Category Archives: Disease – Vegetable

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 UW Plant Disease Facts D0010, 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/bactericides that are labeled for use on cruciferous vegetables may help limit additional disease.  See UW Plant Disease Facts D0062, 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.  See UW Plant Disease Facts D0064, Hot-Water Seed Treatment for Disease Management, for details on this process and the proper temperature and treatment time for specific types of crucifer seeds.  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 70% alcohol (preferable for metal tools because of its less corrosive properties) or 10% bleach.  Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol.  If you use bleach on metal tools, be sure to thoroughly rinse and oil them after use to prevent rusting.

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 runner (vine).  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 on these insects  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 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 cucumber beetles 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 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.  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 rot describes 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 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) and is particularly severe 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?  Soft rot bacteria degrade pectate molecules that bind plant cells together, thus causing plant structure to fall apart.  Woody tissues are not susceptible.  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 species of Pectobacterium [particularly Pectobacterium carotovorum (previously called Erwinia carotovora)], Dickeya species [particularly 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, and severe weather such as hail, as well as 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.  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 (where allowed by local ordinance) 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 of broad-leafed, herbaceous plants in at least 38 families.  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?  Infected plants are typically stunted and twisted, with foliage that is yellow, purple 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 purple/red leaves and form taproots with tufts of small, white “hairy” roots.  Tap roots from infected carrots 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 a 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.  Proper diagnosis of aster yellows is important because management of herbicide and eriophyid mite damaged plants does not require plant removal.

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 Bulletins A2595 and A3679 (available at or contact your county Extension agent.