Successful management of late blight includes the following: Scouting, early detection by a Plant Clinic, destruction of infected plant material (tomatoes, potatoes, and susceptible weeds), monitoring of favorable conditions, and weekly protective fungicide application. Luckily, a large, five year, national project with government funding was started to investigate late blight, monitor pathogen genotypes, and improve management by developing resistant varieties and a monitoring program. If a preventative disease management approach is not taken, this disease will be extremely difficult to control. This rapid disease progression can cause crop loss and disease epidemics. Depending on favorable conditions and genotype virulence, the infection can quickly progress within 4 to 7 days.
Fruits can also be infected, developing irregular shaped brown blotches before deteriorating rapidly. Foliage can be quickly killed, especially above points where stems have been infected. The undersides of leaves can have a downy white growth when wet. The first symptom to appear will be rapidly enlarging water soaked areas that will form into irregularly shaped greenish black blotches with a "frosted" appearance. This pathogen can infect both young and old foliage. If both mating types of this pathogen become established in the north, late blight could become a very common disease and have a profound impact on tomato and potato production (especially organic). Oospores can survive in the absence of host tissue and in the presence of adverse conditions in the soil. If both mating types are present, sexual reproduction takes place and oospores are produced. This results in the development of fuzzy fungal growth on affected tissue that yield wind dispersed spores (sporangia containing zoospores). If only one pathogen mating type is present, asexual reproduction occurs. The type of pathogen reproduction varies based on mating types present. Phytophthora infestans is an obligate parasite, and currently only known to survive in the US on living host plant tissue or tubers. Typically, this disease can appear from August through October in the Northeastern US. However, this disease has continuously developed several weeks later than normal (in May) in Florida, which suggests that there may be a genotype that is able to tolerate warmer temperatures. This pathogen can be devastating in cool, rainy weather, as it thrives in a wet, humid environment. Through this project, several existing and new genotypes of Phytophthora infestans have been documented in the Northeastern US. Government funding has led to the recent investigation of late blight. Fortunately, this variation allows for easier control of this disease. For example, they have found that one genotype may only affect potato, while other genotypes may be more aggressive on tomato.
Through chance mutation or recombination during sexual reproduction, Phytophthora infestans can produce multiple genotypes that may vary in aggressiveness on tomato and potato.
Historically, this disease has not been a problem in Illinois. In the Northeastern U.S., tomato and potato are commonly grown together and form a "green bridge" contributing the spread of the late blight pathogen. In recent years, this disease has been a problem in Florida and sporadically in Northeastern United States. In most cases, potato crops are more at risk to be infected by this disease, because infested tubers provide the main source of initial inoculum. Phytophthora infestans was first reported in Europe in 1845 and is more famously associated with the Irish potato famine. More recently, the U of I Plant Clinic received a tomato sample from Northern Illinois also found to be infected with this pathogen. Several weeks ago, Purdue University reported tomato infection by Phytophthora infestans, which causes the disease known as Late Blight. Issue 15, SeptemLate Blight ( Phytophthora infestans) found on Tomato in Illinois.
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