Dry bubble disease of Agaricus bisporus and <i>A. bitorquis</i> , and its control by prochloraz — manganese complex

Abstract When soil, casing, and water were tested for the presence of Verticillium fungicola (Preuss) Hassebrauk vaLfungicola on mushroom farms, the most common source of this pathogen was the soil. The stage of cropping at which infection occurred appeared to be important. For example, when crops of Agaricus bisporus (Lange) Imback and A. bitorquis (Quel) Sacco were infected at casing with V. fungicola var. fungicola and V. fungicola var. aleophilum Gams &amp; van Zaayen respectively, a relatively high incidence of the disease was observed, but less disease was observed when crops were infected at spawning or after second flush. Environmental control also influenced disease severity. For example, when air temperature was reduced from 20 to 14°C and relative humidity from 90 to 80% for five days, there was a reduction in disease severity compared to the control crop grown under unaltered conditions. When temperature and relative humidity were returned to the original level, disease severity remained low for the rest of the cropping period. Laboratory trials demonstrated that several isolates of V. fungicola var. fungicola and the isolate of V. fungicola var. aleophilum were tolerant of Benlate and Tecto 90. However, all the pathogenic isolates, except one of another pathogen ( Aphanocladium album (Preuss) Gams) had low EDso values 2 ppm) when treated with prochloraz. Dry bubble diseases of A. bisporus and A. bitorquis were controlled by incorporating prochloraz ‐ manganese complex (50% active ingredient (a.i.) wettable powder) in the casing at a rate of 1.5 g a.i. m ‐2 . At this concentration the levels of prochloraz ‐ derived residues in the sporophores were well below the official maximum residue limit (MRL) established for mushrooms in Australia.