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 of ascospores of Sclerotinia sclerotiorum, the causal agent of Sclerotinia stem rot of soybean. Incidence of Sclerotinia stem rot measured in a soybean field adjacent to a corn field naturally infested with S. sclerotiorum (area source) was highest close to the area source and decreased with distance from the area source. Disease gradients in the soybean field were described nearly equally well by the exponential model, power law model, and logit-log model. Disease incidence in transects in non-infested areas of a soybean field increased with distance from the edges of soybean and com area sources of inoculum, reached a maximum at 10-12 and 18-20 m, respectively, from the edges of the inoculum area sources, then decreased in a manner characteristic of the disease gradient curve between 10-12 and 32 m, and 18-20 and 44 m from the edges of the area sources. In soy7 bean plots, disease incidence decreased with distance from point sources of apothecial inoculum. The results from these studies suggest 1) the potential for field to field dispersal of S. sclerotiorum, and 2) that the majority of ascospores of S. sclerotiorum are deposited close to the source (apothecia). Where a concentrated area or point source of S. sclerotiorum inoculum exists, the general exponential model may provide good fits to Sclerotinia stem rot gradient data.