Scab of cucurbits

C. cucumerinum can attack any above-ground part of the plant. On leaves, slightly water-soaked or pale-green spots are the first symptoms. These spots are usually numerous and can appear on and between veins. Similar, elongate spots may develop on stems and petioles. Spots gradually turn grey or white and become roughly circular to angular, often with yellow margins. They are superficially similar to those caused by angular leaf spot (Pseudomonas syringae pv. lachrymans). Fine veinlets in the spots may remain brown or tan and are distinct against a white background.
In nature, conidiation on leaves tends to be sparse, a similar situation to that seen in some other pathogenic species of Cladosporium that cause leaf spots (for example, C. chlorocephalum). Dead tissue cracks, breaking away until the whole leaf has a ragged appearance. When young plants are infected the stem and leaves rapidly become rotten. In highly humid weather, an olive-coloured mycelial stroma develops over each lesion. Affected leaves close to the tip of the plant may appear stippled, spotted, stunted and crinkled, resembling those infected by mosaic virus.
Scab can cause the most severe damage on fruit. Lesions vary in different crops, depending on their susceptibility. Lesions first appear as small, water-soaked areas, about 3-4 mm diam., similar to insect stings. They become darker with age and may create a cavity in the fruit. A gummy brown substance is exuded in drops from the infected area, especially on fleshy fruit. Secondary soft-rotting bacteria may invade the cavities and lead to foul-smelling decay. In moist conditions, cavities are lined with a dark-green, velvety layer of fruiting fungus. Occasionally, adjoining spots unite, forming a lesion 1.5 cm or more diam. On pumpkin fruit, lesions may appear as sunken craters that could be confused with those caused by anthracnose. On resistant fruit, the fungus is blocked from deep invasion by the formation of successive layers of callus and lesions appear as raised blisters or superficial pimples.