Horticulture News

DATE: May 20, 2008

HARDIN COUNTY COOPERATIVE EXTENSION SERVICE
201 Peterson Drive
Elizabethtown, Kentucky 42701-9370

BY: Amy Aldenderfer
County Extension Agent for Horticulture

Tomato Early Blight a Problem, What to do about This Disease

Tomato is queen in many Kentucky vegetable gardens.  This vine-ripened fruit is a delight for any table, from a formal dinner to backyard barbeque.  Gardeners often have friendly competition for the "bragging rights" that go with the first ripe tomato of the season.  And commercial tomato production is a growing part of our agricultural industry.

So a tomato crop disease is regarded with alarm.   Early blight is perhaps the most common and damaging tomato disease in the home garden.  This fungal disease causes lower tomato vine leaves to dry up and die; the premature defoliation results in poor tomato fruit production and sun scald of existing fruit.

(While the tomato is sometimes considered a vegetable, botanically it's a fruit because the seeds are contained in a developed fleshy ovary.)

The warm, moist weather typical of a Kentucky summer promotes tomato early blight.  Moisture from dew or rain favor the disease, which most affects plants with poor vigor.

So what's a tomato grower to do?

Home gardeners and commercial growers should choose tomato varieties with more tolerance to early blight.  Because the fungus overwinters on diseased plant parts, sanitation is essential at season's end.  Remove and destroy all tomato plants and parts from the garden or field. Maintain proper fertility. A soil test can easily determine if you need to add fertilizer.  Spray foliage with fungicide at first sign of disease and as needed (weekly during hot, humid weather) thereafter; use chlorothalonil, maneb, mancozeb or fixed copper. (Good coverage is needed.) Make second planting in midsummer for fall crop.

Early blight is caused by a fungus Alternaria solanik that generally is first observed on older tomato plant leaves.  The fungal spores often are blamed as the cause of allergy episodes.  However many Alternaria fungi thrive in gardens and contribute to mold allergy problems that torment many Kentuckians.

There are a couple of more interesting things about tomato early blight.

Blight-infected tomato leaves develop circular brown spots and turn yellow before they shrivel up and die.  The dark brown spots have concentric rings characteristic of this disease.  If you closely look at the pattern, you'll notice each ring could represent a day's fungal growth on the  leaf, much as tree rings signify age in years.

It also appears that fungal growth in the spot is affected by alternating  light and darkness because concentric rings in the dead leaf spot appear to correspond to the daily rhythms of the light and darkness cycle.

For more information on tomato early blight or other garden or field crop diseases, contact your Hardin County Cooperative Extension Service at 765-4121.


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