The effects of moisture and oxygen availability on rhizomorph generation by Armillaria tabescens in comparison A-gallica and A-mellea

Compared with other Armillaria species, natural melanized rhizomorphs of A. tabescens are rarely observed. Growing in autoclaved Vitis stem segments, A. tabescens isolates from the Ozark Mountains in the central USA formed fully melanized rhizomorphs, thinner and shorter than those observed for other Armillaria species under field conditions, and only under conditions of both high oxygen availability, greater than or equal to 2 mug cm(-2) min(-1) and moisture near saturation. Conducive conditions were used to compare the rhizomorph generation capacities of A. tabescens, A. gallica, and A. mellea, which have overlapping host and geographic ranges in central North America. While the rhizomorphs of A. tabescens were significantly shorter than those of the other two species, A. gallica and A. tabescens produced similar numbers of rhizomorph initials. Finally, we demonstrated the ability of melanized A. tabescens rhizomorphs to span woody food bases and thereby establish viable infections by penetration of intact bark of Vitis stem segments. We hypothesize that A. tabescens rhizomorphs form under conditions of periodic saturation which promote rapid water movement through naturally occurring lacunae in the soil. Thus, the role of A. tabescens rhizomorphs in the spread of the fungus should be re-evaluated.