THE INFECTION PROCESS AND SYMPTOM EXPRESSION OF VERTICILLIUM DISEASE OF AGARICUS-BISPORUS

Verticillium fungicola, the cause of verticillium disease of Agaricus bisporus, produces several symptoms in its host, including dry bubble, necrotic lesions, and stipe blowout. Interactions between the pathogen and its host leading to these symptoms were studied. Naturally infected mushrooms were observed by scanning electron microscopy. There was no evidence of specialized penetration structures or of direct penetration. However, V fungicola was closely associated with the surface of the sporophore and with the internal hyphae of its host. Primordia, developing sporophores, and maturing sporophores of four cultivars of A. bisporus were inoculated with 5-muL droplets of spore suspensions of V. fungicola containing 2 x 10(5), 2 x 10(6), and 10 x 10(6) spores mL-1. The kind of symptom produced depended on the age of the sporophore at the time of inoculation, inoculum density, and post-inoculation incubation time. Dry bubble developed after inoculation of primordia. Necrotic lesions occurred in all treatments. Stipe blowout developed only in sporophores inoculated with the highest dose of spores. The incidence of sporophores with symptoms before harvest increased with spore concentration. At the lowest spore concentration, 90% of the inoculated mushrooms developed symptoms after harvest. The occurrence of infected mushrooms without symptoms at harvest may affect the epidemiology and control of the disease and may lead to reduced eye-appeal of mushrooms at retail markets.