Downy mildew on Squash and Pumpkin

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.

Factsheets

Diagnostic Resources

Images of Downy mildew on Pumpkin

disease symptoms on upper side of leaf

Initial symptoms of downy mildew on pumpkin (Cucurbita pepo) include small,faint, light yellow, angular spots. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

 

Sometimes dark sporulation is visible on the underside of the leaf. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

symptoms on leaf

Tiny white spots often with a yellow border and/or dark (necrotic, dead) center visible upon close examination. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

disease symptoms on leaf

Close up view of tiny white spots often with a yellow border and/or dark (necrotic, dead) center visible. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

disease symptoms on leaf

Small, orange-yellow, angular spots on upper leaf surface of pumpkin. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

disease symptoms on leaf

Small, orange-yellow, angular spots on upper leaf surface of pumpkin. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

disease symptoms on upper leaf surface

Small, orange-yellow, angular spots on upper leaf surface of pumpkin. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

disease symptoms on leaf

Small, orange-yellow, angular spots on lower leaf surface of pumpkin. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

disease symptoms on leaf

Small, orange-yellow, angular spots on lower leaf surface of pumpkin. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.

Images of Downy mildew on Squash

downy mildew symptoms on squash leaves

Symptoms of downy mildew on butternut squash begin as tiny spots with a yellow border. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University

downy mildew symptoms on leave

Spots can occur in clusters. Very early symptoms tend to lack the angular appearance common for older symptoms: look at the individual yellow spots on the left side of the image. The spots in the cluster on the right side of this image are exhibiting some angular edges where their expansion was stopped by major veins in the leaf. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University

Upper surface of a squash leaf with a cluster of spots where their expansion was stopped by major veins in the leaf. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University

Lower surface of a squash leaf with a cluster of spots where their expansion was stopped by major veins in the leaf. Image provided by Dr. Margaret McGrath, Vegetable Pathology, Cornell University.