. 2000. Spread of Sclerotinia stem rot of soybean from area and point sources of apothecial inoculum. Can. J. Plant Sci. 80: [389][390][391][392][393][394][395][396][397][398][399][400][401][402]. 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 corn 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 soybean 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. Ascospore dispersal is important for dissemination and survival of the pathogen. Previous studies (Hartill 1980;McCartney and Lacey 1991;Suzui and Kobayashi 1972a) have shown that the majority of ascospores of S. sclerotiorum are deposited within a few metres of the source. Boland and Hall (1988) found that apothecia of S. sclerotiorum and Sclerotinia stem rot of soybean were spatially aggregated, and numbers of apothecia and disease incidence were correlated. They concluded that disease resulted primarily from inoculum produced within the field. Suzui and Kobayashi (1972a) found that in fields of kidney bean (P. vulgaris), ascospores of S. sclerotiorum were deposited in large amounts near point sources of apothecial inoculum, and the number of ascospores decreased as the distance from the source increased.