Bacterial soft rot

Symptoms

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.

Treatment

There are no effective chemical products and infected organs cannot be cured. Infected bulbs, tubers and plants should be removed and destroyed, avoiding spreading the foul-smelling slime, and the store and tools should be thoroughly disinfected. Because the destruction of the tissue results from enzymatic breakdown by the bacteria, this process cannot be reversed — the diseased organ will not 'regenerate', so what counts is quick action to limit losses. Rotting bulbs and tubers are picked out and removed away from the crop and the store (not onto a compost heap right next to the store), and healthy stock is separated from the infected. Spread within the store is stopped above all by lowering the temperature and humidity and improving ventilation to dry the surface of the organs; crates, shelves, tools and floors should be thoroughly washed and disinfected. Copper products have only a limited, mainly preventive effect against the bacteria and no longer cure tissue that is already infected. In field cultivation, diseased plants are removed, overhead irrigation is suspended, and one does not work among wet plants, so as not to spread the bacteria from plant to plant.

Prevention

The key is to avoid mechanical injuries during harvest and storage, because wounds are entry points for infection. It helps to harvest in dry weather, to dry the organs thoroughly before storage, to store at a low temperature with good ventilation, and to avoid overwatering and waterlogged sites. After harvest, onion and garlic are worth curing and drying well so that the neck and skin heal over, while damaged, partly rotten or immature organs should be discarded before they are put into store. Controlling pests that pierce and nibble the tissue matters greatly — insects and nematodes create wounds and also carry the bacteria themselves. In the field, a crop rotation with a break in growing susceptible plants, good drainage and an airy site, moderate nitrogen fertilisation (lush, watery tissue is more susceptible), and planting healthy, certified material all help. The store, crates and tools are disinfected before every season, and during storage a stable, low temperature without fluctuations is maintained to prevent water from condensing on the organs — a moist film on the surface is a precondition for the bacteria to multiply and penetrate.

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Sanitation — removing and destroying infected organs	Picking out rotting bulbs, tubers and plants and disinfecting tools eliminates the reservoir of bacteria and curbs the spread of the infectious slime.	Continuously during harvest and during inspections in the store
Careful harvest and drying of organs (curing)	Wounds are entry points for infection — gentle harvesting in dry weather and curing the skin closes the route by which the bacteria enter.	Harvest at full maturity in dry weather; drying before storage
Control of storage conditions	The bacteria need warmth and water — a low temperature, low humidity, good ventilation and no condensation curb tissue maceration.	Throughout the whole storage period
Agronomic prevention in the field	Crop rotation, good drainage, moderate nitrogen fertilisation and healthy planting material reduce the inoculum pressure and the susceptibility of the tissue.	—

Chemical methods

Method	How it works	When to use
Disinfection of the store, crates and tools	Disinfectants (e.g. oxidising or chlorine products) reduce the number of bacteria on surfaces before the season — a biocidal, not a curative, action.	Before every storage season
Copper products (contact)	Copper acts on the surface and bacteriostatically — its effectiveness against soft rot is limited and preventive; it does not cure infected organs.	—

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.