Leaf mold occurs sporadically on Long Island, and elsewhere in the USA. This disease develops under humid conditions. Therefore it is more common in gardens and in protected environments (greenhouses and high tunnels) than in commercial production fields, where better air movement reduces humidity.
The fungal pathogen needs high humidity (at least 85% RH) in order to be able to infect. In can infect when the leaf surface is wet. Leaf mold can develop over a wide temperature range (39 – 90 F). Optimal temperature is 72 – 75 F.
Symptoms mostly develop on leaves, appearing first on older leaves. Color of leaf mold spots on upper leaf surfaces progresses from pale green to yellow. Spots are not distinct as they lack a clearly defined border, in contrast with most other foliar diseases. Thus affected leaves may look like they have a general yellowing due to a nutrient deficiency.
Spores form on the lower surface of leaves below the yellow spots. Initially they appear greenish brown, becoming darker brown with age and developing a velvety appearance. Severely affected leaves die prematurely.
Symptoms sometimes also occur on stems, petioles, blossoms and fruits. Infection can kill blossoms. A black, leathery rot can develop on the stem end of green and ripe fruit.
An important initial source of the fungus causing leaf mold is spores from other infected plants, including susceptible weeds in the nightshade family. The spores are easily moved by wind and splashing water. This pathogen can be seed-borne. Leaf mold can reoccur when tomatoes are planted again in the same location because the pathogen can survive in soil at least one year as spores or in infested plant tissue from the previous planting.
Management practices for leaf mold include managing humidity, changing the location where tomatoes are grown, selecting resistant or less susceptible varieties, applying fungicides, and removing tomato plant debris after last harvest or incorporating it deeply into the soil.
Managing humidity is an especially important practice for this disease. Improve air movement around plants by planting with good spacing between plants, locating rows parallel to the prevalent wind direction in an open area, staking or trellising plants, and pruning excess branches and dead leaves. Provide water to the base of plants rather than using sprinklers that wet leaves.
Growing tomatoes in full sun will facilitate drying of leaves after rain. Avoid excessive amounts of nitrogen fertilizer. Resistance has been bred into a few varieties adapted to the protected environment of greenhouses and high tunnels. Resistant varieties can vary in susceptibility to leaf mold because the resistance is not effective for all 12 known races of the pathogen.
Two images below: Symptoms of leaf mold on nightshade, a weed related to tomato.