Mycosphaerella cucumis (Didymella bryoniae) is a fungal plant pathogen that causes gummy stem blight.
Life cycle and appearance of Gummy stem blight
Mycosphaerella cucumis survives in crop debris and produces the sexual ascospores and asexual conida simultaneously. The spores germinate when free water is present on the plant, at high relative humidity (RH) and on wounds. Leaves can be infected directly, through wounds, hydathodes (the end of leaf veins) or the hair bases. Stems are infected through wounds and by the infected leaves through the petioles. Fruits are infected through the flowers. The newly produced ascospores are dispersed by air and the conidiospores by water drops, tools and hands of workers. The ascospores are the main source of dispersal of disease upwards in the crop and the conidia for spread of the disease downwards.
Mycosphaerella cucumis in cucumber causes brown lesions on the foot and higher up on the stem. On the lesions, golden-brown gummy drops occur, hence the name gummy stem blight. On the lesions, black specks, the new fruiting bodies with spores, become visible. On the leaves, round and extending lesions may occur. In cucumber, the most important symptom is internal fruit rot, which can only be recognized from the outside by the small narrowing of the tip of the mature fruits. The symptoms on stems, leaves and inside the fruits may occur together or individually.