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Ecology and Epidemiology

Effects of Isolate Virulence, Plant Age, and Crop Residues on Seedling Blight of Sunflower Caused by Alternaria helianthi. K. K. Jeffrey, Graduate research associate, Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691, Present address: Buckman Laboratories, Inc., 1265 N. McLean Boulevard, Memphis, TN 38108; P. E. Lipps(2), and L. J. Herr(3). (2)(3)Assistant professor, and professor, respectively, Department of Plant Pathology, Ohio State University, Ohio Agricultural Research and Development Center, Wooster 44691. Phytopathology 74:1107-1110. Accepted for publication 7 May 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-1107.

Isolates of Alternaria helianthi obtained from infested sunflower seed, wild Helianthus annuus, and a commercial sunflower hybrid varied in virulence as determined by the number of lesions produced on leaves. Differences in the number of lesions per leaf caused by the various isolates could only be detected at the highest conidial concentration used (1,500/ml). Young sunflower plants, ranging in age from 1 to 21 days, were equally susceptible to stem infection when conidia (20,000/ml) were deposited on the stem base. Conidia of A. helianthi were produced on sunflower stem residue that overwintered in the field. A greater proportion of stem residue (92%) yielded conidia when collected from the soil surface on 20 May compared to residues that were buried (13%) or those that were collected on 27 July from the soil surface (7%) or buried (0%). In greenhouse mist-chamber tests, infested residue placed on the soil surface resulted in a high amount of seedling disease, whereas residues buried above or below the seed resulted in very little and no disease, respectively. In a field trial, extensive seedling death occurred when early planted (14 June) seedlings emerged in plots covered with infested sunflower residues as compared to the level of disease that occurred in plots without residues or in plots planted later in the season (12 July). Results indicate that crop residues are a significant source of inoculum for Alternaria blight and that reduced inoculum levels could be achieved by clean plowing, 1-yr crop rotation, and/or delayed planting.