EFFECTS OF POTATO CULTIVAR AND TIME OF HARVEST ON THE SEVERITY OF SILVER SCURF

Economic losses due to silver scurf of potato have increased, possibly because of the development of benzimidazole resistance in the causal agent, Helminthosporium solani. In an effort to develop new disease-ma nagement strategies, the relative resistance of potato cultivars to si lver scurf and the effect of harvest date on disease severity were eva luated. In field studies at two locations, there were significant diff erences in disease severity (tuber surface area with symptoms) among c ultivars. However, differences appeared to be related to physiological maturity of the cultivars: disease severity was greatest on early-mat uring cultivars, intermediate on cultivars of medium maturity, and lea st on late-maturing cultivars. In resistance assays using mature, deta ched tubers, there were differences among cultivars in disease severit y, but not in sporulation. In contrast to field trials, disease severi ty was not related to cultivar maturity. In separate field experiments , disease severity increased with later harvest dates for all cultivar s. However, disease was present on tubers of some cultivars at the ear liest harvest date, when vines were still green. These results indicat e that disease severity at harvest is correlated to the length of time that tubers are exposed to inoculum in the field. Mature tuber assays may be a more reliable method than field tests for screening potato b reeding clones for silver scurf resistance.