Pest Of The Month: Leaf Mold

Leaf Mold

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Identification

Symptoms typically begin on older, lower leaves. Initially, pale green spots with diffuse margins appear on the upper leaf surface that later turn yellow. The spots are often so diffuse that the yellowing looks like nondescript mottling. The most distinctive symptom is on the underside of leaves, where patches of olive-green, fuzzy mold consisting of masses of conidia and conidiophores can be observed.

As lesions coalesce, infected leaves curl up, wither, and may eventually drop from the plant. Fruit infections result in black, leathery lesions primarily on the stem end.

Survival And Spread

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High relative humidity and warm to high temperatures favor disease development. To infect tomato plants and spread in the field, the fungus requires very high humidity — in excess of 85% — for prolonged periods. These conditions occur frequently in greenhouses with poor air circulation, especially at night, but less frequently in the field.

Management

Several precautions will help avoid problems from leaf mold. Sanitation is important. Clean up and destroy debris from infested fields and greenhouses to discourage overwintering of the pathogen. In the greenhouse, it is advisable to sterilize production areas once debris has been removed, either with steam or with an approved disinfectant.

Since the disease may be seedborne, it pays to purchase quality seeds and transplants that have been certified disease-free. Resistant cultivars are available, but the pathogen continues to develop new races. Humidity control is important in greenhouses. Leaf mold can be controlled effectively with fungicides applied on a preventive basis.

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