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Brownish-green spots appear on the leaf margins and leaf tops. Later, large areas of the leaves turn brown completely. During wet weather, lesions on the lower side of the leaves may be covered with a gray to white moldy growth, making it easier to distinguish healthy from dead leaf tissue. As the disease progresses, the foliage runs brown, curls and dries. In some cases, the sharply delimited brown spots and the white covering also appear on the stems, branches, and petioles. Greyish-green to dirty-brown and wrinkled stains appear on the fruits. At these spots, the fruit flesh is hardened.
At this point, there is no biological control of known efficacy against late blight. To avoid spreading, remove and destroy plants around the infected spot immediately and do not compost infected plant material.
Always consider an integrated approach with preventive measures together with biological treatments if available. Use fungicide sprays based on mandipropamid, chlorothalonil, fluazinam, mancozeb to combat late blight. Fungicides are generally needed only if the disease appears during a time of year when rain is likely or overhead irrigation is practiced.
The risk of infection is highest in midsummer. The fungus enters the plant via wounds and rips in the skin. Temperature and moisture are the most important environmental factors affecting the development of the disease. Late blight fungi grow best in high relative humidities (around 90%) and in temperature ranges of 18 to 26°C. Warm and dry summer weather can bring the spread of the disease to a halt.