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.
Two mating types are needed to produce sexual, persistent soil-borne oospores. The population is largely clonal outside its center of origin in the Toluca Valley of Mexico, relying on production of asexual sporangia for persistence. In the U.S., clonal lineage (also referred to as genotype or strain) US-1 (A1 mating type) was the predominant clonal lineage until the late 1980s-early 1990s, when US-8 appeared. US-8 was the opposite mating type (A2) and was insensitive to mefenoxam, a fungicide with exceptional activity against oomycetes, but with a specific mode of action that effectively selects for insensitivity. New clonal lineages have predominated epidemics in recent years with varying levels of mefenoxam resistance. Late blight pathogen populations in the U.S. have and continue to experience major genetic changes or evolution. The end result is the production of pathogen isolates with unique genotypes and epidemiological characteristics. As such, continued investigation of this pathogen is necessary to maintain best management strategies in susceptible crops.
Our objective was to monitor for late blight on a state-wide basis and characterize P. infestans in a timely manner to inform appropriate management recommendations and enhance understanding of the pathogens introduction and persistence in Wisconsin.