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Ecology and Epidemiology

Use of Petal Infestation to Forecast Sclerotinia Stem Rot of Canola: The Influence of Inoculum Variation over the Flowering Period and Canopy Density. T. K. Turkington, Department of Biology, University of Saskatchewan, Saskatoon S7N 0W0, Canada, Present address: Research Station, Agriculture Canada, P. O. Box 29, Beaverlodge, AB T0H 0C0; R. A. A. Morrall, Department of Biology, University of Saskatchewan, Saskatoon S7N 0W0, Canada. Phytopathology 83:682-689. Accepted for publication 17 February 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-682.

The relationship in canola (oilseed rape) between the percentage of petals infested with Sclerotinia sclerotiorum and the incidence of Sclerotinia stem rot was investigated relative to changes in petal infestation during flowering and to canopy density. Using an agar-plate test, percent petal infestation was assessed in commercial crops of Brassica napus and B. rapa (=B. campestris) in various regions of Saskatchewan. Crop height, stem thickness, percent light penetration of the canopy, leaf area index, and the number of plants per square meter also were assessed at some locations. The incidence of stem rot was determined before harvest. Substantial changes in petal infestation were observed between early and late bloom. Most changes were increases; in 1989, however, infestation generally decreased. The changes were related to variation in rainfall, which probably influenced ascospore production and release. Higher disease incidence in denser crop canopies was attributed to more favorable microenvironmental conditions. Multiple regression analyses with petal infestation at early, full, and late bloom, light penetration, leaf area index and crop height as independent variables accounted for 5598% of the variation in disease incidence. In general, disease incidence was positively correlated with petal infestation when the measurement coincided with favorable conditions for infection. For practical on-farm disease forecasting, changes in petal infestation could be accounted for by sampling at early, full, and late bloom. The effects of host and environment also require consideration, but simple measurements of these factors may be difficult to obtain.