Botrytis allii is a plant pathogen, a fungus that causes neck rot in stored onions (Allium cepa) and related crops.

[4] There are seven different species of Botrytis associated with onions in storage, but the rot induced by B. allii and B. aclada causes the greatest commercial loss.

The infection is present in the field but does not manifest itself until after harvest, however there may be a falling off of vigour while the crop is still growing, particularly in cool, moist weather.

A mass of grey conidiophores and conidia develop on the mycelium, and blackish sclerotia form at the site of the initial infection.

[3] In onion crops grown for the production of seed, Botrytis allii can cause spotting and girdling of the stipe (stem) and develop on the sheath that protects the inflorescence and on the flowers themselves.

In the seedling, infection with B. allii does not produce any symptoms, but the fungus spreads between plants as the conidiophores release spores into the air.

The leaf tips are invaded first, the infection spreading down the leaves and into the neck of the bulb, where its presence only becoming apparent when the foliage dies down at the end of the season.

It may also infect wild Allium species and can grow saprophytically on decaying crop residues such as cereal, pea and bean straw.

Detection of this disease is difficult due to the morphologically indistinguishable characteristics of Botrytis species cultured on agar.

[10] These enzymes are able to degrade onion cell wall components such as sodium polypectate, citrus pectin, lupin galactan, araban, xylan and carboxymethyl cellulose.