Potato

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.

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.

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.

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.