Late Blight Rots Eastern Tomato Crops

Instead of plucking juicy, perfect tomatoes from their vines, local gardeners have watched in horror as tomatoes rotted on the vine before even ripening, and a strange fungus sickened the entire crop.

A Widespread Problem

Because infested tomato plants were traced to a garden center chain with stores all across the country, diseased plants turned up in vegetable gardens in Alabama, Connecticut, Delaware, Georgia, Kentucky, Maine, Maryland, Massachusetts, New Jersey, New Hampshire, New York, Ohio, Pennsylvania, Rhode Island, South Carolina, Vermont, Virginia and West Virginia.

History Repeats Itself

This is not a new scourge upon modern crops. Rather, late blight is one of the most devastating diseases of potatoes and tomatoes worldwide, responsible for the Irish Potato Famine of the 1840s. It was never eradicated.

Since 1990, late blight has caused widespread damage across the United States and Canada. If left unmanaged — as history reminds us — this disease can result in complete destruction of potato or tomato crops.

The Fungus Effect

Erik Draper, Horticulture Extension Educator in Ohio, reports on the rapid infectious abilities of the fungal pathogen, Phytophthora infestans (commonly called Late Blight) in Buckeye Yard and Garden Online. This fungus, under cool, moist conditions, transforms lightly infected plants into leafless, brown stems in as little as three to five days.

This disease only infects tomatoes and potatoes and needs live plant tissue in order to survive. This fungus cannot overwinter due to freezing temperatures, which usually kills plant tissue.

However, the fungus can overwinter on potato culls, tomato fruits or large pieces of stems tossed into the compost pile. If these large chunks of plant tissues are not properly composted and decomposed by next year, they will begin the infection cycle all over again.

Stop the Cycle Now

The Ohio State University Extension Office recommends totally destroying all plant parts by burning them, burying them deep in the soil, or placing them into plastic bags and disposing of plants in the landfill. Do not compost plants infected by the fungus. Good sanitation and horticultural practices like crop rotation should eliminate the threat of a repeat performance by this detestable blight next year.

If you’re one of the lucky tomato growers to have escaped infection, keep watch. It can occur without warning, and there’s nothing you can do to prevent it (unless you have commercial-grade chemical spray.)

If your tomato crop is infected now, should you trust volunteer tomato plants next Spring? No. Draper recommends that you pull them. Volunteer potatoes and tomatoes can be a significant source of spores of the late blight fungus. All volunteers should be destroyed as quickly as possible by herbicides, chopping, or cultivation.

Recognize the Signs

To recognize late blight, consider these descriptions from the OSU Extension Office: It appears on potato or tomato leaf edges as pale green, water-soaked spots. Lesions are surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge and turn dark brown to purplish-black. During periods of high humidity and leaf wetness, a cottony, white mold growth appears on lower leaf surfaces at the edges of lesions. In dry weather, infected leaf tissues dry up and white mold growth disappears. Infected stems appear brown to black.

On potato tubers, late blight appears as a shallow, coppery-brown, dry rot that spreads from the surface through the outer tissue margins. On tuber surfaces, lesions are brown, dry, and sunken, while infected tissues immediately beneath the skin appear granular and tan to copper-brown. Secondary bacteria and fungi result in a slimy breakdown of entire tubers.

Late blight can also develop on green tomato fruit, resulting in large, firm, brown, leathery-appearing lesions concentrated on the sides or upper fruit surfaces. If conditions remain moist, white mold growth develops on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy, wet rot of the entire fruit.

It may be too late to escape the detestable blight this season, but preparation now is your best defense for next year’s tomato crop.