![]() Leaves are penetrated either directly through the cuticle or through intercellular spaces around the bases of trichomes. Free moisture on leaves for at least 1 hr is necessary for infection, and further continuous leaf wetness is required for lesion expansion. Peak ascospore dispersal occurs after rain and during dew periods at night. Moisture is more important for disease development than temperature. The optimum temperature for infection is 20☌ in melon and 24–25☌ in watermelon and cucumber. The pathogen survives between seasons on diseased vines and crop debris and may be seedborne. The fungus grows in culture between 12 and 32☌, with an optimum temperature of 24☌. No physiological specialization has been reported. ![]() The upper cell is usually wider than the lower cell. The ascospores are 14–18 × 4–6 µm, hyaline, monoseptate with a constriction at the septum, and rounded on the ends. The numerous bitunicate asci usually produce eight ascospores per ascus. Dark pseudothecia, 125–213 µm in diameter, may also form, especially on stems. The conidia are hyaline, cylindrical with rounded ends, non- or monoseptate, and 6–13 µm long. Ascochyta cucumis Fautrey & Roum.), 120–180 µm in diameter, form on the surface of leaves, stems, and fruits. Dark pycnidia of the anamorph, Phoma cucurbitacearum (Fr.:Fr.) Sacc. Gummy stem blight is caused by Didymella bryoniae (Auersw.) Rehm (syn. In the tropics infection can occur through fresh blossom scars and is manifested as a rapid decay of inner fruit tissue, which proceeds from the blossom end to the stem end and then moves to the fruit surface. The spots contain conspicuous black fruiting bodies. Small, water-soaked spots develop on infected fruit, enlarge to indefinite size, and exude gummy material. Cankered vines usually wilt after mid-season. If infection occurs in older plants, lesions develop more slowly on stems near the center of a hill. 14.) Stems may be girdled and seedlings killed.įig 2. (See Compendium of Cucurbit Diseases, Table 7 and Part V, Fig. Small fruiting bodies (pycnidia or perithecia) may appear as black specks on the cankers (Fig. Stem cankers develop in the cortical tissue, and a brown, gummy exudate is commonly produced on the surface (Fig. Water-soaking may occur on hypocotyls and leaves. Circular, black or tan spots appear on the cotyledons and stems of young plants. SymptomsĬircular, tan to dark brown spots appear on the leaves, often first at the margins, and enlarge rapidly until the entire leaf is blighted (Fig. It is also found in temperate regions, especially on winter squash and pumpkin, and on greenhouse-grown cucumber. The pathogen is most common in the southern United States and in subtropical and tropical areas of the world. The disease has become increasingly severe as fungicide usage and genetic resistance have resulted in the effective control of other cucurbit diseases. On fruit the disease is known as black rot. It affects the leaves, stems, and fruits of all cucurbits. Gummy stem blight was first reported in 1891 in France, Italy, and the United States.
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