Effect of water potential on radial colony growth of Armillaria mellea andA-gallica isolates in culture

Increased incidence of Armillaria root disease on orchard and landscape tre es in northern California may be related to changes in irrigation practices . Continuously moist soil from sprinkler irrigation may favor survival and growth of the fungus. The effects of water potential on radial colony growt h were tested for two species of Armillaria. Isolates were tested on malt e xtract agar amended with either KCl or sucrose to control osmotic potential and produce six different water potentials from -0.5 to -8.5 MPa. Eleven A rmillaria mellea isolates from orchard crops and hardwood forest trees and three isolates of A. gallica from hardwood forest trees in California grew well between -0.5 and -1.5 MPa water potential. Rates of mycelial growth we re 7-10 mm/wk. Below -1.5 MPa mycelial growth of all isolates declined with decreasing water potential. In this study, A. mellea, and to a lesser exte nt A. gallica, were sensitive to changes in water potential in culture.