Sweet corn

The soil-borne fungus infects the root system and enters the vascular bundles, blocking the flow of water within the plant. The plants wilt, first on hot days and often on one side or from the lower leaves, then turn yellow and die despite moist soil. A cross-section of the stem reveals a characteristic brown discolouration of the vascular bundles. The disease intensifies in warmth — the pathogen develops best at high soil temperatures (about 25–30°C), which is why the first symptoms usually appear in midsummer. In the early phase the plants wilt during the hot midday and temporarily recover at night or after watering; over time the wilting becomes permanent. Yellowing progresses from the oldest, lower leaves upwards and is often distinctly one-sided — sometimes only half a leaf or one side of a shoot is affected, on the side along which the blocked vascular bundles run. The plants are stunted (growth is inhibited), and in seedlings fusarium wilt appears as damping-off and toppling of the transplants. Brown discolouration of the vascular bundles is the most reliable diagnostic feature — in fusarium wilt it is usually intense and reaches deep into the stem and the roots. Under moist conditions, especially at the base of the stem or on the cut surface, a delicate pink-salmon or white bloom of fungal sporulation may appear. Fusarium oxysporum occurs in specialised forms (formae speciales), each of which infects only a particular species or genus of plant (e.g. tomato, cucumber, pea, carnation, banana), which helps to distinguish it from pathogens with a wide host range.

Rusty-orange to brown, powdery clusters of spores on the underside of leaves, matched on the upper side by yellow spots. Heavily infected leaves turn yellow and drop prematurely, and the plant weakens. It affects roses and mallows among others, and some rusts have an alternate host (for example pear rust and juniper). The pustules (clusters of summer spores, the so-called uredinia) break through the leaf skin and release an orange-rust powder that rubs off onto the finger - this is the most reliable identifying feature. In high summer they appear mainly on the underside of the blade, along the veins and on the leaf stalks, and in some species also on green shoots and sepals. In late autumn the clusters darken to brownish-black - these are the overwintering spores (telia) by which the fungus survives winter on plant debris. Rusts are highly host-specific: a given rust usually infects only one species or genus of host, which is why the rusts of rose, mallow and marsh mallow, bean, leek and garlic, mint, pelargonium or lawn are different, non-interchangeable fungi. The general symptom - yellowing and premature leaf drop - weakens the plant and limits flowering and yield, though it rarely kills it within a single season.

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.

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.

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.