Leaf Mould of Tomato
Symptoms commonly occur on both sides of the foliage and sometimes on fruit. Older leaves are infected first and then the disease slowly moves up towards young leaves. On the upper leaf surface small, diffuse, pale green or yellowish spots with indefinite margins appear. On the underside, olive green to grayish purple and velvety patches develop below the leaf spots. These are composed of spore-producing structures and spore masses (conidia). Over time, as the spots enlarge, the color of the infected leaf changes from yellowish (chlorosis) to brown (necrosis) and the leaf begins to curl and dry. The leaves will drop prematurely, leading to defoliation in severe cases. Occasionally, this pathogen causes disease on the blossoms or fruits with various symptoms. The blossoms may turn black and will be killed before fruit set. Green and ripe fruits will develop smooth black irregular area on the stem end. As the disease progresses, the infected area becomes sunken, dry and leathery.
The symptoms are caused by the fungus Mycovellosiella fulva, whose spores can survive without a host for 6 months to a year at room temperature (non-obligate). Prolonged leaf moisture and humidities above 85% favor the germination of spores. The temperature must be between 4° to 34 °C for spores to germinate, with an optimum temperature at 24-26°C. Dry conditions and the absence of free water on leaves impair germination. The symptoms usually start to appear 10 days after inoculation with the development of spots on both sides of the leaf blade. On the underside, a large number of spore-producing structures are formed and these spores are easily spread from plant to plant by the wind and water splashing but also on tools, clothing of workers and insects. The pathogen usually infects the leaves by penetrating through stomata in a high humidity level.
Seed treatment with hot water (25 minutes at 122 °F or 50 °C) is recommended to avoid the pathogen on seeds. The fungi Acremonium strictum, Dicyma pulvinata, Trichoderma harzianum or T. viride and Trichothecium roseum are antagonistic to M. fulva and could be used to reduce its spread. In greenhouse trials the growth of M. fulva on tomatoes was inhibited by A. strictum, Trichoderma viride strain 3 and T. roseum by 53, 66 and 84% respectively. In small arms, apple-cider, garlic or milk sprays and vinegar mix can be used to treat the mold.
Always consider an integrated approach with preventive measures together with biological treatments if available. Applications should be made prior to infection when environmental conditions are optimal for the development of the disease. Recommended compounds in field use are chlorothalonil, maneb, mancozeb and copper formulations. For greenhouses, difenoconazole, mandipropamid, cymoxanil, famoxadone and cyprodinil are recommended.