More than 40 different diseases infect cucumbers, and they're susceptible to weeds and insect pests when growing conditions aren't right. Recognise some of the common diseases help you prevent loss of fruit. Your local county extension office has information about treating plant diseases in your area.
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Angular leaf spot is a bacterial infection that first appears as small, water-soaked lesions on the bottoms of leaves. The lesions, which only show up between the leaf veins, turn yellow and eventually decompose. Bacteria ooze from the lesions if they're wet, and the liquid dries into a white crust. Cucumber stems and fruits also develop lesions. Those on the fruit turn white and crack open, allowing other infections to develop. Leaf spot is more likely to break out when the temperature is between 23.8 and 26.6 degrees C and it's raining steadily.
Cucumbers are susceptible to downy and powdery mildew. Both fungal diseases are characterised by a whitish growth of mould on the leaves. However, downy mildew develops on the bottom sides of leaves while powdery mildew appears on the upper sides of leaves and on the stems. Downy mildew may turn grey or purple in wet weather. Downy mildew develops when the humidity is greater than 95 per cent, while powdery mildew develops when the humidity is less than 45 per cent. Powdery mildew also infects the fruit.
Striped or spotted cucumber beetles spread bacterial wilt. The bacteria overwinter in the beetle and spreads to the cucumber when the beetles feed on seedlings. The symptoms, which include wilting and collapsing vines, don't usually appear until the cucumbers are half grown. By then, it's too late to prevent the disease. Infected plants must be destroyed.
Scab is a fungus infecting all parts of the plant growing above the ground. A brownish mat of spores forms on the fruit, and it develops scablike lesions. The leaves develop light-green, water-soaked lesions that first turn brown and then become grey or white. Lesions also grow on the stems and petioles. This disease breaks out during cool, humid weather.
Anthracnose is a fungal infection that attacks the leaves, stems and fruits of cucumbers. Early symptoms include small, yellowish water-soaked areas on the leaves. As the disease progresses, these areas dry out and turn dark. Brown lesions also develop on the stems and petioles, and black cankers appear on the fruit. Black specks appear on damaged areas of the fruit, and these specks ooze salmon-coloured spores.
Cucumbers can be infected with papaya ringspot virus, zucchini yellow mosaic virus and cucumber mosaic virus. Viral infection symptoms include green and yellow uneven spotting of the leaves, wrinkled leaves, warty fruit and stunted growth.
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- University of Hawaii Extension; Field Cucumber Production Guidelines for Hawaii; Hector Valenzuela, et. al
- Cornell Cooperative Extension: Growing Cucumbers
- University of Illinois Extension: Cucumber
- University of Illinois Extension: Aphid
- University of Illinois Extension: Spotted and Striped Cucumber Beetle