Plumed cockscomb

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.

Tiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.