Potato silver scurf affected by tuber seed treatments and locations, and occurrence of fungicide resistant isolates of Helminthosporium solani

The effectiveness of various seed-tuber treatments was evaluated for control of silver scurf on potato (Solanum tuberosum), caused by Helminthosporium solani, at four locations in Washington and Oregon using seed-tubers from the same source. Disease incidence was determined at harvest and following storage, and differed significantly among locations. The highest incidence of disease was observed at Redmond, OR and the lowest was at Hermiston, OR. Significantly less silver scurf occurred on progeny-tubers, regardless of location, when seed was treated with the fungicide treatments fludioxonil, fludioxonil + quintozene, azoxystrobin, or thiophanatemethyl + mancozeb compared with the nontreated control. A sample of H. solani from seed planted in the Columbia Basin of Washington and Oregon was assessed for resistance to thiabendazole (TBZ) and thiophanate-methyl (TPM). Sensitivity of 20 isolates of H. solani to TBZ was determined on V8 media amended with TBZ. Four isolates, selected as a subset from the 20 isolates tested for TBZ sensitivity, were tested for resistance to TPM, mancozeb, and TPM + mancozeb on amended V8 media. Isolates differed significantly in sensitivity to both chemicals. Thirteen isolates out of the 20 tested were resistant to TBZ. One out of the four subsets of isolates was resistant to both TBZ and TPM. TBZ sensitivity was not related to geographic origin of the isolates. Variation in resistance was evident on a small scale as seen with one resistant and one sensitive isolate collected from the same tuber. This is the first report that silver scurf incidence is affected by location despite the same tuber-seed source and also the first report of TBZ and TPM resistant isolates of H. solani from the Columbia Basin (Oregon and Washington) production area.