Pest Of The Month: Gray Leaf Spot
February 1, 2007 
Identification
Leaf lesions first appear as small blackish-brown specks on lower leaves. These develop into grayish brown, glazed-looking lesions about 1â„10 of an inch or more in diameter and are often surrounded by a yellow halo. If numerous lesions develop, they may coalesce and kill large areas of the leaf blade. In severe cases, yellowing of the leaf occurs, followed by leaf drop and defoliation. Gray leaf spot is almost always restricted to the leaf blades and occasionally the petioles and tender portions of stems if conditions for development are especially favorable.
The disease is favored by extended periods of leaf wetness from rains or dew, and by moderate to warm temperatures. Spores germinate quickly in the range of 75ËšF to 86ËšF and develop rapidly on suitable hosts producing visible symptoms in as little as five days. The pathogen can survive in the soil or on plant debris from one season to the next. The fungus can also persist on volunteer tomatoes and alternate hosts, such as other solanaceous crops and weeds. The fungi are excellent saprophytes and grow readily on necrotic tissues. Gray leaf spot is commonly spread on diseased transplants. Fungal spores are spread from infected tissues by wind and splashing rain.
Management
Resistance is available in most commercial cultivars, and the widespread use of resistant tomato varieties has reduced the importance of this disease in commercial production. Fungicidal sprays may be used for susceptible cultivars. Fungicides containing chlorothalonil are recommended for control of gray leaf spot.
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