Tag Archives: Cabbage

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 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.