My Cucumber's Leaves Are Dying, but Fruit Are on the Vines

Powdery Mildew

• The powdery mildew fungus attacks cucumber leaves but leaves the vine's fruits intact. The disease begins mid-growing season as talc-like white spots on the tops of mature leaves. The fungus advances quickly in hot, dry weather, causing the plant's leaves to turn yellow and wither. Though powdery mildew doesn't directly infect cucumber fruits, cucumbers can suffer from sunburn as leaves around them die off, exposing them to too much sunlight. Fungicide sprays with benomyl and dinocap can control powdery mildew. Spray your cucumber plants as soon as you see the fungus.

Alternaria Blight

• Unlike powdery mildew, Alternaria blight thrives in cool, wet weather. The blight appears as circular, watery spots on mature leaves near the plant's center. The spots turn brown, grow up to 1/2 inch in diameter and develop target-like concentric rings. Vines can lose their leaves within weeks once infected with the blight. If your cucumber plants also suffer from poorly conditioned soil, a bad growing climate or other diseases, the blight can spread to the plants' fruits and cause sunken, moldy spots. To avoid Alternaria blight, treat cucumber seeds before planting with a fungicide containing thiram or captan. For infections in existing plants, apply a spray such as Dithane or Manzate or one containing anilazine.

Bacterial Wilt

• Cucumber beetles carry the microorganisms that cause bacterial wilt. The wilt is difficult to control, because cucumber plants are infected with the disease well before symptoms appear. Affected vines will begin to wilt when their fruits are about half-grown; the vines die shortly after, and the fruit will eventually follow. A cucumber patch likely has the disease when random plants suddenly wilt and die. It's impossible to stop bacterial wilt once it infects a plant. To control the disease, attack cucumber beetles -- which are black and yellow and 1/4 inch long -- as soon as they appear on seedlings. Pesticides that won't damage delicate cucumber plants include rotenone, methoxychlor and carbaryl.

Pest-Resistant Varieties

• Minimize leaf-related cucumber diseases by planting infection-resistant varieties. Plant scientists at the University of Illinois recommend the Marketmore 76, Fanfare, Salad Bush and Carolina varieties. The University of California-Davis recommends Liberty Hybrid, Saladin, Sweet Success and Sweet Slice, among others. Disease-resistant varieties are vigorous growers and bear fruit in 49 to 68 days.

Other Tips

• Growing conditions can also affect cucumber plants' leaves. Cucumbers require lots of water and moist soil. However, watering plants from overhead can leave foliage too wet and encourage moisture-loving diseases such as Alternaria blight, so water plants only at the soil. Cucumbers also grow best in soil amended with compost or manure and covered with plastic mulch early in the growing season. Cucumbers need a dose of nitrogen-heavy fertilizer when seedlings begin to vine. If you have persistent problems with dying cucumber leaves, consider rotating to an unrelated crop such as cabbage for a season or two. The break will rid the soil of cucumber-reliant fungi and bacteria.