Lettuce

A white, powdery coating on leaves, shoots and buds. The leaves turn yellow, become deformed and drop prematurely. The coating develops on both sides of the leaf blade, is often clearly visible from above, and can easily be rubbed off with a finger — it is the superficial mycelium of an ectoparasite that draws nutrients from the epidermal cells by means of feeding structures (haustoria). At first single, round, white spots appear, which in time merge and cover the entire leaf surface and the young growth. Young leaves and the current season's shoots are the most susceptible; heavy infection causes them to pucker, stiffen and stop growing, and on buds it leads to deformation or death of the flowers. On the fruit of some species (gooseberry, apple, cucurbits) the coating leaves russeting or a corky skin. Towards the end of the season, small, dark fruiting bodies (chasmothecia, formerly called cleistothecia) — the pathogen's overwintering structures — appear within the coating. The disease affects many plants, including roses, oak, privet, lilac, cucurbits, cereals, grapevine, apple, gooseberry, phlox and coneflowers, with each of these hosts being infected by a specialised species of fungus.

On the stems, at the base of the plants and on parts in contact with the soil, a watery, soft rot appears, covered with a dense, white, cottony fungal mycelium. Within the tissue and on its surface, black, hard resting bodies (sclerotia) the size of a grain form, which are a diagnostic feature. Infected plants wilt, snap and die; the disease attacks many species of vegetables and ornamentals. The infection usually begins as a pale, watery, almost translucent spot that quickly enlarges into a soft rot; in humid air it becomes overgrown with the characteristic, cotton-white mycelium. As the disease progresses, the infected stem becomes girdled and constricted and loses its rigidity, so that the plant above the point of infection wilts and dies, while the tissue, once dried out, bleaches and turns dry, stringy and frayed. The inside of affected stems (the pith) is often hollow and filled with black sclerotia the size of cereal grains — this is the most reliable feature distinguishing this disease. Sclerotinia infects a very wide range of hosts (including lettuce, beans, cucumbers, carrot, brassicas, sunflower, oilseed rape and numerous perennials and ornamentals), and the rot also develops after harvest, in stored roots and heads. In spring, small, cream-brown, cup-shaped fruiting bodies (apothecia) grow from the sclerotia lying in the soil and release ascospores, which infect plants most often through spent petals and weakened tissues.

Yellow or angular (vein-bounded) spots on the upper leaf surface, matched underneath by a grey-violet or brown coating of spores. It differs from powdery mildew, whose white coating sits on the upper surface. It develops in humidity and cool nights and is dangerous to cucumbers, onions and grapevines. The spots are initially light green or yellowing, and their angular, mosaic-like shape arises because the pathogen's spread is halted by the larger leaf veins. The delicate, felt-like coating on the underside (hence the name 'mildew') is the sporulation of the oomycete — best seen in the morning or after a period of high air humidity. Over time the spots turn brown, dry out and merge, the leaf dies prematurely, which weakens the whole plant and lowers the yield. The disease is highly host-specialised: cucumbers and squashes are attacked by Pseudoperonospora cubensis, onion by Peronospora destructor, grapevine by Plasmopara viticola, and lettuce by Bremia lactucae — so infection of one species usually does not spread to unrelated crops.

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.

Slugs and shelled snails that feed at night and after rain. Symptoms: irregular holes in the leaves and young seedlings eaten off completely, silvery slime trails on leaves and soil. The greatest damage occurs in damp, shaded spots — particularly on lettuce, strawberries and hostas. The most damaging in gardens are the slugs (which lack an external shell): the Spanish slug (Arion vulgaris, formerly described as A. lusitanicus) and the grey field slug (Deroceras reticulatum). The leopard slug (Limax maximus) can be morphologically similar to them, but it feeds mainly on decaying organic matter and fungi and may even prey on other slugs, so it is of little importance as a pest of living plants. Shelled forms — grove snails (Cepaea) or the protected Roman snail (Helix pomatia) — usually cause less damage. The body is soft, elongated and permanently covered in slime; on the head there are two pairs of retractable tentacles — the longer upper pair bears the eyes, the shorter lower pair serves a tactile and olfactory function. Slugs and snails are hermaphrodites: they lay spherical, transparent or pearly eggs in clusters, hidden in damp soil, under stones and in leaf litter. A single Spanish slug can lay several hundred eggs; both eggs and young slugs overwinter, and feeding lasts from spring to late autumn, intensifying on warm, damp nights. The damage has smooth edges and often appears in the middle of the leaf blade, not only from the margin; seedlings and transplants are sometimes eaten down to bare soil, and on strawberries and ripening vegetables the slugs gnaw hollows into the fruit. The decisive diagnostic sign is the dried, silvery glistening slime trail — no insect leaves one.

Larvae of click beetles: narrow, hard and shiny, yellow-brown (hence the name 'wireworms'), up to 2-3 cm long, with three pairs of short legs just behind the head. They develop in the soil for 3-5 years. Symptoms: narrow tunnels bored into potato tubers, carrot roots, bulbs and the base of stems, together with gnawed roots of seedlings that wilt and fail in patches - especially on ground freshly broken from turf. The adult beetles (click beetles) are elongated, dark, 7-12 mm long, known for their characteristic ability to spring up with a click after being flipped onto their backs - they themselves cause no significant damage to crop plants; only the larvae are the pest. Females lay their eggs shallowly in the soil, preferably under grassed, moist turf, which is why the threat is greatest in the first 2-3 years after ploughing up a long-established lawn, fallow or field infested with couch grass. The larvae move through the soil profile depending on temperature and moisture: in spring and autumn they feed shallowly, in the root zone, while in heat and drought they descend deeper. The damage takes the form of characteristic narrow, smooth channels and tunnels bored into tubers, storage roots and bulbs; it lowers the market value of the crop and opens the way to secondary bacterial and fungal rots. In seedbeds and young seedlings (maize, vegetables, bedding plants) the larvae chew through the roots and the base of the stem at the root collar, causing the plants to wilt and die off in patches, leaving empty gaps in the rows (breaks in the stand). The damage intensifies on heavy, moist, humus-rich and heavily weed-infested soils.

Large, fleshy larvae of cockchafers (common and forest cockchafer), growing to 4-6 cm, cream-white and C-shaped, with a brown head, three pairs of legs and a darker, translucent tip to the abdomen. They develop in the soil for 3-4 years. Symptoms: gnawed roots, tubers and bulbs, wilting and yellowing plants that lift easily from the soil (roots chewed off), and patchy dieback of lawns and seedlings. The full cycle lasts 3-4 years: the beetles emerge during the May-June flight, the females lay their eggs in the soil, readily beneath dense turf and along field margins, and the hatched larvae pass through three developmental stages. The youngest grubs feed on fine rootlets and cause little damage, whereas larvae in their second and third year, when they are largest, gnaw thick roots, tubers and underground plant parts, causing the characteristic patchy loss of lawn and seedlings; the damaged sward can then be rolled up like a carpet because it has been stripped of roots. White grubs gather in nests, so damage appears in focal patches rather than evenly across the whole area. Cockchafer larvae are sometimes confused with the grubs of the rose chafer (Cetonia aurata) - these, however, are beneficial, breaking down dead organic matter in compost and humus, have a distinctly smaller head and fine legs and, when placed on a flat surface, move on their backs; they should not be destroyed.

Thick, fat caterpillars of noctuid moths, grey-brown or earth-coloured, with smooth skin, feeding at night. During the day they hide shallowly in the soil, curled into the characteristic shape of the letter C. The host plants are vegetables, transplants, perennials and strawberries. Symptoms: cutworms gnaw young plants right at the soil surface, severing seedlings as if with a knife, and in addition eat roots, tubers and the lower leaves. The name 'cutworms' covers several noctuid species — most often the turnip moth (Agrotis segetum), the heart and dart moth (A. exclamationis) and the black cutworm (A. ipsilon). The adults are inconspicuous nocturnal moths with grey-brown forewings and a wingspan of about 3.5–4.5 cm, active from May; the females lay eggs singly or in groups on the soil, plant debris and weeds. Young caterpillars feed at first on leaves low to the ground, and only the older ones (growing to 4–5 cm) move down into the soil and switch to gnawing plants at the base. Usually one to two generations develop per year, and the caterpillars overwinter in the soil. The greatest damage occurs on light, dried-out soils and in dry, warm summers, especially on freshly planted beds and on fields established just after a ploughed-in fallow or lawn. A characteristic sign is a young plant cut through and toppled just above the ground, and after scraping away the soil at its base — a curled, fat caterpillar.

Microscopic roundworms (0.3–2 mm), invisible to the naked eye, feeding inside the roots. The diagnostic sign is the characteristic knotty swellings (galls) on the roots, formed in response to larval secretions. Affected plants grow weakly, wilt on hot days despite moist soil, turn yellow and yield poorly. Most damaging under cover (tomato, cucumber) and on carrot, lettuce, strawberry and houseplants. The mobile, infective stage is the invasive larvae (second-stage juveniles, J2) present in the soil: they penetrate the young root tips just behind the apex and induce the formation of specialised feeding cells (giant cells). The female feeding there swells into a pear shape, stays inside the root and lays hundreds of eggs in a gelatinous sac that often protrudes onto the surface of the gall. Warmth speeds up development — the optimum soil temperature is about 25–30°C, which is why in tunnels and greenhouses, and in light, quickly drying sandy soils, the nematode produces several generations per season and does the most damage. On tomato and cucumber the galls are spindle-shaped thickenings fused with the root, giving it a beaded, knotty form; they cannot be rubbed off with the fingers — this distinguishes them from the beneficial nodules of nitrogen-fixing bacteria on the roots of legumes. On carrot, parsley and parsnip the feeding causes forking, shortening and hairiness of the storage root, which lowers its market quality even without obvious galls. The above-ground symptoms — stunting, wilting during the hottest hours despite moist soil, chlorosis and nutrient-deficiency signs — result from damage to the root system, which no longer takes up water or mineral salts; in the field the weakened plants occur in patches (nests). The pest spreads with infested soil, run-off water, on tools and footwear, and above all with infected seedlings and cuttings; in the soil it survives as eggs for many months, even without a host.