Tomato

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.

Dark-brown, spreading spots on the leaves (often with a lighter margin and a whitish coating on the underside in humid conditions), brown streaks on the stems, and hard, brown rots on tomato fruit. In wet, warm summers it develops with lightning speed and can destroy a crop in a few days. The first symptoms are small, watery, grey-green spots that usually appear on the edges and tips of the leaves and within a few days enlarge into extensive, brown-black, dying areas - the spots do not stop at the leaf veins and cover the whole blade. The characteristic whitish, delicate coating on the underside of the leaf (this is the pathogen's sporulation - the sporangiophores) forms at the edge of the spot in high humidity, most often in the morning or after rain. The stems and leaf stalks become covered with brown, darkening streaks, at which the shoot turns brittle and easily broken. On potato tubers, bluish-rusty, sunken discolourations of the skin appear, beneath which the flesh turns into a rust-brown, granular, hard rot penetrating irregularly into the interior - often secondarily colonized by bacteria, which develops into a foul-smelling wet rot. On tomato fruit (including green, unripe fruit) hard, greasy, brown, irregular spots form. Under favourable conditions (cool nights, warm and humid days) an infected plantation can 'burn up' within a few days, giving off the characteristic smell of rotting haulm.

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.

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.

A characteristic mosaic appears on the leaves — irregular, interweaving light and dark green or yellow patches and streaks. The leaves are often deformed, wrinkled, curled and smaller, and whole plants become stunted and fruit poorly. The symptoms are most visible on young leaves, and the fruit of cucumber, courgette or tomato may be blotchy and misshapen. The disease affects many vegetables, perennials and ornamental plants. The disease picture depends on the specific virus, the host species and the temperature — the same pathogen produces different symptoms on different plants. A distinguishing feature is that the mosaic is systemic (it affects the whole plant and develops from the youngest leaves) rather than localised like fungal leaf spots — there are no clearly delimited spots with a margin, no mycelial coating and no fruiting bodies. Besides the mosaic, there is vein clearing (a lightening of the tissue along the veins), striping and blister-like bulges of the leaf blade. Severe narrowing and thread-like tomato leaves (the so-called 'shoestring' symptom) is associated mainly with CMV (cucumber mosaic virus), occasionally with tobamoviruses, whereas TMV/ToMV mostly produce mosaic and mottling of the leaves. On the fruit you can observe blotching, concentric rings, wart-like bulges (cucumber) and uneven ripening (tomato), and on the flowers of ornamental plants — colour breaking, i.e. streaks and flecks of discolouration on the petals. In high temperatures (above ~28–30°C) the symptoms may become masked and temporarily disappear, only to return once it cools down — this does not mean the plant has recovered.

Numerous small, round spots appear on the leaves with a lighter, greyish centre and a darker, brown margin. Within the spots tiny black dots are visible – pycnidia, the fruiting bodies of the fungus, which distinguishes this disease from other leaf spots. The infection progresses from the lower, oldest leaves upwards; with heavy infection the spots merge, the leaves yellow, dry out and drop prematurely, weakening the plant. Affected plants include tomato, currant, phlox and many perennials. The spots are usually 2–5 mm across; the decisive identifying sign is the dark pycnidia at their centre – best seen through a hand lens, and in high humidity exuding whitish 'tendrils' of spores. Leaves densely peppered with spots look as if dusted. The disease is favoured by warm (about 20–25°C), humid weather and water lingering on the leaves for a long time (rain, dew, overhead watering). On tomato the infection is confined almost exclusively to the leaves and leaf stalks – the fruit usually stays healthy – but the loss of the photosynthetic apparatus leads to smaller fruit and exposes it to sunscald.

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 (1-2 mm) white insects resembling minuscule moths that rise in a cloud when the plant is disturbed. They feed on the underside of leaves, sucking sap and excreting honeydew. This causes yellowing and weakening. Common on tomatoes, peppers, cucumbers and pelargoniums, especially in polytunnels and greenhouses. The wings of the adults are covered with a white, waxy bloom (hence the name) and at rest lie flat, roof-tile fashion over the body. Females lay their eggs on the underside of young leaves, often in characteristic arcs or circles, on short stalks. The eggs hatch into mobile first-instar larvae (so-called crawlers), which, once they find a feeding site, lose the ability to move and, as flat, oval, translucent greenish 'scales', attach themselves to the leaf until pupation. Feeding symptoms are pale spots, yellowing and wilting of the leaves and - most troublesome of all - copious honeydew, on which sooty mould (sooty mould fungi) develops, disfiguring leaves and fruit and limiting photosynthesis. In heated greenhouses the whitefly develops without interruption, producing many overlapping generations throughout the year; in the Polish climate it does not overwinter outdoors (it is killed by frost), so the source of infestation is protected cultivation and introduced seedlings, while in summer the insects may fly out into the open ground. The whitefly can also be a vector of plant viruses (criniviruses, e.g. tomato chlorosis virus).

A beetle 6–11 mm long with yellow, convex wing cases bearing ten black longitudinal stripes. The larvae are brick-red to orange, hump-backed, with two rows of black dots along the sides; the orange-yellow eggs are laid in clusters on the underside of leaves. Symptoms: larvae and beetles strip the leaves of potato, as well as aubergine, tomato and pepper, down to the veins — under heavy infestation the plant is almost completely eaten. Also characteristic is the orange, black-spotted pronotum (the shield behind the head). The beetles overwinter in the soil at a depth of 10–30 cm and emerge in spring once the surface layer warms to about 14–15°C, usually in May, straight onto the emerging potatoes. A female lays several hundred eggs in total, in clusters of 20–60; after several to a dozen or so days the larvae hatch and pass through four instars (L1–L4). The most voracious and most damaging to the yield are the larvae of the later instars (L3–L4), especially when they feed during tuber initiation and growth. Fully grown larvae move down into the soil and pupate there; under Polish conditions there are usually 1–2 generations per year (up to three in warmer regions). A heavily damaged crop can be seen from afar as patches of stripped plants with remnants of leaf veins and dark larval droppings.

Very small (1–2 mm), slender, elongated insects with fringed wings, coloured yellow, brown or black; the larvae are pale yellow and wingless. They suck sap from the cells of leaves, petals and buds. Symptoms: silvery to silvery-grey streaks and blotches on the leaves, tiny black specks of excrement, and distortion of young leaves, buds and flowers. They are especially damaging under glass and on houseplants, and some species transmit plant viruses. The silvery sheen appears once the pest has sucked out the contents of the epidermal cells — the empty, air-filled cells reflect light like metallic streaks; they are accompanied by glistening black specks of excrement, which distinguish thrips feeding from that of most other sap-sucking pests. The mouthparts are piercing-sucking and asymmetric (only a single mandible), so the insect first rasps the tissue and then sucks up the exuding sap. On flowers, feeding produces white or brownish discolouration and distortion of the petals — severe on roses, chrysanthemums, orchids and cyclamens. The life cycle is short and, in warm conditions, the generations overlap: the females lay eggs inserted into the plant tissue, the larvae (two instars) feed on the leaves and then drop to the soil or litter, where they pass through the immobile prepupal and pupal stages. Among the most damaging are the western flower thrips (Frankliniella occidentalis) — a key greenhouse pest and vector of tospoviruses (including tomato spotted wilt, TSWV) — and the onion thrips (Thrips tabaci). Only the larvae acquire the viruses, while the adults transmit them, so reducing the larvae also lowers the risk of viral infection.

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.