Common stock

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.

Seedlings and young transplants suddenly wilt, topple over and die, often in patches. At the base of the stem, just above the soil surface, the root collar turns black, softens and constricts, so the plant loses its support and falls over. Seeds may also rot before emergence, which shows as sparse, uneven emergence of vegetable transplants, annual flowers and herbs raised from seed. Two phases are distinguished: pre-emergence damping-off (rotting of the germinating seeds, absence or gaps in emergence) and post-emergence damping-off (toppling of seedlings that have already emerged). The exact picture depends on the pathogen. With infection by oomycetes (Pythium) the rot is watery, translucent and soft — the constricted stem looks as if 'cut with a knife', and the seedling falls flat, especially in cold and excess water. Rhizoctonia solani produces a brown to rusty-red constriction, the tissue is drier and more pinched than watery, the seedling may remain standing for some time, and a delicate, brown, cobweb-like mycelium is sometimes visible on the surface of the medium and on the collar. Fusarium causes wilting with yellowing and discoloration at the base and of the roots. The focus spreads in a circular, patchy way from the first infected seedling, because the pathogen travels through the moist medium and water; the roots of diseased seedlings are brown, poorly developed, with sloughing bark.

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 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.