Oilseed rape: Sclerotinia risk forecasting

Abstract

Twenty-two "depots" of buried sclerotia of Sclerotinia sclerotiorum were established in crops, or individual plots of oilseed rape throughout England in September 1991. The depots were monitored for the production of apothecia from the end of February 1992 until July. The experiment was repeated in 1992 when 25 depots were monitored between March and July 1993.

A detailed examination of meteorological data at six sites in 1992 and 1993 indicated that the initiation of apothecial production was preceded by (i) a dry/drier period of weather and/or (ii) mean daily temperatures (on a weekly basis) starting to rise above 10°C. Germination of sclerotia increased thereafter reaching a peak after a period of wet weather. Apothecial numbers declined during late June to early July when the weather became dry and the mean daily temperatures reached > 15.3°C for the week. Sclerotinia infection was detected at eight of the sites in 1992 and nine in 1993.

The initiation of germination of sclerotia coincided with flowering, or the presence of fallen petals during pod-ripening, but did not occur at a specific crop development stage. Any scheme for predicting the time of germination of sclerotia and, therefore, ascospore release, could not rely on development stage alone. However, it seems possible that sclerotial germination can be predicted by recording rainfall and temperature data at a site close to the oilseed rape crop. The data can be interpreted simply by examining the rainfall pattern in the weeks preceding the mean daily air temperature, calculated on a weekly basis, reaching 10°C. The remote prediction of the emergence of apothecia can be incorporated readily into forecasting systems which take account of criteria for infection of oilseed rape by Sclerotinia sclerotiorum.