Pacific Pests & Pathogens - Mini Fact Sheet Edition

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Cucumber downy mildew (143)

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Photo 1. Yellow spots of downy mildew, Pseudoperonospora cubensis, on the upper surface of a leaf; they are often angular and restricted by the leaf veins.

Photo 4. Yellow irregular patches caused by downy mildew, Pseudoperonospora cubensis, on the upper surface of a melon leaf.

Photo 5. Downy mildew, Pseudoperonospora cubensis, on the lower surface of a melon leaf showing irregular brown patches where spores are produced.

Photo 3. Underside of a zucchini leaf showing brown patches where downy mildew is forming spores. The white patches are powdery mildew (see Fact Sheet no. 63).

Photo 1. Drying and loss of leaf canopy in a crop of cucumber caused by downy mildew, Pseudoperonospora cubensis. Note the exposure of the fruit could cause sunscald.

Summary

• Worldwide distribution. On members of the cucumber family (cucurbits): important on cucumber, rockmelon and zucchini, but also on gourd, melon, pumpkin, squash, watermelon, zucchini.
• Not a fungus, a water mould, or oomycete. Spots yellow, then brown, merge; leaves dry and die.
• Spread is by wind and water splash, and long distance in air currents when humidity is high. Survival is on volunteer plants and weeds.
• Cultural control: avoid overhead irrigation; space plants to allow air movement; avoid planting in warm and wet times of the year; tolerant varieties available (melon, cucumber, watermelon); weed.
• Chemical control: copper, chlorothalonil, mancozeb (protectants); metalaxyl, phosphorus acid (systemics).

Common Name

Cucumber downy mildew

Scientific Name

Pseudoperonospora cubensis

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