Squash Downy Mildew

Pseudoperonospora cubensis, a fungus-like water mould

Downy mildew on muskmelon
Downy mildew on muskmelon [Credit: Jerzy Opioła]
Downy mildew on muskmelon [Credit: Jerzy Opioła]
Downy mildew on watermelon
Downy mildew on watermelon [Credit: David B. Langston, University of Georgia]
Downy mildew on watermelon [Credit: David B. Langston, University of Georgia]
Downy mildew on cucumber
Downy mildew on cucumber [Credit: Jerzy Opioła]
Downy mildew on cucumber [Credit: Jerzy Opioła]
Angular yellow markings are tell-tale signs of the start of downy mildew on cucumber family plants
Angular yellow markings are tell-tale signs of the start of downy mildew on cucumber family plants [Credit: Christian Hummert (Ixitixel)]
Angular yellow markings are tell-tale signs of the start of downy mildew on cucumber family plants [Credit: Christian Hummert (Ixitixel)]


Host Plants:

On Crops: Cucumbers, melons, pumpkins and squash

Where Found:

Worldwide, especially in cool climates

Description:

A disease that only occurs following periods of cool, wet weather, downy mildew begins with light yellow angular spots on leaves. Beneath the spots, a furry gray to purplish mold develops. The ideal temperature range for cucurbit downy mildew is 59-68F (15-20C).

Damage:

If plants are healthy when this disease hits, they will likely produce a modest crop. Plants that have lost more than one third of their leaves to downy mildew are not likely to produce well.

Preventing Problems:

Choose resistant varieties, which are widely available. Make sure plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying. Rotate cucumber family crops to a fresh site to prevent disease buildup, and use mulch to keep soil from splashing onto plants during heavy rains. Avoid using sprinklers or other overhead irrigation methods if conditions are cool enough for downy mildew.

Managing Outbreaks:

Clip off and compost affected leaves, and then spray plants with 1 teaspoon baking soda mixed with one quart of water. Cucurbit downy mildew usually disappears if the weather become warm and dry.

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