How to know if your garden is infected with late blight


COLUMBUS — Following the confirmation of late blight at an Ohio garden center June 25 and recent wet, cool weather conditions — which create a favorable environment for the spread of the fungal pathogen that causes this potentially devastating disease of tomatoes and potatoes — gardeners across the state are now wondering if their backyard crops may be infected and what to do about it.

Responsible for the Irish potato famine of the 1840s, late blight has caused widespread crop damage across the U.S. and Canada since 1990. If left unmanaged, this disease — which is transmitted via spores — can result in complete destruction of tomato or potato plants.

Inquiries increasing

The disease has been found in only a few gardens in Ohio so far, but the number of inquiries from home gardeners and farmers is starting to increase, said Nancy Taylor, director of Ohio State University’s C. Wayne Ellett Plant and Pest Diagnostic Clinic in Columbus.

Late blight thrives under weather conditions such as those that prevailed in most of Ohio recently: nights in the 50s Fahrenheit and days in the 70s Fahrenheit, accompanied by rain, fog or heavy dew, said Sally Miller, a plant pathologist and vegetable crops specialist with the Ohio Agricultural Research and Development Center and Ohio State University Extension.

Under those conditions, lesions may appear on leaves within three to five days of infection, followed by white mold growth soon after. Spores formed on the mold are spread readily by irrigation, rain and equipment.

Another cycle

These spores can also be easily dislodged by wind and rain and can be blown into neighboring gardens or fields within five to 10 miles or more, beginning another cycle of disease.

The current outbreak was first reported on tomato plants sold at garden centers and on tomatoes at home gardens and farms in New York, Virginia and Pennsylvania; it was also found on potatoes from Pennsylvania.

Because infested tomato plants were traced to a garden center chain that has numerous stores across Ohio and the country, diseased plants may have already turned up in people’s vegetable gardens.

What to look for

Late blight appears on potato or tomato leaves as pale-green, water-soaked spots, often beginning at leaf tips or edges. The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissue.

Lesions enlarge rapidly and turn dark brown to purplish-black. During periods of high humidity and leaf wetness, a cottony, white mold growth is usually visible on lower leaf surfaces at the edges of lesions.

In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears.

Infected areas on stems appear brown to black, and entire vines may be killed in a short time when moist weather persists.

Potato tubers

On potato tubers, late blight appears as a shallow, coppery-brown, dry rot that spreads irregularly from the surface through the outer 1/8-1/2-inch or more of tissue.

On tuber surfaces, lesions appear brown, dry and sunken, while infected tissues immediately beneath the skin appear granular and tan to copper-brown.

Late blight can also develop on green tomato fruit, resulting in large, firm, brown, leathery-appearing lesions, often concentrated on the sides or upper fruit surfaces.

If conditions remain moist, abundant white mold growth will develop on the lesions and secondary soft-rot bacteria may follow, resulting in a slimy, wet rot of the entire fruit.

What to do

If late blight is found in your garden, destroy plants already infected — pulling out the entire plant, placing it immediately in a plastic bag and disposing of the closed bag in the garbage.

Miller and Taylor advised against putting infected plants on a compost pile or in a composter, or leaving them lying around.

“Live plant tissues serve as a source of inoculum, and uprooted plants may support active spores of the pathogen for some time,” Miller pointed out.


Healthy-looking plants should be protected with a fungicide. Conventional gardeners can use fungicides containing chlorothalanil or copper; several brands are available in garden centers and other retail outlets.

Organic gardeners can use copper-based fungicides; several Organic Materials Review Institute-approved copper-based fungicides and formulations are available.

Gardeners who spot potential late blight disease symptoms and are unsure of identification can submit samples to the C. Wayne Ellett Plant and Pest Diagnostic Clinic for a nominal fee.

Learn more

To learn more, log on to or call 614-292-5006. A fact sheet with photos of late blight symptoms and additional disease information can be found at

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  1. An excellent product I have used for years and with great success even this year with all the rain for my tomatoes is “Serenade”. I use it as a general anti-fungal because it treats a wide variety of diseases including late blight. Happy gardening.


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