Onion

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.

Infected fleshy organs — bulbs, tubers, storage roots and succulent stems — rot wet, turning into a slimy, watery, soft mass with a characteristic, unpleasant, putrid smell. The tissue disintegrates, and the skin often remains as a thin bag filled with decomposed flesh. The disease progresses rapidly in warmth and high humidity, especially in storage; it affects onion, garlic, tulip, daffodil, hyacinth and root vegetables. The first symptom is small, translucent, seemingly water-soaked spots that quickly turn brown and soften; at the site of infection the flesh becomes macerated — the bacteria break down the substances that cement the cell walls, so the tissue loses its cohesion and dissolves into a creamy-brown mush. In cross-section the boundary between healthy and diseased tissue is watery and blurred, and a cloudy, slimy ooze often seeps from the injury. In bulbs and the neck of bulbous plants the infection usually starts from the top or from a wound and descends deeper, producing a soft, slippery 'neck'; in potato, soft rot of the tubers links up with blackleg — the blackening, rotting base of the shoot. The bacterium itself does not form mycelium or a visible bloom — this distinguishes it from fungal rot — and the intense stench comes from secondary decomposition by other microorganisms. Symptoms progress fastest at temperatures above about 20°C, with poor ventilation and a film of water present on the surface of the organs.

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.

An inconspicuous, ash-grey fly 6-7 mm long, resembling a small housefly; the damage is done by its larvae - legless, white, glossy maggots up to 8 mm long. The larvae bore into the bulbs and the base of the stem and tunnel through them, causing the plants to yellow and wilt, while infested bulbs rot and become secondarily infected with bacteria. It feeds on onion, garlic, leek and shallot; two to three generations usually develop over the season, and the first, in spring, attacks the youngest seedlings and is the most damaging. The fly overwinters in the soil as a puparium (a barrel-shaped, brown pupa) a few centimetres deep, most often where onions were grown the previous year. The flight of the first generation usually begins at the turn of April and May and is traditionally signalled by the flowering of dandelions and early fruit trees; the females then lay small, white, elongated eggs singly or in small groups at the base of the plants and in soil cracks right next to the bulb neck. The hatched larvae travel downwards and bore into the basal plate and the fleshy part of the bulb. In the bed, an infestation is recognised by the progressive yellowing and drying of the oldest onion leaves, the wilting of whole plants, and the fact that diseased plants can be pulled out of the ground very easily - tunnels and white larvae are visible in a cut-open bulb and at the base of the stem. The damaged tissue quickly turns into a wet, foul-smelling bacterial rot, so that a single larva can destroy an entire bulb, and the focus spreads to neighbouring plants in the row. The later generations (summer and late-summer) damage bulbs that have already formed, reducing their storage life.