Effects of Wounding and Wetting Duration on Infection of Potato Foliage by Colletotrichum coccodes. Dennis A. Johnson, Plant Pathologist, Washington State University Irrigated Agriculture Research and Extension Center, Prosser 99350. E. R. Miliczky, Research Technician II, Washington State University Irrigated Agriculture Research and Extension Center, Prosser 99350. Plant Dis. 77:13-17. Accepted for publication 21 August 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0013.

Dark brown to black lesions developed on leaves, petioles, and stems of Russet Burbank potato after wounding by sandblasting and inoculating with either conidia or sclerotia of Colletotrichum coccodes in the greenhouse and with conidia in the field. Conidia from 10 isolates and four collections of sclerotia of C. coccodes from various areas of Washington and Idaho were pathogenic on potato foliage. Significantly (P < 0.01) more lesions developed on foliage that was wounded than on foliage that was not wounded just before inoculation. Inoculated plants wilted, and lower and middle leaves became chlorotic and sometimes blighted. C. coccodes was reisolated from lesions and blighted areas and from internal stem tissues of inoculated plants. Disease symptoms did not develop on plants that were wounded but not inoculated. Successful infections decreased significantly (P < 0.001) as the interval between wounding and inoculation increased from 0 to 7 days, with most of the reduction occurring at an interval of 2 days. Lesion numbers on foliage of wounded and inoculated plants increased significantly (P < 0.001) as the duration of the postinoculation wet period increased from 2 to 48 hr.