Deposition of Ascospores of Sclerotinia sclerotiorum In and Near Potato Fields and the Potential to Impact Efficacy of a Biocontrol Agent in the Columbia Basin

Abstract

The relative importance of ascospores of Sclerotinia sclerotiorum originating from within and external to potato fields on the development of white mold and the effectiveness of a commercial preparation of Coniothyrium minitans (Intercept®) in reducing white mold in potato fields were determined in the Columbia Basin. Such information would aid in developing disease management strategies. To determine the spatial origin of ascospores, Petri dishes containing a medium semi-selective for S. sclerotiorum were exposed on plant foliage for 25–30 min intervals for multiple days over a 6-week period. Petri dishes were placed along transects extending from 30 m outside of fields to 80 m within fields of four center irrigated potato fields each of 2 years. Numbers of trapped ascospores varied significantly among the four fields during both years of the study. Significantly more ascospores were trapped 30 m outside than at the edge of fields in 2003. Numbers of ascospores trapped 30 m outside of field did not differ significantly from those at 40 and 80 m within fields. Significant ascospore deposition gradients from the interior to exterior of circles were not detected using regression analysis. Number of apothecia, number of ascospores of S. sclerotiorum deposited in exposed Petri dishes, frequency of blossom infection, and incidence of stems with lesions did not differ between sections of the fields treated and not treated with Intercept®. The results suggest that airborne ascospores of S. sclerotiorum originating from external-field sources likely played an important role in initiating white mold of potato in the Columbia Basin of Washington State and therefore compromised the effectiveness of C. minitans in reducing inoculum of apothecia of S. sclerotiorum within the potato field.