University of Illinois

No. 24/November 7, 1997

White Mold Update

Soybean researchers at Illinois and Iowa have confirmed that Sclerotinia sclerotiorum, causal agent of white mold, can be seed-transmitted. It has been known for many years that the pathogen can be moved as sclerotia (small black structures about the size of rodent droppings) mixed with the seed, but the role of infected seed has not been described. The seed infection was very evident in 1996 when seed samples were evaluated for warm germination on blotters. In several seed lots, the characteristic white mycelium and sclerotia were produced from normal seed. Without the characteristic sclerotia, infected seeds may resemble seeds infected with Phomopsis longicolla and/or Diaporthe phaseolorum (phomopsis seed decay). The level of infection was fairly low, usually below the 1-to-2% range. The infected seeds did not germinate; hence, seedling infection does not appear to be important. However, the development of sclerotia on the blotters indicates this may be a possible means to introduce the pathogen into new fields. In 1997, we designed a study to determine if infected seeds could produce sclerotia and apothecia (the small mushrooms that produce spores that infect soybeans) under field conditions. Results of that study demonstrate that the infected seeds do produce sclerotia and the sclerotia produce apothecia in the field.

What does this mean to the grower? First, Sclerotinia sclerotiorum has a very wide host range and will infect many broadleaf weeds. In addition, the sclerotia can survive in the soil for up to 5 years. Therefore, the importance of seed infection is not known at this time. However, researchers at Iowa State University and University of Illinois are studying this problem. Second, if seed is saved from infected plants, it is important to have the seed cleaned and sized to remove sclerotia and small infected seeds. To determine if a seed lot is infected, it is essential to have a germination test done and ask if white mold was observed. Illinois Crop Improvement is currently doing this test. Finally, if an infected seed lot is to be used for seed, the application of a fungicide seed treatment containing TBZ (thiabendazole) will control this type of infection.

Wayne Pedersen, (217)333-3847, and Glen Hartman, (217)244-3258, Department of Crop Sciences