Distribution and Development of Black Dot, Verticillium Wilt, and Powdery Scab on Russet Burbank Potatoes in Washington State. Dennis A. Johnson, Plant Pathologist, Washington State University, Prosser 99350. Eugene R. Miliczky, Research Technician, Washington State University, Prosser 99350. Plant Dis. 77:74-79. Accepted for publication 5 September 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0074.

Plant samples were collected from 10 commercial fields of Russet Burbank potatoes in south-central Washington and assayed for Colletotrichum coccodes, Verticillium dahliae, and Spongospora subterranea 13 times between 21 May and 29 August 1990. Three fields were likewise surveyed seven times between 30 May and 20 August 1991. Colony-forming units of C. coccodes and V. dahliae were quantified in sap expressed from stem sections. Powdery scab galls were counted on roots. C. coccodes was detected 38 and 15 days after plant emergence in 1990 and 1991, respectively. V. dahliae was detected 10–15 days after plant emergence both years, and galls caused by S. subterranea were first seen 66 and 47 days after plant emergence in 1990 and 1991, respectively. C. coccodes was isolated from plants in all fields by 29 June 1990 and 18 June 1991. C. coccodes and V. dahliae were isolated from 70–100% of the randomly sampled plants by mid-July and mid-August, respectively, both years. Colony-forming units of C. coccodes and V. dahliae per centimeter of aboveground stem tissue increased as the season progressed. Infection of potato by C. coccodes did not appear to depend on concurrent or preceding infection by V. dahliae.