Within the last week, late blight has been found in Adams and Juneau counties on potatoes and in Sauk County on tomatoes. Our cool, wet spring likely contributed to this early arrival.
Any of you who have grown tomatoes or potatoes are most likely aware of late blight and the damage it can do to these crops; those of you just starting out growing these crops may not know. However, given the destructiveness of late blight, anyone who grows these crops should diligently learn the symptoms of this disease, regularly inspect plants for signs of the disease, and act to control any late blight if and when it is found. For late-blight disease diagnosis, take infected samples to your local University of Wisconsin-Extension office.
Late blight is a fungal-like disease caused by Phytophthora infestans. This pathogen can kill plants very, very quickly; entire crops of tomatoes and potatoes can be wiped out in just a few weeks from the time that the first symptoms are seen. Symptoms of 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. Continual infections from release of more spores acts to infect more leaves on the same plant and any nearby plants. Fruits of tomatoes with late blight have yellow to brown hard spots with distinct rings. Any plant part can have fuzzy, white-gray growths from reproducing spores.
The destruction doesn't stop with the above-ground parts; potato tubers can be infected when rain or irrigation water washes the spores down through the soil from the leaves and/or stems of infected plants. Furthermore, because tubers are living tissue, the pathogen can remain within stored potatoes over winter, and be ready to start the infection cycle over again in spring.
As part of a control program, those who grow tomatoes and potatoes should monitor the conditions for late blight in Wisconsin. This can be done by accessing the Vegetable Crop Plant Update newsletter at www.plantpath.wisc.edu/wivegdis/. When conditions are favorable for late blight, control efforts should include spraying with a broad-spectrum contact fungicide (before infection occurs), inspection of plants on a daily basis, and application of a fungicide specifically labeled for late blight if and when late blight is found. Information on fungicides for late blight, including for organic production, is available at the website listed, or you can contact me at 715-346-1589.
If late-blight control efforts are not successful, all plant parts should be pulled from the soil, placed in black garbage bags, and left in the sun for several days to kill the spores. The garbage bags should then be placed in the trash. It is important that any infected plants are disposed of; infected plants can release spores that could infect your neighbors' plants.