Advice
RHS Help & Advice
Onion neck rot
Common name: Onion neck rot
Latin name: Botrytis allii
Plants affected:
Most onion cultivars, shallots and garlic. In general onion cultivars with white bulbs are more likely to be attacked than those which have red or yellow bulbs. Leeks and some lines of A. fistulosum (Japanese bunching onions) are moderately resistant at all stages of growth. Seedling resistance has been noted in A. schoenoprasum (chives) and in a number of wild species of Allium.
What is it
Onion neck rot is a fungal disease.
Symptoms
Scales of diseased onions first become soft and brown giving the bulbs a ‘cooked’ appearance. They soon become covered with a dense grey mould consisting of innumerable fungal spores which spread the disease to other bulbs. Affected tissues gradually dry up so that the bulb suffers from a dry rot and eventually becomes shrunken and mummified. Fairly large crust-like resting bodies (sclerotia) of the fungus develop amongst the mould, particularly at the necks of the bulbs.
Cause/biology
The fungus can be transmitted through infected seed or sets and can live within the bulbs producing symptoms until the crop is stored. It can be contracted from infected onion debris remaining in the soil after a previous crop and by airborne spores during the growing season, the plants being most susceptible when the leaves are dying down. The fungus forms sclerotia which survive in the soil. These may fall off into the soil and remain viable for a long time. When they do germinate, they produce spores which can start new infections in stored or growing bulbs.
Neck rot is mainly seen on stored onions. Bulbs with thick necks which only dry off with difficulty and those which are soft as a result of downy mildew disease on the foliage during the growing season are very susceptible to this rot. Poor storage conditions will encourage the spread of the disease once it has developed on one or two bulbs.
Control
A four year crop rotation is recommended. Aim to produce hard, well-ripened bulbs. Applications of fertilisers, which have a stimulating effect on growth, should not be applied after July. In addition, manuring should be balanced to avoid excessive nitrogen. Damaged or thick-neck bulbs should be immediately used as they are more susceptible. Ensure the plants are dry before storing in a cool dry place. The disease is seed-borne so buy plants from reputable firms.
There are no chemicals available to the amateur gardeners for the control of this disease.