Late blight

Late blight on tomato foliage. Photo courtesy Meg McGrath, Cornell University

Late blight, a disease that strikes tomatoes and potatoes, can quickly ruin an entire crop — and provide a source of infection for other plants. So far this growing season, the disease has been confirmed in Florida and up the East Coast up to Pennsylvania and New York, as well as in California.

It is critical that gardeners understand that late blight is not like other tomato and potato diseases. Many other diseases affect these crops in home gardeners, but most of them only affect leaves or cause limited damage to fruit, and while they may reduce the harvest, they generally don’t cause a total loss. Furthermore, because most pathogens are not readily dispersed by wind, their effects are localized. Late blight, on the other hand, kills plants outright, and it is highly contagious. Its occurrence in your garden can affect other gardens and farms due to the wind-dispersed spores.

The USA Blight web site tracks the occurrence of late blight in real time, so check the site regularly during the growing season. When late blight is detected in your region, consider a weekly preventative spray. Research is ongoing in determining the best organic options. Cornell plant pathologist Meg McGrath suggests that organic gardeners use Actinovate (which contains the beneficial bacteria Streptomyces lydicus) as a preventative spray, adding a copper-based product, such as our ready-to-spray Copper Fungicide, when late blight is present. Once a plant is infected, it can’t be cured. Your best bet is to take steps to prevent infection in the first place and keep minor infections from spreading. Heavily affected plants must be destroyed.

It’s important for gardeners to be aware of this disease so they can act quickly and responsibly. For details on ways to prevent, monitor for, and treat late blight, including how to distinguish late blight from other common diseases, read How to Prevent Late Blight.

Suzanne DeJohn, Gardener’s Supply