A Forecasting Model for Fire Blight of Pear

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Abstract
Bactericide applications for control of fire blight, caused by Erwinia amylovora, were forecast by using daily mean orchard temperatures. Bacteria were detected in pear flowers in 11 of 12 orchards in 1977 within 2 wk after the mean temperature in the orchard exceeded a prediction line drawn from 16.7.degree. C on 1 March 1 to 14.4.degree. C on May 1. During 1974, 1975 and 1976, bacteria were detected in 93% of the orchards 22 days or more after the crossing of the prediction line. These epiphytic bacteria were isolated from healthy pear flowers before any fire blight appeared in the orchard in 100% of the orchards monitored during 1974-1977. Treatments initiated soon after the mean temperature exceeded the prediction line gave control equal to a spray program initiated at 10% bloom, which entailed more applications. Reduction in the number of applications depends on spring temperatures. In 1974 and 1975, the number of applications in test orchards was reduced by > 60% (from 16 to 6, 14 to 6, and 7 to 3); in 1977, however, the reduction averaged only 1 application. Modifications of this technique may be applicable to other geographic areas where pears are grown.

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