See:
Cucumber (Cucumis sativus) - Alternaria Leaf Spot
Cause The fungus Ulocladium cucurbitae has beeen reported in Washington. Research in the mid- to late 1970s in Washington showed this disease did not reduce yields under Washington conditions. Alternaria alternata f. sp. cucumerina causes a similar leaf spot in Oregon and Washington. Ulocladium survives for more than a year on infested plant debris. Conidia are formed on infested debris and can be moved by wind.
Symptoms Small dark brown spots and the centers turn lighter with a dark brown ring and brown halos, giving them a "bulls'-eye" appearance. Coalescing spots may involve large areas of the leaf but the light brown centers. Fruit is not normally attacked.
Cultural control
- Use resistant varieties.
- Rotate to non-host crops for 2 years.
- Plow under crop debris soon after harvest.
Chemical control Is not usually warranted in the Pacific Northwest.