Zucchini

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.

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.

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).