Leaf mold occurs occasionally in tunnel or greenhouse tomato production. According to a plant pest advisory from Rutgers University, the fungus will cause infection during prolonged periods of leaf wetness and when relative humidity remains above 85%. If the relative humidity is below 85%, disease will not occur. Therefore, it is important to regularly ventilate high tunnels and greenhouses well.
The pathogen can overwinter as a saprophyte on crop debris or as sclerotia in the soil. Conidia (spores) of the fungus can also survive for up to a year in soil.
The leaves of infected plants will develop distinct pale green or yellow spots. A dense, olive-green to brown mass of spores will develop on the underside of infected leaves.
Leaf mold management begins with recognizing early symptoms, applying preventative fungicides, and removing all infected plant material from the field, greenhouse, or tall tunnel, and crop rotation. Protective fungicides used in weekly spray programs help control leaf mold. For organic growers, regular copper applications can help suppress the disease.
For more information on leaf mold control options in polytunnels and high-rise greenhouses, see Table E-11 of the 2022/2023 Mid-Atlantic Commercial Vegetable Production Recommendations Guide.
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Watch for leaf mold in greenhouse tomato production