Late blight on tomatoes is a major concern for both the home gardener and commercial tomato farmers. Late blight usually strikes late in the season, when plants are full of fruit, and can cause catastrophic losses. This disease can spread rapidly and easily become an epidemic under certain conditions. Prevention and control are essential to keep late blight at bay, and ensure a healthy crop of tomatoes.
One of the most famous diseases in history, late blight was responsible for the Irish potato famine in the mid-19th century, and the subsequent migrations of Irish immigrants to the United States. Late blight originated in central Mexico, and was introduced to the U.S. in the 1840s. By the 20th century, it had spread worldwide. In 1946, U.S. tomato farmers lost millions of dollars in crops due to late blight. In the early 1990s, several more severe and aggressive strains were found to have entered the U.S. from Mexico.
Late blight is caused by a fungus-like oomycete, Phytophthora infestans. P. infestans spreads rapidly during periods of high humidity and temperatures between 60 to 80 degrees Fahrenheit. It can develop in very wet conditions with high daytime temperatures and moderate nighttime temperatures. It is spread by rain and wind, and overwinters on infected plant parts and in the ground. Phytophthora infestans also attacks other members of the solonaceae family, such as eggplant, peppers and petunia.
Late blight first appears on tomato leaves as light brown to purplish spots which grow rapidly. A white fungus can sometimes be spotted on the underside of the leaves in the early morning. Tomato fruit develop mahogany to purple blotches. These blotches can form a ring pattern on the fruit. The rotten fruit has a foul smell caused by secondary infection following the late blight.
Start with disease-free transplants, and space tomato plants far enough apart so that they dry quickly during the day. Clean all debris from the garden in the fall, and destroy all infected plants. Do not compost infected plant parts.
Rotate tomato crops to another location every year. It is best to leave four years between tomato plantings in a spot where late blight has been a problem in the past.
Sterilize soil with solarization to prevent late-blight pathogens from surviving in the soil. Solarization is most effective during the summer months, when temperatures are highest.
The best control is sanitation, but apply fungicides if needed. If possible, remove all diseased plant parts and destroy them. Apply protectant fungicide treatment early in the season. Apply fungicides at regular intervals during the growing season to maintain protection and prevent rapid spread. Adjust fungicide application to the weather conditions, and apply more frequently when conditions are favorable to the spread of late blight.
Inspect tomato plants regularly and apply controls quickly when the disease symptoms first appear. Fungicides are needed most in mid-season when plants have dense vegetation that prevents proper air flow. Use the most effective fungicides you can during this period.