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Canopy Density and Microclimate Effects on the Development of Aerial Stem Rot of Potatoes. M. R. Cappaert, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902; M. L. Powelson, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902. Phytopathology 80:350-356. Accepted for publication 17 October 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-350.

Field plots of potato cultivar Russet Burbank were established in four center-pivot irrigated fields in Oregon over 2 yr to evaluate canopy density and microclimate effects on the development of aerial stem rot caused by Erwinia carotovora subsp. carotovora. Plant densities were 13, 26, and 52 ? 103 plants per hectare. Four treatments consisted of factorial combinations of within-row spacings of 23 or 46 cm and between-row spacings of 86 or 173 cm. Alteration in between-row spacings had a greater effect on disease onset and area under the disease progress curve (AUDPC) than did changes in within-row spacing. Area under the leaf area index curve (AULAIC) values were significantly higher (P? 0.01) with a decrease in either between- or within-row spacing. Disease onset in dense plantings occurred 5?34 days earlier than in sparse plantings. AUDPC values were significantly higher (P? 0.01) with 86 cm compared with 173 cm between-row spacings at four sites. Based on linear regression, AULAIC accounted for 95?99% of the variation in AUDPC. Plant density had no significant effect on epiphytic population size of soft rot erwinias, although some seasonal trends were noted. Populations of the epiphytes were not detected early in the season, but increased to peaks at mid- or late-season and generally declined by the end of the season. Average duration of leaf wetness was proportional to the leaf area index and periods of leaf wetness were of a longer duration in dense than in sparse plantings. Peak leaf area index values and longest periods of leaf wetness preceded extensive symptom development by 2 wk for all plant densities.

Additional keywords: epidemiology, epiphytic bacteria.