Effect of Water Potential on Radial Colony Growth of Armillaria mellea and A. gallica Isolates in Culture

Abstract

Increased incidence of Armillaria root disease on orchard and landscape trees 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 growth were tested for two species of Armillaria. Isolates were tested on malt extract 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 Armillaria 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 were 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 extent A. gallica, were sensitive to changes in water potential in culture.