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Effects of Host Density and Number of Disease Foci on Epidemics of Southern Blight of Processing Carrot. V. L. Smith, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, Present address: New York State Agricultural Experiment Station, Department of Plant Pathology, Geneva 14456; C. L. Campbell, S. F. Jenkins (deceased), and D. M. Benson. Associate professor, Professor, and Professor, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 78:595-600. Accepted for publication 13 November 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-595.

Epidemics of southern blight of processing carrot, caused by Sclerotium rolfsii, were influenced positively by high host density and by high initial number of disease foci per row. Disease severity, estimated as area under the disease progress curve (AUDPC), was highest in rows with high host density (52 plants per meter) and the highest number of initial foci (10 foci per row). Host tissue available for infection apparently was limiting in plots with eight to 10 foci per row, as disease progress curves of these plots converged at the end of the season at 32% due to focus extinction. AUDPC for disease severity and the rate of disease increase, derived from a linear model, were directly related to the initial number of disease foci per row. Rate of focus expansion, measured as centimeters per focus per day, was influenced by fluctuations in the number of hours of relative humidity greater than 95% in the plant canopy, near the crown of the plants. Variations in canopy density, obtained through combinations of thinning the plants and removal of foliage from a portion of the carrot plants, affected disease severity, but the relationship between change in the number of foci per row and change in disease severity was not constant over the different canopy densities. In rows where carrot plants were thinned and foliage removed, disease severity with 10 initial foci per row was almost equal to that at 0 and 4 foci per row. Removing the foliage of the plants enhanced interplant mycelial growth in rows that were not thinned. Disease severity over time was significantly influenced by the number of foci over time, suggesting that quantitative evaluations of epidemics of southern blight of processing carrot may be possible with a rating of disease incidence (number of foci) rather than disease severity.