Spread of Sclerotinia stem rot of soybean from area and point sources of apothecial inoculum

Field experiments were conducted from 1995 to 1998 to quantify the spread o f ascospores of Sclerotinia sclerotiorum, the causal agent of Sclerotinia s tem rot of soybean. Incidence of Sclerotinia stem rot measured in a soybean field adjacent to a corn field naturally infested with S. sclerotiorum (ar ea source) was highest close to the area source and decreased with distance from the area source. Disease gradients in the soybean field were describe d nearly equally well by the exponential model, power law model, and legit- log model. Disease incidence in transects in non-infested areas of a soybea n field increased with distance from the edges of soybean and corn area sou rces of inoculum, reached a maximum at 10-12 and 18-20 m, respectively, fro m the edges of the inoculum area sources, then decreased in a manner charac teristic of the disease gradient curve between 10-12 and 32 m, and 18-20 an d 44 m from the edges of the area sources. In soy bean plots, disease incid ence decreased with distance from point sources of apothecial inoculum. The results from these studies suggest 1) the potential for field to field dis persal of S. sclerotiorum, and 2) that the majority of ascospores of S. scl erotiorum are deposited close to the source (apothecia). Where a concentrat ed area or point source of S. sclerotiorum inoculum exists, the general exp onential model may provide good fits to Sclerotinia stem rot gradient data.