Modelling inoculum dispersal and Sclerotinia stem rot gradients in canola fields

Dispersal of Sclerotinia sclerotiorum ascospores and sclerotinia stem rot (SSR) incidence were studied in commercial canola fields in two North Dakota locations from 2005 to 2007. Ascospores were collected on dishes containing a semi-selective medium that were placed on the soil, within plant canopies, at various distances from an area source of inoculum. SSR incidence was recorded at the end of the growing season. SSR incidence and inoculum gradients declined with distance from the area source in 2005 and 2007. In 2006, neither ascospores nor SSR were detected. The number of S. sclerotiorum colonies per dish ( COL) declined between 30% and 75% within 40 m from the source, whereas SSR incidence declined by 50% within 12-17 m from the source. Ascospore dispersal and SSR gradients across years and locations were best described by Lambert's model (pseudo-R-2 >= 0.98; P < 0.001) although the negative exponential model also provided a good fit for both (R-2 >= 0.81; P <= 0.001). Every unit increase in the weekly mean COL represented an increase in SSR incidence that ranged between 0.5 and 0.75% (R-2 = 0.96; P <= 0.0001) in 2005 and between 0.13 and 0.17% (R-2 = 0.96; P <= 0.0001) in 2007. The existence of short distance dispersal, disease gradients, and the close association between inoculum concentration and disease incidence highlight the importance of within-field produced inoculum and provides support for the use of control tactics that emphasize reduction of sclerotia in canola fields as an effective way to reduce inoculum and control this disease.