Optimum timing of fungicides for control of onion white rot (<i>Sclerotium cepivorum</i>)

Abstract

Onion white rot caused by Sclerotium cepivorum continues to be a major fungal disease of onions in New Zealand The onset of the disease in the Pukekohe area can be predicted by the use of cumulative average daily soil temperatures above a base temperature (white rot degree days) This prediction model was used to determine the optimal timing for fungicide application Analysis of data from trials on timing of fungicide applications conducted between 19892007 showed a correlation between the efficacy of fungicide programme and accumulated degree days with later start times in relation to disease onset equating to a consistent drop in efficacy The disease normally occurs when white rot degree days reach 250 Fungicide programmes commenced above this value achieved relatively poor control Spray programmes should start before the disease is observed in the field with start times between 100 and 200 white rot degree days achieving the best control