Kohlrabi

On the roots of brassicas (cabbage, cauliflower, oilseed rape, radish), characteristic swellings and thickenings form, deforming the root system. The damaged roots no longer take up water and nutrients properly, so the plants wilt during the day, especially in heat, grow poorly, turn yellow and become stunted. Over time the galls rot, releasing huge quantities of resting spores into the soil. The galls take the form of spindle-shaped, club-shaped or knobbly thickenings that often merge into large, tuberous masses on the taproot and the lateral roots. The disease affects only brassicaceous plants (Brassicaceae), including weeds such as shepherd's purse and wild mustard, which also harbour and spread the pathogen. The above-ground symptoms are non-specific: wilting (at first recovering at night, later permanent), stunted growth and dwarfing, small, loose heads, and the leaves are sometimes discoloured blue-green or reddish, so a reliable diagnosis requires lifting the plant from the soil and examining the roots. Young seedlings may die off completely. The rotting galls give off an unpleasant smell and release billions of resting spores, which contaminate the soil for as long as a dozen or more, up to twenty years. The development of the disease is favoured by acidic, warm and moist soils.

Tiny (1–3 mm) beetles with black, or black and yellow longitudinally striped, wing cases and strongly thickened hind femora, with which they jump away like fleas when disturbed. They attack brassicas as well as radish, rocket, Chinese cabbage and kohlrabi. Symptoms: they chew numerous small, round holes in the leaf blades like a sieve, which is especially dangerous for seedlings and young plants, whose growth heavy feeding can completely halt. They cause the greatest damage in warm, dry weather. The name 'flea beetles' covers several species of the genus Phyllotreta — on brassicas in Poland most often Phyllotreta undulata, P. atra, P. nemorum and P. nigripes. Adult beetles overwinter in the upper layer of the soil, under plant debris and in leaf litter along the edges of fields, gardens and field margins. In spring, as it warms up (activity rises markedly above about 15°C), they emerge from their hiding places and fly onto the youngest brassica plants — which is why the first flights coincide with the emergence of the vegetables. Most species have one generation per year: the females lay eggs in the soil, and the larvae feed on the roots and fine rootlets (in most Phyllotreta doing little damage; the exception is P. nemorum, whose larvae mine the leaves). A new generation of beetles appears in summer, feeds up and moves into overwintering. The characteristic damage consists of shallow feeding pits and holes 1–2 mm in diameter clustered on the upper side of the leaf; under heavy pressure the blade looks as if riddled like a sieve, browns around the wounds and dries out. Most dangerous is feeding on the cotyledons and first true leaves and on the growing point — a damaged seedling stops developing, and the wounds may become entry points for secondary infections. The beetles are most active in the sunny, warm midday; cold, rain and high humidity clearly suppress their feeding.

A large day-flying butterfly with a wingspan of 50-65 mm; the wings are white, the forewings with black tips, and in the females additionally with two black spots. The yellow eggs are laid in groups on the underside of leaves. The caterpillars are yellow-green with black spots and yellow stripes, and feed gregariously. Symptoms: the caterpillars strip the leaves of brassicas (cabbage, broccoli, cauliflower, oilseed rape, nasturtium) down to the thick veins and foul the heads with droppings. Two, or in warm years three, generations usually develop per year; the insect overwinters as a pupa, pupating in sheltered places - on trunks, walls or fences. Butterflies of the first generation appear from May, but the summer generation, feeding from July to September, is the most numerous and the most damaging. The yellow, barrel-shaped eggs are laid in clusters of several dozen; young caterpillars at first skeletonize the leaf, leaving a network of veins, while older ones disperse and eat the whole blade down to the thick veins - a full-grown caterpillar reaches about 40 mm. Besides brassica vegetables (including Brussels sprouts, kale and kohlrabi), the hosts also include nasturtium, mignonette and wild cruciferous weeds, which form a reservoir for the pest. The dark green droppings that foul the heads lower the market value of the crop and encourage secondary rots; warning signs are white butterflies circling over the crop and yellow egg clusters on the underside of leaves.

Small (1–3 mm) soft-bodied insects, green, black or pink, feeding in clusters on young shoots and the underside of leaves. They excrete sticky honeydew. The diagnostic feature that distinguishes aphids from other small sucking insects is the pair of siphunculi (tubular projections) at the rear of the abdomen together with a tail-like cauda; a single colony usually holds wingless forms (parthenogenetic females) alongside winged migrants that fly onto new plants in summer. In summer aphids reproduce parthenogenetically and viviparously over many generations, so the population builds up explosively; in autumn many species produce a sexual generation that lays overwintering eggs on the host, and host-alternating species move between a winter host (often a tree or shrub) and a summer host (a herbaceous plant). Feeding symptoms include curling, crinkling and distortion of young leaves, stunted growth, yellowing, and the shedding of buds and fruitlets. Abundant honeydew coats the leaves, and the sooty mould fungus that develops on it forms a black film which limits photosynthesis; the honeydew is fed on by ants, which in return defend the colony against predators. Aphids are also important vectors of plant viruses (mosaics among them), transmitting them from plant to plant even during a brief probing puncture.

A greyish fly resembling a small housefly, which lays its eggs at the base of brassica plants and on the moist soil around them. The white, legless larvae that hatch feed in the roots and root collar of cabbage, cauliflower, broccoli, radish and turnip. Damaged plants wilt in the sun, grow poorly, take on a bluish tinge and topple over easily or die off completely, while in radish and turnip the tunnels bored by the larvae are visible. The greatest damage occurs on seedlings and young plants shortly after transplanting. The adult fly measures about 5-6 mm, is ashy grey, with dark bristles on its body and legs; males have a dark longitudinal stripe on the upper side of the abdomen. The eggs are white, elongated, about 1 mm long, laid singly or in small groups right by the root collar and in the crevices of moist soil; a single female usually lays over a hundred of them. The larvae are white, legless and headless maggots, tapering towards the front, growing to about 8 mm, which chew irregular tunnels in the roots and root collar, and in the fleshy roots of radish and turnip bore galleries that render them inedible. They pupate in the soil in brown, barrel-shaped puparia. The cabbage root fly overwinters as a puparium in the soil and under our conditions usually produces two to three generations per season; the first, most damaging flight of the flies occurs at the turn of April and May, as the soil warms up (it is sometimes linked phenologically to the flowering of dandelion and fruit trees). Damaged roots become an entry point for bacterial and fungal rots, so infested plants often additionally rot at the base and are easily pulled out of the ground without roots.

A small, grey-brown moth with a wingspan of about 12–15 mm; in the resting insect the folded wings form a pale, diamond-shaped (patterned) mark along the back, from which the English name 'diamondback moth' is derived. The damage is done by small, green, spindle-shaped caterpillars up to 10 mm long, feeding mainly on the underside of leaves: they scrape away the lower skin, leaving transparent 'windows', while older larvae chew irregular holes. It attacks brassicas — cabbage, broccoli, cauliflower and also radish; in warm, dry summers it produces several generations and can be exceptionally numerous. The moths are active at dusk and at night, resting hidden among the leaves during the day; when disturbed they make short, nervous flights. The female lays yellowish eggs singly or in small groups on the underside of leaves, along the veins. A very useful identifying sign is the behaviour of the caterpillars: when touched they wriggle violently and often drop on a thin thread of silk — a trait rare among other brassica caterpillars. Pupation takes place in a loose, net-like cocoon attached to the leaf. Under favourable conditions the full cycle takes only 2–3 weeks, so 3–5 overlapping generations develop over the season and the moth flight is spread out over time. The pupae overwinter on brassica debris; in mild winters the adults also survive, and in spring the populations are supplemented by an influx of moths migrating from southern Europe. The most serious damage is injury to the heart (growing point) of transplants and to the curds of broccoli and cauliflower; the characteristic 'windowing' is most easily seen by holding the leaf up to the light.

A nocturnal moth with a wingspan of about 35-45 mm, with brown-grey forewings bearing a characteristic pale kidney-shaped spot. The caterpillars are large (up to 40 mm), at first green, later grey-green to brown, very voracious and polyphagous: they chew irregular holes in the leaves and, in cabbage, bore into the interior of the head, contaminating it with frass and opening the way to rot. Besides brassicas it feeds on beet, lettuce, onion and many other vegetable and ornamental plants; usually two generations develop per season, of which the summer-autumn one is the most damaging. The females lay eggs in characteristic, closely packed clusters of several dozen on the underside of the leaves; the eggs are hemispherical, at first pale, yellowish-green. The youngest caterpillars feed gregariously on the underside of the leaf blade, scraping it away and leaving a thin, transparent skin (so-called windowing), and in time they disperse and move on to chewing holes and boring deep into the plants. Older caterpillars are active at night and by day hide at the base of the plant, among the leaves or shallowly in the soil, which makes them hard to detect and means the damage is often noticed only after it has been done. The key symptom distinguishing the cabbage moth from caterpillars that feed only on the surface is the contamination of the interior of the head with moist, dark frass - this, alongside the tunnels bored in the head, is what makes the crop unmarketable and opens the way to rot-causing bacteria. The species overwinters as a pupa in the soil; the first moths usually fly from May to June, and the second, more numerous and more damaging generation in July and August.