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The profuse growth of mold on leaves, shoots, pods or fruits is the most striking symptom. Initially, sparse, tan or brown and water-soaked lesions appear on plant parts that are in contact with the soil or that have been injured. Profuse, hairy and gray to brown patches of fungal growth develop on these tissues. Later on, the mold grows on younger fruits and pods, giving them a fuzzy appearance. Alternatively, the symptoms appear during storage. In both cases, severe losses can be expected. Basal leaf and stem rots cause a total crop failure in the case of susceptible varieties of horticultural crops (damping-off). Occasionally, dieback of branches and canker formation have been observed on woody crops.
The symptoms are caused by the soil-borne fungus Botrytis cinerea, which can grow and infect all plant parts. Moist weather, with frequent rainfall and cool temperatures favors the appearance of the symptoms. The reported optimal temperature range for the development of the fungus, the colonization of the plant and the progression of the disease is 15 to 20°C. Symptoms first appear on leaves or plant parts that have been injured mechanically during field work or through hail or frost. Lower leaves are most vulnerable. Excessive irrigation and dense canopy may increase the level of the disease by providing a humid, dense environment that is favorable for fungal growth.
Biofungicides containing the competitive fungus Trichoderma harzianum are effective against gray mold on a wide range of crops. Products based on Streptomyces griseovirides are also available for use on lettuce.
Always consider a integrated approach with preventive measures and biological treatments, if available. Control is difficult to achieve because the fungus may colonize the host plants close to harvest time, thus preventing the application of chemicals that leave toxic residues. In case of early infections, foliar sprays with benomyl or chlorothalonil can be applied to control its spread. Other fungicides based on dicloran, fluazinam, and thiophante-methyl can also work. The development of resistance is common when fungicides are used intensively.