Late blight is a potentially destructive disease of potatoes and tomatoes caused by the fungal-like organism, Phytophthora infestans. This pathogen is referred to as a ‘water mold’ since it thrives under wet conditions. Symptoms include leaf lesions beginning as pale green or olive green areas that quickly enlarge to become brown-black, water-soaked, and oily in appearance. Lesions on leaves can also produce pathogen sporulation which looks like white-gray fuzzy growth. Stems can also exhibit dark brown to black lesions with sporulation. Tuber infections are dark brown to purple in color and internal tissues are often reddish brown in color and firm to corky in texture. The time from first infection to lesion development and sporulation can be as fast as 7 days, depending upon the weather.
Soil Science Extension
University of Wisconsin Madison