NPIPM:Sclerotinia stem rot on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Sclerotinia stem rot, also called white mold, was first reported in the U.S. in 1924. In 1970s, the disease was a chronic but locally isolated problem in soybean Michigan, Minnesota and Wisconsin. In 1990s the disease became more prevalent in other soybean growing regions of the North Central U.S. Currently, Sclerotinia stem rot is considered a serious annual problem to soybean production in the Northern Plains.
Causal Organism
Sclerotinia stem rot is caused by the fungus Sclerotinia sclerotium. The fungal pathogen infects more than 400 species of plant, including crops and weeds. Crops affected by S. sclerotium include soybean, sunflower, dry bean, peanut, chickpea and numerous others.
Symptoms and Signs
On infected plants, symptoms and signs of Sclerotinia stem rot appear during early reproductive stages. At growth stages R3-R4, upper leaves of infected plants start to wilt and eventually die. Leaf wilting is caused by stem lesions. The pathogen can grow between nodes, creating white encircling lesions on the stem. Under high humidity conditions, white fluffy mycelium grows on and cover the lesions. Black sclerotia (dark-pigmented hard clump of fungal structures about 0.1-1.5 in. or 3-40 mm long), a characteristic sign of S. sclerotium infection, are then produced on the lesions or within the infected stem. When the plant matures, infected stems appear bleached and easily shredded. Resulting pods appear white with embedded black sclerotia. The seeds within infected pods are moldy and shriveled.
Disease Impacts
Field trials indicate that for each 1% plant mortality due to sclerotinia stem rot at the R6-R7 plant stages, there is a decrease of 17-49 kg/ha in yield. Sclerotia of S. sclerotium are often mixed with harvested soybean seeds. These sclerotia are not toxic to livestock.
Life Cycle and Epidemiology
The fungal pathogen survives in the soil as sclerotia. Saturated soil, cool temperatures, and full canopy cover are favorable for development of apothecia, fruiting bodies of the pathogen, than in turn produce spores. The spores are airborne and can be distributed to adjacent fields by wind.
The spores enter plant primarily through senescing flowers, but entrance through wounds on other plant parts is also possible. Free water on plant surface must be available for infection to occur. Thus, sclerotinia infection is likely to occur if cool and wet weather prevails during flowering. More sclerotia develop within infected plants. These sclerotia are later incorporated to the soil with plant debris and become inoculum source for the subsequent seasons.
Management Approaches
Cultural Methods
Make sure to use seeds that are not contaminated by sclerotia. High plant populations and narrow rows can increase the incidence and severity of Sclerotinia stem rot. Thus, fields with a history of Sclerotinia stem rot should be planted at low populations that maintain yield potential and possibly not be planted in narrow rows. In irrigated fields, avoid or decrease the frequency of irrigation during flowering. As the fungus infect a wide host range including some weeds, good weed control help to eliminate inoculum sources. Tillage affects the survival of sclerotia since only sclerotia located within 2 inches (50.8 mm) of soil surface can germinate. Yet, the long survival period of sclerotia generally renders tillage less consistent in managing Sclerotinia stem rot. Fall tillage, for example, may bury new sclerotia but uncover old sclerotia and bring them closer to the surface. Planting soybeans after a crop that is susceptible to Sclerotinia stem rot should be avoided, most importantly, sunflowers, dry beans, and canola
Host Plant Resistance
Soybean varieties vary in their response toward Sclerotinia stem rot, and seed companies usually include the rating against this disease in their catalogs. Short varieties with good lodging resistance may reduce disease potential.
Chemical Control
Foliar fungicide applications are recommended as flowering begins if a field is grown for seed production. Results of chemical control options vary significantly.
