Abstract
Brown blotch disease of Agaricus bisporus is caused by Pseudomonas tolaasii and is responsible for losses of 8-10% worldwide during commercial production. Variability in virulence was observed among 23 P. tolaasii strains. A minimal concentration of 2 X 10(6) cells per cap provided as a 20 microL droplet was required to induce prominent symptoms on susceptible mushroom strains. The optimum screening method involved inoculation of first flush caps with a 20 microL droplet of a 10(8) cfu mL-1 suspension of a virulent strain of P. tolaasii and incubation for 48 h in a moist chamber at 16 degrees C. Cap resistance increased with flush numbers, regardless of the A. bisporus and P. tolaasii strain. The resistance test when applied in situ (on mushroom beds) or on harvested caps gave similar results. Development of lesions did not depend on the morphological stage of the mushroom at inoculation. Wild strains of A. bisporus from France exhibited a wide distribution in disease reaction, ranging from resistant to highly susceptible. A high percentage (65.6%) of wild strains tested were significantly more resistant than B62, a commercial strain. Therefore, natural resistance could be used in a mushroom breeding program.
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