Common Tomato Problems: Gray Leaf Spot

Few things in life are as pleasing to a home gardener as a juicy, red, homegrown tomato. Unfortunately, growing a problem free tomato is a very tricky prospect. The same hot and wet conditions that make a tomato plant thrive are often the cause of tomato problems such as gray leaf spot. Gray leaf spot is a common fungus prevalent in the southeast U.S. and found in all warm areas of the world.

Symptoms of gray leaf spot

Gray leaf spot symptoms are mostly limited to the foliage of the plant. Both mature plants and seedlings may be affected. The foliage of an affected plant first develops brown or black specks on both younger and older leaves. The lesions expand slowly with age into round spots that may develop a gray center and be surrounded by a yellow area. The center of the spot may fall out leaving a shotgun looking hole in the leaf. The spots cause the leaf to turn yellow and eventually die and drop from the plant. Defoliation of the plant may become severe. Although the fungus doesn't directly affect the fruit, severe defoliation can lead to sunscald on developing fruits and reduced production of new fruit.

Causes of gray leaf spot

The disease is caused by the fungi genus Stemphylium. Pathogens survive long periods of time on dead or dying plant debris. It can also be found on alternate hosts such as peppers, eggplants, gladiolas, and solanceous weeds, those belonging to the nightshade family of plants. Infection often begins on seedlings, in seed beds or green houses. The gray leaf spot fungi spores germinate quickly during warm, wet weather and penetrate susceptible plants. The fungus spores are spread by the wind or splashing water.

Control of gray leaf spot

Plant healthy plants free of the disease. Many resistant varieties are available such as Better Boy or Celebrity. Check foliage of seedlings carefully before setting in your garden and space plants far enough apart to allow for air circulation.

Rotate your tomatoes with non-solanceous crops on a three-year or longer schedule. Also, remove all plant debris, tomato volunteers, and weeds. Overhead watering should be avoided or planned to allow for leaf drying before evening dew to reduce periods of leaf wetness.

Fungicides are effective against gray leaf spot, especially as a proactive step, but often the disease can be managed with cultural control strategies alone.

Sources:

http://www.aces.edu/

http://www.avrdc.org/pdf/tomato/gray_leaf_spot.pdf

http://scarab.msu.montana.edu/