Late blight is caused by the fungal-like oomycete pathogen Phytophthora infestans. The primary host is potato and tomato. The first symptoms of late blight in the field are small, light to dark green, circular to irregular-shaped water-soaked spots. 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 appearing greasy. Leaf lesions also frequently are surrounded by a yellow chlorotic halo. The lesions are not limited by leaflet veins, and as new infections occur and existing infections coalesce, entire leaves can become blighted and killed within just a few days. The lesions also may be present on the petioles and stems of the plant. During active growth, especially in cool, wet weather, a white mildew-appearing area is visible at the edge of the lesions or along petioles. This is the area where the late blight pathogen actively produces spores. As the weather changes to warm and dry, these lesions become dry, stop sporulating and become tan.