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Effect of Black Knot Incidence on Yield of Stanley Prune Trees and Economic Benefits of Fungicide Protection. D. A. Rosenberger, Plant Pathologist, New York State Agricultural Experiment Station, Geneva 14456. W. D. Gerling, Agricultural Economics Extension Specialist, Cornell University, Ithaca, NY 14853. Plant Dis. 68:1060-1064. Accepted for publication 14 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/PD-68-1060.

Captan, benomyl plus captan, zineb plus captan, thiophanate-methyl, ferbam plus sulfur, and dichlone, applied to 3-yr-old Stanley prune trees in seasonal spray programs for black knot control, resulted in black knot infection of 14, 20, 5, 36, 14, and 7% of total limb length, respectively, compared with 52% in untreated controls. Yield was measured the season following infection, and a regression equation was developed to relate yield to tree size and severity of black knot infection. Economic benefits of fungicides were calculated after fungicide and application costs were deducted. Zineb plus captan sprays applied during a single epidemic season could save $8,533/ha in subsequent losses attributable to black knot, but economic benefits of fungicides applied on an annual basis would be diminished by the infrequent occurrence of conditions favoring severe black knot epidemics.

Keyword(s): Dibotryon morbosum.