Summer Squash

Powdery mildew is a fungal disease that coats leaves - and often stems, buds, and fruit - in a white-to-gray powder that looks like spilled talcum. It is not one fungus but a large family of them, each fairly specialised to its own group of plants, which is why mildew on one crop rarely jumps to an unrelated one beside it. Unlike most leaf diseases it does not need wet leaves: it thrives on warm dry days followed by humid nights, and it takes hold fastest where plants stand crowded in still air or in shade. The coating blocks light, so heavily covered leaves yellow, brown, and drop early. On many plants that is mostly disfiguring and arrives late enough not to matter; where it strips away the leaves a crop still needed to finish, it costs you size, sweetness, and quality.

Watch for: Round white powdery spots on the upper leaf surfaces that spread and merge until whole leaves look dusted. It usually starts on the oldest, lowest, most shaded leaves and works upward, so check there first. Look at the undersides too, and at stems, buds, and flower stalks, which get coated as it advances. Leaves that yellow, brown, curl, or go crisp and drop early under the coating. New growth that opens small, crumpled, or twisted where the fungus has reached the shoot tips. On plants where it gets onto the fruit, a dull gray film, russeted or scarred skin, and cracking. Late in the season, tiny dark specks scattered through the white felt - that is the fungus setting up to overwinter on the debris.

Prevent: Give plants room. Crowding and still air are the biggest single driver, so space at the full recommended distance, thin seedlings properly, and divide or prune out congested growth until air moves through the plant. Full sun and an open position help; deep shade and a still corner against a wall are where it always starts. Water at the base rather than overhead, and water early in the day so nothing stays damp overnight. Keep plants evenly watered - drought stress at the roots leaves them far more susceptible, even though the fungus itself likes dry air. Go easy on nitrogen, since soft lush growth is the most vulnerable. Where resistant varieties exist for what you are growing, choosing one is the single most effective thing you can do on a plant that mildews every year. Clear infected leaves and stems away at the end of the season rather than composting them.

Treat: Start early: sprays protect healthy tissue, they do not clean up leaves that are already coated. Pick off and bin the worst leaves as soon as you see them, and open the plant up by thinning or pruning so air moves through. On anything that regrows quickly, shearing the whole plant back forces a clean flush and beats spraying. If it keeps spreading, potassium bicarbonate, horticultural oil, or neem applied weekly will hold it - cover both leaf surfaces. Sulfur also works, but it burns the foliage of some plants, should never go on in heat, and must not be mixed with or applied close behind an oil spray. Whatever you use must be labeled for the plant you are treating: follow that label and observe its pre-harvest interval on anything you eat. Do not compost badly infected material. Late-season mildew on a plant that is finishing anyway is usually cosmetic; on a crop still filling out it is worth acting on.

Nearly universal on summer squash by late season. A July succession sowing gives you clean young plants just as the first ones mildew out, which is far more reliable than trying to hold an infected planting together.

Flat, gray-brown shield-shaped bugs that pierce leaves and stems to suck sap, injecting a toxin that wilts and browns foliage. Adults and gray nymphs cluster at the plant base and can overwhelm young plants; they are hardest to kill once mature.

Watch for: Neat bronze egg clusters in the V of leaf veins on the undersides, fast-moving gray nymphs, and adults that scatter when you lift a leaf or a board laid nearby.

Prevent: Scrape egg masses off leaves into soapy water; clear old vines and mulch that shelter overwintering adults; lay a board at dusk and destroy the bugs that gather beneath it by morning; row covers until bloom.

Treat: Hand-pick adults and crush egg clusters daily - this alone controls small plantings. Target young nymphs with insecticidal soap or neem; mature adults shrug off most sprays, so timing on the nymph stage is everything. Use only a product labeled for the crop you are treating, follow the label exactly, and observe the pre-harvest interval before you pick.

Squash bugs love summer squash; scout leaf undersides for egg clusters twice a week from the seedling stage.

The larva of a clearwing moth that looks like a wasp. The moth lays eggs at the base of stems in early summer; the grub tunnels inside and hollows the vine, cutting off water so the plant wilts and collapses seemingly overnight. Squash and pumpkins are the favored hosts.

Watch for: Sudden wilting of a whole vine in midsummer, a small hole near the stem base oozing wet, sawdust-like frass, and orange-and-black moths flying by day.

Prevent: Cover young plants with row fabric until flowering (remove for pollination); wrap or mound soil over the lower stem to block egg-laying; plant a second sowing in early July after the moth's main flight; grow resistant Cucurbita moschata types where borers are severe.

Treat: At the first frass, slit the stem lengthwise, dig out and destroy the grub, then bury that stem section in moist soil to re-root. Bury vine nodes along the row so the plant survives even if the crown is lost. Destroy spent vines in fall.

One of summer squash's most destructive pests - a July succession sowing that matures after the moth flight is the surest home-garden defense.

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.

Watch for: 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.

Prevent: 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.

Treat: 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.

Damages seedlings and can spread bacterial wilt to summer squash, though the crop tolerates the wilt better than cucumbers and melons do.

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.

Watch for: 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.

Prevent: 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.

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

Summer squash is less susceptible than cucumber or melon; a wilting squash plant is more often the vine borer, so check the stem base for frass first.

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.

Watch for: 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.

Prevent: 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.

Treat: 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.

Less devastating on squash than on cucumber, but wet summers can still strip the foliage.

Aphids are small soft-bodied sap-suckers - green, black, gray, pink, or waxy white, and mostly wingless - that gather in dense colonies on the tenderest new growth: shoot tips, buds, and the undersides of young leaves. They breed extremely fast in mild weather, so a few can become hundreds inside a week. Their feeding curls and distorts soft growth, and the sticky honeydew they excrete coats everything below and turns black with sooty mold. Ants often farm and defend the colonies for that honeydew. A few aphids are normal on almost any plant and a healthy plant shrugs them off; only dense, persistent colonies do real damage, and some aphids also spread plant viruses as they feed.

Watch for: Dense clusters of tiny insects packed onto shoot tips, flower buds, and the undersides of young leaves. Curled, cupped, or distorted new growth that never opens properly. A sticky shine on the leaves - and on whatever sits under the plant - that later goes black with sooty mold. Tiny white cast skins left behind where a colony has been feeding. Ants running steadily up and down the stems, which is often the first thing you notice. Mottled, streaked, or misshapen leaves that do not grow out can mean a virus the aphids brought with them rather than the feeding itself.

Prevent: Go easy on nitrogen: the soft, lush growth that heavy feeding produces is exactly what aphids breed on. Check new growth once a week through spring and early summer so you catch colonies while they are still small, and rub the first few out with your fingers. Leave flowering plants nearby and skip broad-spectrum sprays - ladybugs, lacewings, hoverflies, and parasitic wasps do most of this work for free, and they only stay if there is something for them to eat. Keep ants off the plant where you can, since they protect aphids from those predators. Water steadily so plants are not stressed, and check any new plant over before you bring it home or move it indoors, since aphids usually arrive on something else.

Treat: Knock the colonies off with a hard jet of water and repeat every few days - on most plants that alone ends it. Pinch out or prune off the worst-infested tips rather than treating the whole plant. If colonies keep rebuilding, spray insecticidal soap or horticultural oil directly onto the insects, covering leaf undersides, and repeat weekly until they are gone; treat early or late in the day, and never spray a wilted or heat-stressed plant. Avoid broad-spectrum insecticides - they kill the ladybugs, lacewings, and parasitic wasps that were about to solve the problem for you, and aphid numbers usually come back worse afterward. Anything you spray must be labeled for the plant you are treating; follow that label and observe its pre-harvest interval. On anything you eat, wash leaves, heads, and buds well before use, since aphids hide down in crowns, folds, and tight growth.

Rarely serious on vigorous summer squash, but watch new growth for the melon aphid and the mosaic viruses it spreads.