M A Y / J U N E 2 0 1 7 20 POTATO GROWER Late blight is caused by the fun- gus-like oomycete pathogen Phytophthora infestans. The pri- mary host is potato but P. infes- tans can also infect other solana- ceous plants including tomatoes, petunias, and hairy nightshade that can act as source of inoculum to potato. In North America, late blight survives between seasons in infected seed tubers, cull piles and volunteer plants. This potentially devastating disease can infect potato foliage and tubers at any stage of crop development. It is necessary to know the symptoms and disease cycle of this rapidly destructive disease in order to implement management options. Symptoms The first symptoms of late blight in the field are small, light to dark green, circular to irregular-shaped water-soaked spots (Figure 1). These lesions usually appear first on the lower leaves. Lesions often begin to develop near the leaf tips or edges, where dew is retained the longest. During cool, moist weather, these lesions expand rapidly into large, dark brown or black lesions, often appear- ing greasy (Figure 2). Leaf lesions are also frequently surrounded by a yellow chlorotic halo (Figure 3). The lesions are not limited by leaf veins, and as new infections occur and existing infec- tions coalesce, entire leaves can become blighted and killed within just a few days. The lesions may also be present on petioles and stems of the plant (Figure 4). During active growth, espe- cially in cool wet weather, a white mildew-appearing area is visible at the edge of the lesions (Figure 5) or along petioles (Figure 6). This is the area where the late blight pathogen is actively producing spores. As the weather changes to warm and dry, these lesions become dry, stop sporulating and Andy’s Advice: Late Blight In Potato Dr. Andy Robinson, Assistant Professor and Potato Extension Agronomist, NDSU/UMN, Dr. Gary Secor, Professor of Plant Pathology, NDSU, Dr. Neil Gudmestad, University Distinguished Professor of Plant Pathology, NDSU Figure 1. Initial symptoms of late blight are small, light to dark green, circular to irregular-shaped water-soaked spots. (Photo credit: Dr. Andy Robinson, NDSU/UMN)