Abstract:
Four methods for predicting an increase in the incidence and severity of blue mould were compared for their effectiveness as spray warning systems to indicate the need to apply fungicides for control of the disease. Cruickshank's Formula, based on temperature and relative humidity, was found unsatisfactory and was discarded after one season. A modified Cruickshank Formula and a spray warning system based on the occurrence of rain gave promising results when high disease indices were recorded in untreated plots but not when disease indices were low. The Synoptic Forecast system, based on Collins' 'Critical Relative Humidity' gave better results than the other systems but was inferior to weekly treatments. Weekly fungicidal treatment provided the most effective control of blue mould. Three fungicides were used in conjunction with different spray warning systems to investigate possible interaction. No difference was found between 0.2 per cent zineb spray, 15 per cent zineb dust, and 0.2 per cent maneb spray in these experiments. Leaf quality (grade index) was not adversely affected by any of the fungicides at the concentration or frequency with which they were used.