This is an extremely important soil-borne disease in onions. There are several Fusarium species. Fusarium culmorum primarily attacks leek and garlic.
Fusarium basal rot is a major problem in onions worldwide. Incidence has increased in the United Kingdom over recent years.
Symptoms and diagnosis
The first symptoms are usually yellowing and dieback of leaves and wilt of the plant. Infected roots will turn light brown or pink, after which rot will gradually develop at the basal plate. The basal plate will initially appear watery and pale brown, but remain firm. The rot will then spread further up the bulb, resulting in severe soft rotting of the fleshy scales. If the inside of the bulb is affected, white fluffy mycelium will usually appear on the outside. If the infection is mild, the incidence of Fusarium will sometimes not become visible until the bulbs have been in storage for a while.
Fusarium culmorum attacks garlic and leek, causes root rot and damages the basal plate. Symptoms in leek include pink lesions on the leaf sheaths of mature plants and dieback of seedlings.
Conditions for disease development
Infected soil is usually the most important source of Fusarium infection. Pathogens are also seed-borne and can spread via onion sets, shallots and garlic. They can survive for many years as chlamydospores or in other crops and weeds. The chlamydospores germinate in reaction to exudates of onion roots and penetrate the fine roots. They spread through the roots to the root plate and subsequently to the fleshy scales. Fusarium oxysporum causes little damage at temperatures below 15 °C and is encouraged at higher temperatures; the optimum temperature range is 25 to 28 °C. Crop damage will depend on the extent of basal infection, high temperatures and the sensitivity of the cultivars grown. Fusarium species can colonise wounds and damage caused by pest infestation, facilitating penetration of the affected plants by secondary rotting organisms. Seed transmission is possible, to a certain extent, and could be important if seedlings are grown in modular seed trays, although this is not often mentioned in literature. Seed inoculation could play an important part in the long term, when new production areas and new strains of the pathogen develop.
Impact and importance
Severely damaged plants are rendered unsaleable and other infected plants have a reduced yield. The control measures most commonly adopted are varietal selection (the use of resistant varieties) and crop rotation. Fusarium pathogens are difficult to combat and the effectiveness of crop rotation is limited, as they can survive in the soil for a long time. Hygiene measures are needed to limit the spread of infected soil to other fields and farms and to prevent soil-transmitted infection of greenhouses used for propagation. There is some interest in the use of fungicides and soil modifications as a control measure for infected fields. While seed treatments can help disinfect the seed, the advantages have not been quantified. It is unlikely that they will protect transplants against inoculation via the soil.