Cucurbit Family Plant Problems - Powdery Mildew and Downy Mildew

Powdery mildew on cucumber.

Powdery mildew.

Powdery mildew - mold on leaf.

Downy mildew - top surface.

Downy mildew - bottom surface.


The topsides of leaves are covered with a white powdery growth. Areas of the leaves and stems turn brown, wither, and dry. Fruit may be covered with the growth.

ANALYSIS: Powdery mildew

This common plant disease is caused by either of two fungi (Erysiphe cichoracearum or Sphaerotheca fuliginea) that thrive in both humid and dry weather. The powdery patches consist of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungi sap the plant nutrients, causing yellowing and sometimes the death of leaves. A severe infection may kill the plant. Since powdery mildew attacks several different kinds of plants, the fungi from a diseased plant may infect other types of plants in the garden. Under conditions favorable to powdery mildew, infection can spread through a closely spaced planting in a matter of days or weeks.

ANALYSIS: Downy mildew

This common plant disease is caused by the fungus Pseudoperonospora cubensis. Irregular shaped yellowish to brown spots appear on the upper sides of leaves, usually at the center of plants. Under moist conditions, a purplish mildew develops on the underside of leaf spots. Leaves die as spots increase in size. Spread is rapid from crown toward new growth. The causal fungus overwinters in areas with relatively warm climate and can be introduced to other areas by wind. Moist conditions favor disease development.


At the first sign of the disease, treat the plants with a fungicide containing chlorothalonil. Continue treatment at intervals of 7 days for as long as the disease is a problem. Grow varieties resistant to powdery and downy mildew.

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Text and images adopted from theOrtho Problem Solver
Last Updated: Fri Sep 06 10:30:00PM CST 1996

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