The problem with the cucumbers appears to be powdery mildew. Powdery mildew presents as a white powdery growth on both the upper and lower surfaces of the leaves and on the plant stems. Powdery mildew on cucumbers is caused by two primary fungal pathogens, Podosphaera xanthii and Erysiphe cichoracearum. Fungal spores are spread by the wind. This fungal disease is common in warm weather and can be destructive in dry as well as hot seasons. It can grow rapidly with high humidity but, unlike many other plant fungi, is not encouraged by wet foliage.

Powdery mildew can be managed by planting varieties of cucumbers that are resistant to plant pathogens. Space the plants to provide adequate air circulation around plants or grow the cucumbers on a trellis. Cleaning up diseased leaves and disposing of the leaves, do not compost the infected leaves. At the end of the growing season clean up all plant debris and do not allow the diseased plant to over winter in the garden. Apply a fungicide labeled for use on powdery mildew at the first sign of an infestation. As with any fungicide make sure to read and follow the label directions.

More information on powdery mildew can be found in this article from the University of Minnesota Extension: Powdery mildew of cucurbits | UMN Extension

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