Napa Cabbage

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.

Worst on the midsummer seedling stage of the fall crop, exactly when flea beetle numbers peak - row cover sealed at the edges from the day of sowing is close to essential. They disfigure the outer wrapper leaves as well as the seedlings.

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.

A clubbed or checked napa never forms a usable head, so this is an expensive disease on this crop. Where clubroot is known to be present, grow a resistant variety such as Bilko.

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.

The gray waxy cabbage aphid burrows deep between the leaves of the dense head where sprays cannot reach, so deal with colonies while they are still accessible and soak or split-and-rinse before cooking.

The brassica caterpillar complex: velvety green imported cabbageworms (larvae of the white cabbage butterfly) and inch-worming cabbage loopers (a moth larva). Both chew large ragged holes through the leaves, then work into whatever folds up at the center - head, curd, sprout, or growing crown - fouling it with green pellet droppings. Fast leaf crops are usually cut before the damage matters; slow heading crops suffer most. Nasturtium carries the same mustard-oil chemistry that cues the butterflies to lay, so it hosts them too and is often planted nearby as a decoy.

Watch for: Ragged holes in the leaves, dark-green frass in the crown, and green caterpillars that match the leaf and hide along midribs and leaf undersides. White butterflies fluttering low over the bed mean eggs are going down now - look under the leaves for the small pale-yellow eggs, laid one at a time rather than in clusters.

Prevent: Floating row cover from sowing or transplant is the near-complete fix - it stops the adults ever laying. Seal it at the soil edges and lift it only to weed or harvest. Otherwise check leaf undersides twice a week, rub off the eggs, and hand-pick the caterpillars. Let dill, cilantro, or other umbels flower nearby to feed the parasitic wasps that do this work for you.

Treat: Bt (Bacillus thuringiensis kurstaki) is the one to reach for first: it affects only caterpillars and is safe around bees, parasitic wasps, and hoverflies. Spinosad also works, but it is not caterpillar-specific - while the spray is still wet it is toxic to bees and to the parasitic wasps that hunt cabbage worms for you. If you use it, spray at dusk so it dries before bees are out, and never wet open flowers, including flowering companions like dill or nasturtium. Follow the label for both. On a small planting, hand-picking and rubbing off eggs is enough on its own.

A napa head folds in tightly and seals worms inside where no spray reaches, so cover this crop from sowing rather than trying to fix it later. Split the head and rinse the inner leaves before use - especially if the outer leaves showed holes.

Fungal leaf-spotting diseases (Alternaria, Septoria and others) that make angular dark spots on the hairy leaves, especially in wet, humid weather - a very common cosmetic issue on black-eyed Susans.

Watch for: Brown-to-black circular spots with concentric rings and yellow halos, often on outer and older leaves; sunken dark lesions on the head in wet weather.

Prevent: Clean seed, 3+ year rotation, wide spacing for airflow, water at the base not overhead, and clear crop debris; avoid working plants when wet.

Treat: Remove badly affected foliage, keep leaves dry, and apply copper or a biological fungicide early in a wet season. Harvest on time - the roots stay fine even as tops decline.

The tight, water-heavy napa head is vulnerable to spotting and secondary rot in a wet fall - airflow and prompt harvest matter most.