Internal stipe necrosis of Agaricus bisporus is recognized as an emerging and potentially serious disease in the UK mushroom industry. Symptoms are visible only on harvest and appear as a variable browning reaction in the centre of stipes, which may be accompanied by limited collapse of the internal tissues.The hypothesis that this problem is of bacterial origin was investigated, initially by an extensive bacteriological examination of affected mushrooms.The enteric bacterium €winge//a americana was isolated from a t least 93% of symptomatic mushrooms. Various strains of Pseudomonas fluorescens are also usually present in diseased mushrooms, but no single P. f/uorescens biovar was consistently associated with the disease. Typical symptoms were reproduced following infection trials with isolates of E. americans derived from diseased mushrooms. In addition, strains recovered from diseased mushrooms following such trials were shown by RFLP studies to be identical to those applied, thereby confirming Koch's postulates for these strains. The possible contribution of other bacterial species to symptom development is discussed.
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