Late Blight Identified On Cheshire Tomato Plants

By Nancy K. Crevier

Pathologist Dr Sharon Douglass of the Connecticut Agricultural Experiment Station in New Haven confirmed the state’s first case of late blight on a tomato plant grown in a backyard garden in Cheshire. The late blight pathogen, Phytophthora infestans (P. infestans) is the disease responsible for the potato famine of Ireland in 1840, and last year the pathogen devastated tomato crops in New England when infected tomato transplants purchased at chain stores and favorable cool and wet weather conditions rapidly spread the disease from one garden to the next.

The case identified in June in Cheshire was grown from seed by the home gardener, who did not have a late blight problem last summer.

“Our best guess is that it came from a garden with potato tubers that overwintered or were in a compost heap, and some volunteer potato plant that came up,” said Mary Inman, a technician in the Connecticut Agricultural Experiment Station diagnostic clinic. “We really don’t know where it originated, but everyone has been watching for this,” she said.

The late blight pathogen does not survive in the soil or in plant debris; however it can survive in infected potato tubers.

P. infestans produces up to 300,000 sporangia, a white fuzzy growth, each day for a single lesion on the leaf of an infected plant. Each sporangium is capable of spreading the infection, through disturbance due to irrigation, rain, wind, or human activities. As was seen in New England last summer, the epidemic can be catastrophic to commercial and home gardeners.

All parts of the tomato plant that grow above ground are susceptible to late blight. Symptoms appear as water-soaked blotches (lesions) of an olive-brown or black color on stems and leaves. The white growth is visible following a rainfall. If cool and rainy weather is present, the symptoms will spread rapidly, killing the plant in just a few days. Blight can also develop on the fruit of the tomato plant.

Other plants in the nightshade family, including eggplant and peppers, can be infected by P. infestans, as can ornamental hybrid petunias.

According to an update issued by Dr Douglass, management of late blight requires identifying the pathogen, proper sanitation, and the use of fungicide sprays when necessary. Good air circulation and well drained soil for healthy stock is needed, and while P. infestans does not overwinter in the soil, it is still recommended that tomato plants be rotated from space to space in a garden to prevent other pathogens from infecting the crop.

Remove any “volunteer” tomato or potato plants that come up in the garden from previous years.

Inspect tomato and potato plants daily. If symptoms of late blight are observed, pull the plant immediately, place in a plastic bag and seal. Do not compost infected plants.

Avoid overhead watering, which can spread the pathogen. Staking tomato plants helps to keep the leaves dry and reduces infections.

The application of fungicides may be needed if there is a risk of late blight. For Connecticut homeowners the fungicides chlorotholonil and copper are registered for use.

Contact the Connecticut Agricultural Experiment Station at ct.gov/caes if late blight is suspected, for proper identification.