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 plant-aid.org/map/ 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.