Plant Disease: Sclerotinia Stem Rot

Taxonomy:

Distribution:

• This pathogen has an extremely wide host range and can affect more than 400 plant species, including oilseed, vegetable and forage legume crops.


• It has caused a disease known as white mold on dry edible beans in western Nebraska for 40 years, but it did not become a problem in soybeans until the 1990s.


• This fungal pathogen can be found on soybeans in Illinois, Iowa, Michigan, Ohio, Minnesota, Wisconsin, and North Dakota.

Life Cycle:

• The fungal pathogen survives from year to year as hard dark structures called sclerotia.


• It is variously shaped bodies of tightly packed white mycelium covered with a dark, melanized protective coat.


• For sclerotia to produce apothecia, they must undergo a cool, wet period.


• Apothecia are mushroom-like bodies that produce millions of airborne spores almost daily over a 7- to 10-day period.

• These spores are released during favorable weather conditions and can travel to other fields in air currents.


• However, sources of spores under the plant canopy exert the most influence on disease severity.


• Spores infect plants like soybean primarily through colonized blossoms but they can also infect through injured plant tissue.

• Free moisture must be present on the plant surface for infection to occur.


• Flowers on the tips of small pods provide a common entrance for the fungus.

• Invasion of the pod and eventually the stem may lead to lesions covered with sclerotia.


• During harvest these structures are scattered back onto the soil.


• Thus, inoculum for the next three or more seasons has been distributed.

Symptoms:

• Initial symptoms are visible during pod development.


• Leaves will wilt and turn gray-green before turning brown, curling and dying.


• It is important to observe stems and pods for white mycelium and sclerotia to differentiate Sclerotinia stem rot from other diseases.


• Since blossoms are infected first, early stem or pod water-soaked symptoms often result from colonized flowers.


• In a few days diseased stem areas are killed and become tan and eventually bleached.

• This bleached stem will have a pithy texture and will shred easily.


• Infected plant parts generally will have signs of the fungal pathogen as white, fluffy mycelium during humid conditions, and sclerotia on the surface of or embedded in the stem tissue.

• Although stem and pod infection usually occurs about 6 to 14 inches above the soil line, some basal infection also may be found.

Management:

• No single disease management strategy effectively prevents infection; however, various measures can reduce disease severity and minimize yield losses.


• More information about management practices for Sclerotinia can be found at your local Extension office.


• Ask for G95-1270-A, Sclerotinia Stem Rot of Soybeans.