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Gummy stem blight of cucurbits produces a variety of symptoms which are referred to as leaf spot, stem canker, vine wilt and black fruit rot. Lesions on leaves and fruit usually begin as spreading water-soaked areas; in the former these may have a chlorotic halo, become light brown and irregular in outline; leaves can be destroyed. On fruit, dark cracked sunken lesions form, beneath which an extensive rot is found. In the field the first symptoms may be plant collapse where sunken, girdling cankers lead to total loss. Infection also occurs on seedlings. Spots on stems often elongate into streaks (usually starting at the joints) and have an amber exudate of gummy material. The size and colouring of spots vary according to crop. The main characteristic features are the gummy exudate on stem and fruit lesions, and the abundant pycnidia followed by perithecia. The pycnidia, whether on fruit, stem, or leaf, are closely spaced groups of the dark brown to black fruiting bodies, just large enough to be seen without a hand lens. Sometimes they are arranged in rings on the fruit or leaf surface.
During the rainy season lesions can become water soaked and can spread and lead to severe defoliation. Further development can lead to bark scaling and cracking in cucurbit vines and the collar region of watermelon. Gummy exudates may occur from cracks, especially in watermelon and pumpkin. Severe infection often results in death of the plant (University of Hawaii, 2012).
D. bryoniae can survive on seeds, weeds and plant debris from previously infected cucurbit crops. Miller et al. (2001) recommend using only disease-free seed produced in arid locations. Seeds from healthy fruits are free of the disease but D. bryoniae-infected seeds may symptomless, and seed treatment may be necessary if not previously treated. Seedlings should be inspected regularly for signs of infection as the disease is common at this stage.
Weeds are a source of inoculum and should be eradicated from the field before planting cucurbit crops. Crop refuse should be ploughed deeply immediately after harvest to reduce fungus survival. Crop rotation with non-cucurbit crops, so that cucurbits are grown only every 3-4 years, is also recommended, as are routine applications of foliar-protectant fungicides (Miller et al., 2001). Fungicides should be used prevetatively, with applications starting at the early stages of plant growth if weather conditions are conducive to the development of the disease. Avoid wounding the fruits at harvest and store the harvested fruit at 7-10°C to prevent postharvest black rot. Resistant cultivars are currently not available.
Bacillus subtilis is available for use as a biological control agent against D. bryoniae, but its success is dependent on environmental conditions at the time of application.
Gummy stem blight, caused by Didymella bryoniae, is one of the most important diseases of cucurbits in many countries of the world (Lee et al., 1984).