Preventing blight in your garden tomatoes

July 07, 2009|By ROBERT KESSLER / Special to The Herald-Mail

This year's wet weather is causing many disease problems for gardeners. Since tomatoes are the most popular vegetable grown in gardens, we will mention what you should be doing to help prevent potential problems.

Tomatoes will get three major diseases in weather like this. Septoria leaf spot is a fungal disease that usually appears on the lower leaves of the plant and works its way up the plant. The affected leaves turn yellow and drop, reducing the plant's ability to produce food, which will reduce the production of tomatoes on the plant.

Early blight is another disease that can start at the base of the plant. It forms large, dark brown or black spots on the leaves. Rings will develop in the dark spots and form a "target spot" on the leaves. As the disease progresses, the leaves turn yellow and fall off. It can spread rapidly up the plant, causing significant defoliation.


The third disease is late blight. This is the same fungus that caused the famous Irish Famine when it destroyed the potato crop in the 1840s. Late blight appears as dark green to nearly black spots on the leaves. These spots have a greasy or wet appearance. These spots can also form on the fruit. Under the right conditions, it can spread rapidly.

Late blight is a problem when it occurs early in the year as it has this year. Late blight has been discovered in at least five Pennsylvania counties and this is the earliest it has ever been found.

Home gardeners who use chemical sprays need to start spraying their tomatoes to prevent the development of any of these diseases. Chlorothalonil (found in Daconil or Bravo) will prevent these diseases from developing. This is a good preventive measure and spray must be continued for the rest of the season.

Do a thorough job of spraying. Apply the material every seven to 10 days on your tomatoes, especially on the bottom section of your plants. Keep the pressure up on your sprayer so you get good penetration into the interior leaves. Spray top and bottom of the leaves.

Nonchemical practices that vegetable gardeners can follow to reduce diseases include:

· Using straw mulch around your tomatoes to prevent splashing up of spores on the soil.

· Do not water with any overhead irrigation that wets the leaves.

· Don't water in the evening.

· Keep weeds removed from the garden area.

· Remove any diseased leaf as soon as you see it and remove it from the garden area.

· Don't handle the plants when they are wet.

· And next year, to help break the disease cycle, don't plant tomatoes where any potatoes, tomatoes, eggplants or peppers were grown in the past year or two.

Bob Kessler specializes in consumer horticulture and energy for Penn State University.

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