Sclerotia from 6‐week‐old pure cultures ofSclerotium cepivorumgerminated immediately in soil only after abrasion of their rinds, but after burial in soil for a month or more, unabraded sclerotia became capable of germination.Marked stimulation of germination occurred in the presence of host plants (onion, leek and shallot). Little or no germination occurred in soil alone or in the presence of non‐host plants (barley, cabbage, carrot and white clover). Sclerotial germination was observed in a number of soils of widely differing pH and over a wide range of soil water content. Germination of sclerotia on uninjured onion roots was greatest at the tip region. On artificially injured roots sclerotial germination was enhanced but the effect of position was lost.Sclerotial germination was independent of contact between roots and sclerotia. It was induced by root extracts of allAlliumspp. tested, but of no other plants. Boiling or autoclaving root extracts did not destroy the active principle and it is concluded that under field conditions sclerotia are induced to germinate by a thermostable chemical substance fromAlliumroots.The process of germination of sclerotia is desc