

What does aster yellows look like? Symptoms of aster yellows are often mistaken for damage due to herbicide exposure. Infected plants are typically stunted and twisted, with foliage that is yellow or red. Infected plants are often sterile. Floral parts that are normally brightly colored may be green, and petals and sepals may become puckered and distorted. In purple coneflower, secondary flower heads (often in a cluster) may emerge from the primary flower head. In marigolds, flowers are often leafy and a muddy green-orange color. Infected carrots have red leaves and form taproots with tufts of small, white “hairy” roots. Roots from these plants often have a bitter taste.

Where does aster yellows come from? Aster yellows is caused by the aster yellows phytoplasma, a bacterium-like organism that lives in the food-conducting tissue (phloem) of plants. Aster yellows is rarely lethal. Thus, infected perennials can serve as source of the aster yellows phytoplasma for many years. The aster leafhopper (Macrosteles fascifrons), a common insect, moves the aster yellows phytoplasma from plant to plant.

How do I save a plant with aster yellows? There is no known cure for aster yellows. Plants suspected of having aster yellows, including weeds such as dandelions, should be removed immediately so that the aster yellows phytoplasma cannot be spread from infected plants to other non-infected plants in the area. How do I avoid problems with aster yellows in the future?

Some herbaceous plants (e.g., geraniums and impatiens), as well as most woody ornamentals, are not susceptible to aster yellows. Therefore these plants should be used in areas where aster yellows is a problem. In landscape settings, attempts to control aster leafhoppers as a means of controlling aster yellows are typically not effective and are not recommended.

For more information on aster yellows: See UW-Extension bulletins A2595, A3679 and A3788 (available at or contact your county Extension agent.