Radish

Flea beetles are tiny dark beetles - often shiny black or bronze, rarely more than a couple of millimeters long - with enlarged back legs that let them spring away like fleas the moment you lean in. The adults chew countless small round pits and shot-holes through leaves, giving the foliage a peppered, buckshot-riddled look. They overwinter as adults in leaf litter, hedgerows, and crop debris, emerge hungry in spring, and are worst in warm dry spells. The damage is mostly a seedling and transplant problem: a small plant with a few leaves can be checked, stunted, or killed outright, while the same plant shrugs them off once it is large and growing strongly. Different species specialize on different plant families, so the beetles hammering your brassicas are not the ones on your nightshades. Some species also have larvae that feed on roots and tubers underground.

Watch for: Dozens of pinhead-sized round holes peppered across the leaves, thickest on the youngest, most tender growth. Tiny beetles that spring away as you brush the plants or lean in for a closer look - that jump is the identification. Seedlings that go lacy and stop growing, and transplants that sit and sulk instead of taking off. Damage heaviest in warm, dry weather and along the edge of a bed nearest weedy ground or last year's crop debris. On root crops, shallow narrow grooves or channels scarring the skin at harvest, from larvae feeding underground.

Prevent: Floating row cover or fine insect mesh, laid over the bed from the day of sowing or transplanting and sealed at the edges so nothing can walk in underneath, is the single best defense - and on the crops they favor it is close to essential. Leave it on until plants are large and growing strongly, or until flowering on anything that needs pollinating. Push growth: a well-watered, unchecked plant outgrows the damage, so the whole game is getting seedlings through their vulnerable weeks fast. Keep beds weed-free, since related weeds host the beetles alongside your crop, and clear crop debris at the end of the season where the adults overwinter. Rotate away from a bed that grew a related crop last year. Where you can, shift sowing dates - cooler-season and fall plantings often miss the peak of adult feeding entirely.

Treat: Row cover and healthy fast growth carry most crops through, and on an established plant the damage is usually cosmetic and needs nothing at all. Where a young planting is genuinely being stripped, kaolin clay coats the leaves and deters feeding, and spinosad will knock the beetles back. Spinosad is acutely toxic to bees while the spray is still wet - apply it at dusk when bees are not flying, never onto plants in flower, and follow the product label. Anything you spray must be labeled for the plant you are treating, and you must observe its pre-harvest interval on a crop you are picking. Yellow sticky traps are useful for telling you when the beetles have arrived. Where the damage falls on the part you eat, harvesting young often beats them outright; where you are growing for a root or a fruit, leaf damage can simply be tolerated. Larvae feeding underground cannot be reached by a foliar spray, so rotation and clean beds are what work against those.

A top radish pest, because they hammer the brassica foliage. The fast radish crop usually outgrows light feeding under row cover; sow early or in cooler weather, and rotate away from brassica beds.

The larvae of a small gray fly that lays its eggs at the soil line against brassica stems. The white maggots hatch and tunnel down into the roots, scarring and rotting them, so plants wilt on warm days, stunt, take on a blue-green cast, and can die outright; root brassicas are tunneled through and spoiled. It is worst in cool, wet spring soil, on transplants and early sowings, and in ground that grew brassicas the year before.

Watch for: Sudden wilting on the first warm days even though the soil is moist, stunted plants with a bluish cast, and - on pulling a failing plant - brown tunnels and slimy scarring through the roots with small white maggots inside.

Prevent: Cover the crop with floating row fabric from the day you sow or transplant, sealing the edges down to the soil so the fly cannot reach the stems, or fit a cardboard or felt collar disc flat around each stem to block egg-laying. Rotate at least three years away from ground that grew brassicas, and do not plant into fresh manure.

Treat: There is no rescue once the maggots are inside the roots. Pull and destroy badly infested plants rather than composting them, and reset the bed with row cover and rotation. Beneficial nematodes drenched into the soil help suppress the larvae in a bed you have to keep using.

Radish roots are tunneled and scarred brown, which spoils them outright rather than merely checking growth. Early spring sowings into cold wet ground are the worst; row cover from the day you sow is effectively the whole answer.

A soil-borne pathogen (Plasmodiophora brassicae) that infects brassica roots and swells them into distorted, clubbed galls. Affected plants stunt and yellow, wilt on warm afternoons and recover overnight, and never size up properly. It is worst in acidic, poorly drained ground, and the resting spores survive in soil for up to 20 years, moving around on soil, tools, boots, water and infected transplants.

Watch for: Midday wilting that recovers overnight, yellowing and stunting with no obvious cause, and - the giveaway - swollen, distorted, clubbed roots instead of clean ones when a plant is pulled.

Prevent: Rotate at least 4 years out of every brassica, counting the fast salad ones and any brassica cover crop. Lime toward pH 7.0-7.2, since the pathogen dislikes alkaline soil, and improve drainage. Plant only clean seed and transplants, never move infested soil or plants around the garden, and grow resistant or tolerant varieties where the disease is known to be present.

Treat: There is no cure once the soil is infested. Pull and destroy affected plants - bag them, do not compost them - lime the bed, and keep brassicas off that ground for years. Rotation and pH are the whole game here.

Radishes are usually pulled long before clubbing costs you the crop, but they are brassicas. Running radish successions through the same bed all season is a quiet way to build spore load.

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.

Radish is usually out of the ground before aphids matter; check the leaf undersides on any crop you let stand.

The soil-dwelling larvae of night-flying moths. They spend the day curled just under the surface and come out after dark to chew through young stems at soil level, felling seedlings and transplants outright - a toppled plant lying beside its own stump is the signature. Some species also climb to gouge and notch emerging shoots and spears. They are worst in freshly worked ground, in beds that were weedy or grassy the year before, and on the first plantings of spring.

Watch for: Seedlings and transplants toppled at the soil line overnight, or shoots chewed and notched near the base, with the severed tops left lying beside the stumps. Dig an inch down within a few inches of the damage and you will usually find the culprit: a plump, dull gray-brown caterpillar that curls into a C when touched.

Prevent: Push a cardboard, foil or tin-can collar an inch into the soil around each seedling and transplant so the stem cannot be reached. Cultivate the bed a couple of weeks before planting to expose the larvae to birds and drying, and clear grassy weeds and debris, which is where the moths lay their eggs.

Treat: Dig around each fallen plant and destroy the caterpillar - left alone, one cutworm will fell another plant every night. Replant the gaps with collars fitted. Hand-picking after dark with a flashlight works well on a small bed, and a fast-growing crop soon outgrows the vulnerable stage.

Radishes grow fast enough to outpace them, and a few gaps in a thickly sown row hardly show.