Master Gardener: Be on the lookout for late blight
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Master Gardener: Be on the lookout for late blight

Jun 21, 2023

Late blight was confirmed in Yates County on Aug. 1, according to the Cornell Vegetable Team.

It had previously been reported north of us in Ontario, Canada back in July. Fortunately, there aren’t any new reports on the USABlight map as of Aug.17, but with the wet weather we have had, it behooves tomato and potato growers to be on the lookout for it.

The forecasted dry and sunny weather — if it holds true — makes it less favorable for late blight to spread across the region. To check for current outbreak information visit, a national website which acts as an information portal and tracks late blight.

Late blight is a very destructive and highly contagious disease among plants in the Solanaceae (tomato) family. If you grow tomatoes and potatoes, you need to be able to identify late blight and know how to prevent it.

It is capable of wiping out tomato and potato crops within a week if conditions are right. You need to be ready to destroy infected plants in your garden, to limit its spread to your neighbors.

Phytophthora infestans is the pathogen that causes late blight. It is a type of water mold, which is a fungus-like microorganism.

It needs moisture to infect the plant. No moisture, no infection.

This disease can spread rapidly during cool, rainy weather, killing plants within a few days. Daytime temperatures between 60 to 70 degrees Fahrenheit, night temperatures between 50 to 60 degrees F and relative humidity near 100 percent create ideal conditions.

During sunny, dry periods the fungus becomes inactive.

The pathogen produces many spores which are easily dispersed by wind. Spores moving on winds can cause infections up to 30 miles away on cloudy days.

Wind and storms can carry spores north from milder southern climates.

Generally, late blight requires living plant tissue to overwinter in the Northeast such as on potato tubers. If there are multiple active strains, there is an increased risk that it could overwinter as long-lasting “oospores” or create new strains that are resistant to fungicides.

Late blight can spread like wildfire so be vigilant. Check your tomato and potato plants at least twice a week.

It takes three to seven days from infection to seeing symptoms. By the time the first plants are showing symptoms, neighboring plants have already been infected.

Late blight initially appears as irregularly shaped, dark green, water-soaked lesions on the lower leaves.

During periods of high humidity, a white cottony growth may be visible on the underside of the leaf, near the edge of the lesion. Spores produced on leaves continue to infect other parts of the plant such as petioles, stems and fruit.

As the disease progresses, lesions enlarge causing leaves to brown, shrivel and die. Eventually the entire plant will collapse.

Tomato fruit infected by late blight develops brown to dark brown greasy spots. Infected potato tubers develop irregular shaped, firm, sunken, dull brown to purple lesions around the eyes.

The infection can penetrate into the tuber, creating a reddish-brown dry rot.

If you find suspicious symptoms on your tomatoes or potatoes, take a plant sample — put it in a sealed plastic bag — to your local extension office for confirmation. The sample should consist of a minimum of five leaflets with lesions.

It is best to have fresh foliage with smaller lesions.

Late blight cannot be cured once a plant is infected. Early detection and treatment with effective fungicides are critical to slowing down its spread.

As a preventative measure, apply fungicides as the label directs.

Fungicides must be on the foliage before spores land on the plant and start an infection. Thorough spray coverage is crucial, including the undersides of leaves.

Since new growth is not protected and fungicides can wash off, it will be necessary to reapply fungicides.

Chlorothalonil is the most effective conventional fungicide available to home gardeners. Copper products are labeled for organic control of late blight.

Monterey Garden Phos — previously Agri-Fos — and Serenade are two organic and systemic fungicides that are also labeled for late blight protection.

Read the entire label and follow all application directions. Also read and understand the safety information before using any pesticide.

If you choose not to use fungicides it is important to check your plants daily.

Promptly destroy affected plants if late blight is positively identified so that it does not spread to your neighbors. This is best done on a dry, sunny day so that the spores die quickly.

If the weather stays wet, it is better to destroy plants rather than wait for a dry day. Do not leave plants in the garden where spores will continue to be produced until the plant dies.

Put infected plants in garbage bags or pile them up and cover with a tarp. Sunlight hitting the tarp will help kill the plant tissue and the fungus.

Leave garbage bags in the sun for a few days before throwing them out for the same reason.

Never compost plants with late blight. Notify your neighbors so that they can check and protect their plants.

There are other steps home gardeners can take to help manage late blight.

When possible, plant resistant tomato varieties such as Cherry Bomb, Defiant, Mountain Merit, Iron Lady, Mountain Magic, Mr. Stripey, Stellar, Jasper, Lemon Drop and Matt’s Wild Cherry. Potato varieties with tolerance to late blight include Allegany, Jacqueline Lee, Kennebec, and Sebago.

Keep in mind that resistant does not mean immunity.

Transplants should be carefully inspected before buying. Use certified seed potatoes.

While no seed is guaranteed to be pathogen-free, ‘certified’ means less chance of infection. Examine your seed carefully and only use blemish-free tubers.

Let’s hope that late blight does not make a late summer appearance.

Hours and programming

Have a gardening question? Our Helpline is open.

Master Gardener volunteers are in the office 10 a.m. to noon weekdays. You can stop in at our CCE office at 420 E. Main St. in Batavia, call (585) 343-3040, ext. 127, or e-mail it at: [email protected].

Visit our CCE website at

Our next Garden Talk will be at noon on Sept. 7. The talk will cover how to do a garden clean up in the fall while protecting pollinators, butterflies and other beneficial insects.

You can attend this free program in person at the CCE office or online via Zoom.

To register for a Zoom link, visit our events page To register for the in-person option, call the CCE office at (585) 343-3040 ext. 101.

The Genesee County Soil & Water District is hosting a pollinator workshop 9 to 11:30 a.m. on Aug. 26 at the Genesee County Park Interpretive Center. Learn who the pollinators are, what they do, and how you can save them.

Master Gardener Pam M. will be doing a program on monarch butterflies. Other topics for the event include local pollinators, native flower gardens — how and where to start, plus an interactive walk.

This event is those interested are asked to register at or call (585) 343-2362 ext. 5.

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