Sources of Verticillium fungicola on a commercial mushroom farm in England

From 1979 to 1981 Verticillium fungicola, a pathogen to Agaricus bisporus, was detected in 47 out of 485 samples of baled peat and in one out of 126 samples of bagged limestone arriving at an isolated mushroom farm in the UK. The pathogen was not detected in 875 samples of water, soil, mushroom spawn or spawned compost. Casing (a mixture of peat and limestone) prepared and used on the farm contained detectable V. fungicola in 25 out of 108 samples. The primary sources of V fungicola were thus the two constituents of casing, especially peat. Secondary sources of V. fungicola were numerous and included symptomless and diseased mushrooms, pickers and other staff, floors and doors and the principal pests (sciarid flies and mites). Viable airborne spores of V. fungicola were detected with an Andersen sampler in farm buildings at all stages in crop production. The spore load increased with the age of the crop. Few airborne spores were detected immediately outside affected buildings, and very few at the farm entrance 30 m away. There was no evidence that V. fungicola reached the farm from the surrounding air. despite substantial soil disturbance nearby. Selective media for V. fungicola are described.