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A Forecasting Model for Fire Blight of Pear. S. V. Thomson, Associate Professor, Department of Biology, Utah State University, Logan 84322. M. N. Schroth, Professor, Department of Plant Pathology, University of California, Berkeley 94720; and W. J. Moller, Extension Plant Pathologist, Department of Plant Pathology, and W. O. Reil, Staff Research Associate, Department of Pomology, University of California, Davis 95616. Plant Dis. 66:576-579. Accepted for publication 28 September 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-576.

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 orchards exceeded a prediction line drawn from 16.7 C on 1 March to 14.4 C on 1 May. 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 19741977. 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 more than 60% (from 16 to 6, 14 to 6, and 7 to 3); in 1977, however, the reduction averaged only one application. In 1978, estimates in California indicate a savings of three spray applications per grower on about 16,200 ha of pears for a total savings of $1.2 million. Modifications of this technique may be applicable to other geographic areas where pears are grown.