FACTORS INFLUENCING A PETAL-BASED FORECASTING SYSTEM FOR SCLEROTINIA STEM ROT OF CANOLA
Turkington, Thomas Kelly
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Studies from 1987 to 1990 investigated the influence of the following factors on the relationship between the percentage of canola petals infested with Sclerotinia sclerotiorum and the incidence of sclerotinia stem rot: canopy density; diurnal and other weather mediated fluctuations in petal infestation; and long-term changes in infestation during flowering. Disease incidence was significantly related to canopy density,as indicated by percentage light penetration through the canopy, and increased as light penetration decreased. Higher disease incidence in the denser canopies was attributed to more favorable micro environmental conditions. A general pattern of increasing petal infestation from morning to afternoon was observed in 1987 and 1988. Infestation was found to be significantly higher during the afternoon than the morning in 4 of 7 crops. Increases from morning to afternoon ranged from 0.3 to 46%; reductions ranging from 0.5 to 14%were also observed on a few days. The pattern in petal infestation probably resulted from periodicity in ascospore discharge related to several environmental factors. Long-term fluctuations of up to 94% petal infestation were observed during flowering. These changes were mostly increases; however, in 1989, infestation generally decreased during flowering. These changes were related to variation in rainfall which influenced inoculum production. In general,disease incidence was significantly related to petal infestation at flowering stages when the presence of inoculum coincided with favorable host and environmental conditions. Multiple regression analyses with infestation atearly, full and late bloom, light penetration,leaf area index and crop height as independent variables accounted for 55-98% of the variation in disease incidence. For practical on-farm disease forecasting, long-term changes in petal infestation may be more important than diurnal fluctuations for determining disease risk and incidence. However, as a precaution against underestimating infestation, petals should be sampled in the afternoon and not after substantial rain fall or dew. Sampling petals more than once during flowering will be essential to account for long-term variations in infestation. The influences of the host and environment are also important; however, assessing their effect may be difficult, especially for on-farm disease forecasting.