If you notice holes in leaves of squash, beans, cucumbers, and other vegetables, you may be seeing damage from the cucumber beetle.
Cucumber beetles are very common pests in vegetable gardens and may also attack ripening stone fruit. The most common in California is the western spotted cucumber beetle. Adult beetles are about 0.25 inches long, greenish yellow with twelve black spots on their back and long antennae.
Larvae feed exclusively on roots, but do not generally damage garden plants, although corn may occasionally be damaged.
Management of cucumber beetles is difficult. Most older plants can support substantial numbers without serious damage. The best strategy for most vegetable gardens may be to place protective cloth over emerging plants and remove it when plants are old enough to tolerate damage.
Various natural enemies and general predators are known to attack cucumber beetles.
Powdery mildew is caused by fungi. All members of the cucurbit family (squash, pumpkin, cucumber, melon) are susceptible to powdery mildew. Powdery mildew first appears as pale yellow spots on stems and leaves. These spots enlarge, white, fluffy growth appears on plant surfaces, and spores are produced. Affected leaves become dull, yellow, and may wilt in the afternoon heat; eventually they become brown and papery. Plants may die.
All powdery mildew fungi grow on living plant tissue; most grow on the surface of the affected plant part. Spores are the primary means of dispersal, and are carried by the wind to new hosts. All powdery mildew species can germinate and infect in the absence of water. Moderate temperatures and shady conditions are generally the most favorable for development of infection. Spores are sensitive to extreme heat and direct sunlight.
The disease is less severe in hot, dry weather. Control weeds and keep the garden clean; avoid overhead sprinklers. Powdery mildew can be eradicated by application of horticultural oil or neem oil. Be careful not to apply oils when the temperatures are high (above 80 degrees F). Another option, although less effective than oils, is to use a bacterial-based spray such as Serenade.
Read more about Powdery Mildew.
Read more about problems with cucumbers.