Cucumber

The number-one cucumber pest because it vectors incurable bacterial wilt - protect seedlings hardest, since young plants are most vulnerable.

Ragged holes in leaves and flowers, scarred fruit rind, and the beetles themselves - striped or spotted, about a quarter inch - diving into blossoms as you approach.

Hand-pick in the cool early morning when beetles are sluggish; kaolin clay deters feeding; spot-treat heavy pressure at flowering with an evening insecticide application to spare bees. Use only a product labeled for the crop you are treating, follow the label exactly, and observe the pre-harvest interval before you pick. Controlling beetles is really about preventing the wilt they spread.

Row covers from planting until flowering are the single best defense (remove at bloom for pollination); yellow sticky traps; delay-sow past the first beetle surge; clean up crop debris in fall and choose wilt-tolerant varieties.

Small yellow beetles marked with black stripes or spots that chew leaves, flowers, and fruit rind. Their real danger is disease: they carry and spread the bacterium that causes cucurbit bacterial wilt and also transmit mosaic viruses, so even light feeding can be costly.

The classic sticky-thread test: cut a wilted stem, touch the ooze, and slowly pull away - a fine strand confirms bacterial wilt over borer or drought.

Individual runners wilting in midday sun and reviving at night, then whole-plant collapse; a cut wilted stem oozes sticky sap that strings into a fine thread when you touch it and pull away.

None - remove and destroy wilting plants before beetles carry the bacteria to healthy vines. Prevention through beetle management is the only real control.

There is no cure, so the whole strategy is beetle control: row covers until flowering, prompt beetle knockdown, and wilt-tolerant varieties. Cucumber beetles carry the bacterium over winter and infect through feeding wounds.

A bacterial disease (Erwinia tracheiphila) spread by cucumber beetles that plugs the plant's water-conducting vessels. Infected vines wilt in the heat, recover overnight at first, then collapse and die. Cucumbers and muskmelons are most susceptible; watermelon is essentially immune.

Cucumbers are the most susceptible cucurbit - downy mildew often ends the cucumber season in humid regions.

Angular yellow blotches on the upper leaf surface bounded by the veins (a blocky, mosaic look), with a gray-purple fuzzy growth on the underside in morning humidity; leaves brown and die from the oldest inward.

Remove affected leaves early and harvest ahead of a wet forecast; protectant copper or approved fungicides slow it if applied before infection takes hold. Once established in warm, humid weather it moves faster than sprays can keep up.

Grow resistant varieties; maximize airflow and sun; water at the base early so leaves dry fast; and watch the regional downy-mildew forecast maps that track the disease moving north each summer.

A fast-moving, wind-borne water mold (Pseudoperonospora cubensis) specific to the cucurbit family. Cool, wet, humid weather triggers it, and it can defoliate a planting within a week or two. Unlike powdery mildew it needs leaf moisture to spread.

Mildew-resistant cucumber varieties are widely available; trellised, well-spaced plants with resistant genetics often outrun it until frost.

Round white powdery patches on the upper (then both) leaf surfaces, starting on older shaded leaves and spreading until whole leaves yellow, brown, and go crisp.

At the first spots, spray weekly with potassium bicarbonate, horticultural oil, or a milk-and-water solution and remove the worst leaves. It rarely kills the plant but shortens the harvest, so start early - sprays protect new growth, they don't cure old.

Plant resistant varieties; space for airflow and full sun; avoid heavy nitrogen; water at the base in the morning and scout weekly from midsummer.

The defining late-season disease of cucumbers, squash, pumpkins, and melons: a fungus (Podosphaera xanthii) that coats leaves and stems in a white talcum-like powder. Unlike most fungi it thrives in warm, dry days with humid nights, and it spreads fast late in the season, shading out the leaves so fruit ripens poorly and sunscalds.