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Peach Rusty Spot Epidemiology: Incidence as Affected by Distance from a Powdery Mildew-Infected Apple Orchard. S. M. Ries, Assistant Professor, Department of Plant Pathology, University of Illinois, Urbana, IL 61801; D. J. Royse, Graduate Research Assistant, Department of Plant Pathology, University of Illinois, Urbana, IL 61801, Present address of junior author: Department of Plant Pathology, The Pennsylvania State University, University Park, PA 16802. Phytopathology 68:896-899. Accepted for publication 17 November 1977. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-896.

Peach fruits were collected at distances ranging from 10-90 m from an apple (cultivar Jonathan) orchard affected with Podosphaera leucotricha, the suspected causal agent of rusty spot. Disease incidence as evidenced by numbers of fruit bearing rusty spot lesions decreased with increasing distance from the apple orchard. Incidence of fruit infection on four sampling dates ranged from a maximum of 43 to 50% at 10 m down to 3 to 10% at 90 m from the nearest P. leucotricha- infected apples. Coefficients of determination (R2) for the calculated gradient always were greater for number of lesions per fruit than for percentage of fruit infected at each sampling date. Lesion enlargement paralleled fruit growth as measured by increase in fruit surface area. There was a significant weight reduction of infected fruit at 2 wk (P = 0.01) and 6 wk (P = 0.05) after floral tube abscission. Repeated attempts to produce rusty spot symptoms on peach fruit by inoculation with P. leucotricha conidia collected from infected apple leaves or with trichomes scraped from infected peach fruit were unsuccessful.