Category Archives: Disease – Herbaceous Ornamental

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 https://learningstore.extension.wisc.edu/) or contact your county Extension agent.