Downy mildew on cucumber

disease symptoms on a leaf

Cucurbit Downy Mildew on a leaf. Image provided by Chris Smart, Cornell University.

Pseudoperonospora cubensis

Downy mildew causes angular, yellow to brown lesions on top of the leaves that are restricted by leaf veins. When conditions are favorable, for example early in the morning when moisture is higher, lesions observed on the underside of the leaf may be covered in dark “down”. This downy growth is masses of pathogen spores called sporangia, which can be seen in the field with a 20X hand lens. In the presence of water, sporangia can directly germinate or differentiate into swimming zoospores that infect leaves. Identification of downy mildew in cucumber can generally be done in a field setting due to the characteristic angular lesions and pathogen sporulation in the underside of the leaf. However, in crops such as watermelon and cantaloupe where lesions do not always have the characteristic angular shape, sporulation can be scarce and diagnostics can be more difficult. 

cucumber field with disease present

Cucurbit Downy Mildew in a cucumber field. Image provided by Chris Smart, Cornell University.

Factsheets

Diagnostic Resources

APS Diagnostic Guide

Disease Control Information

disease symptoms on cucumber leaf

Angular, yellow lesions on topside of the leaves that are bound by leaf veins. Dr. Lina Quesada, NC State Vegetable Pathology Lab

Dark spores on underside of the leaf. Dr. Lina Quesada, NC State Vegetable Pathology Lab