In vitro responses of different Armillaria taxa to gallic acid, tannic acid and ethanol

Twenty‐one isolates from four Northern Hemisphere species of Armillaria ( A. mellea, A. ostoyae, A. bulbosa. and the unnamed North American Group V) and two Southern Hemisphere species ( A. luteobubalina and A. novae‐zelandiae ) were grown on media with or without ethanol. gallic acid, or tannic acid. Ethanol increased dry weights for all isolates except one of A. bulbosa and colony diameters for all isolates of the Northern Hemisphere species except one of North American Group V. Ethanol decreased colony diameters for some isolates of the Southern Hemisphere species. Rhizomorph production increased for most isolates of Northern Hemisphere species on medium amended with tannic acid rather than gallic acid and on ethanol‐amended medium with or without gallic acid. Both Southern Hemisphere species formed rhizomorphs in abundance, regardless of the medium. For most isolates colony diameters and dry weights were reduced by gallic acid, with or without ethanol. but were increased by tannic acid, with or without ethanol. Acid and ethanol effects on colony weights and diameters were not consistent, however, and variation among isolates within each species was great enough for these media to be of little value in distinguishing among the species. Newly defined species of Armillaria have thus retained the attribute of extensive variation in cultural characteristics that is a feature of A. mellea, sensu lato , which necessitates evaluating the responses of several isolates before determining effects of any compound on growth of any species.