Lecanicillium fungicola is a pathogen of the commercial white button mushroom (Agaricus bisporus) and is the causal agent of dry bubble disease, which can cause severe economic losses to mushroom growers. Volatile compounds were measured by GC/MS techniques over pure cultures of mycelia on agars, over microcosms of growing mushrooms, and over harvested mushrooms to identify compounds that might give an early warning of the disease. The mushroom strain tested was Agaricus bisporus, strain Sylvan A15; either deliberately infected with L. fungicola or water as a control. Over microcosms, the appearance of β-copaene, β-cubebene, and α-cedrene coincided with, but did not precede, the earliest visual signs of the disease. Mushrooms with dry bubble symptoms also had high levels of β-barbatene and an unknown diterpene (UK 1821). Over some harvested mushroom sets, high levels of cis-α-bisabolene developed as a defence reaction to infection.