Horseradish

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.

Brown to dark-brown spots appear on the leaves with characteristic concentric, zoned rings resembling a "target board", often surrounded by a yellow halo. The spots enlarge and merge, and the leaves dry out and die, starting with the oldest, lower ones. On fruit, tubers and roots, dark, dry, sunken rot spots form, and the infected tissue becomes covered with a dark-olive, velvety coating of spores. The concentric rings within a spot are the most reliable diagnostic feature — they arise from the alternating, day-and-night growth of the mycelium and are best seen against the light. The picture depends on the host and the fungal species: on potato and tomato (Alternaria solani, "early blight") small dark specks on the lower leaves enlarge into oval, zoned necroses; on brassica vegetables (A. brassicae, A. brassicicola) the spots may be surrounded by a broad chlorotic halo and spread onto the seed pods; on carrot (A. dauci) infection begins at the margins of the older leaves, causing blackening and dieback of the foliage that hampers mechanical harvest. Older spots often die and crumble away in the centre, giving the blade a "shot-hole" appearance, and heavy infection leads to yellowing and premature defoliation, which exposes the fruit to sunscald. On stems and leaf stalks elongated, dark, sunken streaks form, and in seedlings the fungus can cause damping-off at the base of the shoot.

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.