Late blight is a potentially destructive disease of tomatoes and potatoes caused by the fungal-like organism, Phytophthora infestans. This pathogen is referred to as a ‘water mold’ since it thrives under wet conditions. Symptoms of tomato and potato late blight include leaf lesions beginning as pale green or olive green areas that quickly enlarge to become brown-black, water-soaked, and oily in appearance (Fig. 1 and 2). Lesions on leaves can also produce pathogen sporulation which looks like white-gray fuzzy growth (Fig. 1 and 2). Stems can also exhibit dark brown to black lesions with sporulation (Fig. 2). Tomato fruit symptoms begin small, but quickly develop into golden to chocolate brown firm lesions or spots that can appear sunken with distinct rings within them (Fig. 1); the pathogen can also sporulate on tomato fruit giving the appearance of white, fuzzy growth. On potato tubers, late blight symptoms include firm, brown, corky textured tissue (Fig. 2). The time from first infection to lesion development and sporulation can be as fast as 7 days, depending upon the weather (1). Control of late blight in the field is a critical component of long term disease prevention, as infected plant parts, if unexposed to winter killing frost conditions, can carry the pathogen from one growing season to the next (Fig. 3).
Soil Science Extension
University of Wisconsin Madison