Giant onion

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.

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.