Several pathogens attack the cucumber family.
Powdery mildew
A disease affecting all cucurbits, it is caused by several species of related fungi. The two most important have wide host ranges. They overwinter as resting structures on dead plants, releasing in spring airborne spores that infect leaves. These form a white dusty coating of a second spore type that spreads the disease throughout the season. Infections are worse when plants are drought-stressed, or when humidity is high. Water regularly and ensure good ventilation. Green or yellow sulphur (Vitax) may give some control. Destroy or compost old plant material.
Viruses
Cucumber mosaic virus (CMV) (with a wide host range) and zucchini yellow mosaic virus (cucurbits only) are commonest in gardens. Symptoms include yellow mosaic patterns on the leaves, distortion and stunting (below). Both are transmitted by aphids; CMV may also be seed transmitted. Destroy infected plants promptly.