Occurrence and cross-infection of Phytophthora infestans on hairy nightshade (Solanum sarrachoides) and potato (Solanum tuberosum) in Maine

Late blight, caused by Phytophthora infestans, is a significant disease in all potato-growing regions of the world. Identification of additional hosts is critical for understanding potential sources of inoculum. In 2004, late blight was observed on leaves and stems of hairy nightshade (Solanum sarrachoides), a common weed in many potato fields in Maine. Diseased nightshade plants were detected in 2 of 18 locations having potato late blight. Lesions from diseased nightshade leaves ranged from 0.25 to 9 cm(2) and, on diseased stems, the range of lesion length was 4.5 to 13.8 cm. Isolations from diseased samples and microscopic observations confirmed the presence of P. infestans. Based on allozyme analysis, 27 representative isolates of P infestans from hairy nightshade were identified as the US 8 genotype. Pathogenicity tests and Koch's postulates were satisfied by inoculating intact plant and detached nightshade leaves with R infestans isolated from hairy nightshade. Cross-infection of nightshade, potato leaves ('Shepody'), and potato tubers ('Yukon Gold') occurred when the inoculum source was either hairy nightshade or potato. When the inoculum source was hairy nightshade, the mean infection frequency at 18 degrees C was 50% and 67% on hairy nightshade and potato leaves, respectively. Inoculation with Phytophthora infestans isolated from potato resulted in average infection frequencies of 73% and 83% on nightshade and potato, respectively. These results suggest that hairy nightshade may be an inoculum source for late blight in potato fields. Management practices for controlling hairy nightshade would benefit potato production.