Variations in isolates of Mycogone perniciosa and in disease symptoms in Agaricus bisporus

Eight isolates of Mycogone perniciosa, five from Agaricus bisporus and three from Agaricus arvensis, were studied. One isolate of Mycogone rosae was also included. Aleuriospore and phialospore morphology varied among the isolates as did other characteristics, but M. rosae was the only isolate to produce a red colouration of the medium. Growth was also variable, with three isolates of M. pemiciosa growing at about half the rate of the fastest. The slow‐growing isolates contained virus‐like particles, 36 nm diameter, and produced sclerodermoid mushrooms. The fast‐growing isolates did not contain virus‐like particles and caused cap spotting, a symptom not previously described for M. perniciosa. M. rosae produced characteristic cap spots and no scierodermoid mushrooms. A comparison of two isolates of St. perniciosa. one from A. bisporus and one from A arvensis, showed a much greater yield reduction as a result of symptoms caused by the isolate from A. bisporus. The isolate of M. rosae had no significant effect on yield. Restriction fragment banding patterns of ribosomal DNA showed no differences among the seven isolates of M. perniciosa from England, but the isolate from China was slightly different. The single isolate of M. rosae was distinct from M. perniciosa.