Late Blight

Late blight is a fungal disease that attacks the foliage, fruit, and tubers of tomatoes and potatoes. The fungus Phytophthora infestans (genus translates as “plant destroyer”) flourishes in cool, wet conditions and can wipe out robust plants. Traveling by air (though it can be seed-borne in potatoes as well), the spores swiftly proliferate in wet conditions between 60 and 80°.

Infected plants develop greyish-black lesions on leaves and stems, often accompanied by fuzzy white fungal growth. Left uncontrolled, the blight can spread to the tubers as well.

Here are a few tips for healthy crops:

  • Plant only seed that is certified disease-free (that’s us).
  • Consider planting late-blight resistant varieties. If a variety shows field resistance to late blight, we mention it in the description. View Late Blight Resistant varieties.
  • Consider beginning the season with preventive measures. Regalia, formulated with an extract from giant knotweed, induces systemic resistance to Phytophthora and other pathogens. With translaminal action, spraying the tops of leaves extends coverage to the bottoms as well. Spray every 7–14 days to protect new growth. Cease colonizes leaf surfaces with beneficial bacteria that inhibit the growth of predatory fungus. Also consider Monterey Complete Disease Control. Cueva copper soap can be applied as a preventative spray.
  • As a last resort, if lesions appear, apply Bonide Copper.
  • Avoid overhead irrigation just before dusk, as prolonged wet vegetation is a prime target for the blight.
  • Hill potatoes well to reduce the transmission of infection from leaf to tuber. Growing potatoes in plastic mulch may help.
  • Do not compost any infected plant material. Freezing kills spores; be sure plant tissue is thoroughly frozen. The best method to sterilize your field for next year may be to leave sick plants on the surface to freeze.

Stay up-to-date by watching MOFGA’s bulletins or contact your local Cooperative Extension Service for more information.

In Maine you can call the Late Blight Hotline (207-760-9ipm) for daily reports on late blight incidence in Maine. Nationally, you can go to to see an interactive up-to-date map of Late Blight occurrences across the country.

If you are trying to identify a pest or need to send an insect or plant sample to a lab for diagnosis, go to UMaine Cooperative Extension, a useful website of the plant disease diagnostic laboratory, or contact your local Cooperative Extension Service.