ECONOMICS OF REDUCING FUNGICIDE USE BY WEATHER-BASED DISEASE FORECASTS FOR CONTROL OF VENTURIA-INAEQUALIS IN APPLES

In seven field trials conducted over four seasons, black spot incidence was consistently greater in treatments where fungicide use was reduced by timing sprays with weather information compared to standard calendar-based treatments. Fungicide use was measured as the total number of label-rate applications of fungicides with activity against black spot and the individual components of mixed applications of fungicides were counted separately. Overall trials, the mean disease incidence at harvest increased logarithmically as the number of fungicide applications decreased. A regression equation describing disease increase in terms of number of fungicides and number of infection periods was used in an economic analysis of apple production. When the savings from reduced fungicide use were weighed against the increased harvesting and grading costs and revenue losses from increased disease, there appeared to be scope to reduce fungicide use by up to 25% before increased disease adversely affected profitability. Where disease risk was lower because of fewer infection periods, savings of up to 56% appeared to be feasible. It was concluded, however, that because fruit harvesting and grading costs are a high proportion of total costs, because they increase with increasing disease incidence and because cost savings from reducing fungicide use are a relatively small proportion of total costs, there is at present little economic incentive for apple growers to reduce fungicide use.