White rot is caused by the soil-borne fungus Sclerotium cepivorum. It can infect all plants in the
Allium family (including leeks and chives,) but garlic and onions are the most susceptible. Currently,
there are no available resistant varieties
Above ground symptoms include death of leaves, starting at the outer leaves and continuing inward.
It eventually leads to premature plant death. Diseased areas can be seen as patches of dead leaves
or plants, usually any time from mid-season to harvest. Below ground symptoms include a semiwatery
rot of bulbs and roots. There may be fuzzy white material present in the rotted area, which
is the fungal mycelium. You may also see sclerotia (the overwintering structure of the fungus), which
are tiny round structures that resemble poppy seeds, around the size of the head of a pin. Infected
bulbs are not marketable.
White rot spreads and overwinters as sclerotia, which are small round, black fruiting structures similar in appearance to poppy seeds. The sclerotia are highly resistant to adverse temperatures and conditions, and can remain alive in the soil for thirty or more years, even in the absence of a host. A very small number of sclerotia can cause significant disease, and it is very difficult to control. Multiple controls are needed to produce a sufficient yield in infected fields. Ideally, the disease needs to be controlled both by reducing the number of sclerotia in the soil, and slowing fungal growth (i.e. with both a sclerotia germination stimulant and fungicides.)