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-management strategies, the relative resistance of potato cultivars to silver scurf and the effect of harvest date on disease severity were evaluated. In field studies at two locations, there were significant differences in disease severity (tuber surface area with symptoms) among cultivars. However, differences appeared to be related to physiological maturity of the cultivars: disease severity was greatest on early-maturing cultivars, intermediate on cultivars of medium maturity, and least on late-maturing cultivars. In resistance assays using mature, detached tubers, there were differences among cultivars in disease severity, but not in sporulation. In contrast to field trials, disease severity was not related to cultivar maturity. In separate field experiments, disease severity increased with later harvest dates for all cultivars. However, disease was present on tubers of some cultivars at the earliest harvest date, when vines were still green. These results indicate 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 breeding clones for silver scurf resistance.