Incidence of Perenniporia fraxinophila and its effects on green ash ( Fraxinus pennsylvanica) woodlands in eastern Montana, USA

Green ash ( Fraxinus pennsylvanica) woodlands are declining throughout the Northern Great Plains of North America. The heart rot disease of ash caused by Perenniporia fraxinophila is common in these woodlands, but its importance in woodland decline is not known. We sampled the visible incidence of P. fraxinophila stem decay on green ash in 17 native woodland stands in east-central Montana, USA. Randomly chosen trees with and without P. fraxinophila sporocarps were measured for age, height and girth. Incidence of sporocarps on green ash trees ranged from 0 to 67% within stands with a mean of 38%. Sporocarps of other wood-rotting basidiomycetes were not observed in sample stands. P. fraxinophila sporocarps were more frequent on larger, faster growing trees. Rot was detected in basal cores from 23% of the sample trees; 74% of these also had sporocarps. Trees originating from multiple-stump sprouts were not more likely to have sporocarps, suggesting that P. fraxinophila does not usually enter sprouting trees directly from infected stumps. Mean canopy dieback ranged from 22 to 50% and averaged 54% for trees with sporocarps compared with 37% for trees without sporocarps. However, 27% of ash trees lacking both sporocarps and rotten cores had mean canopy dieback ≥50%, indicating that P. fraxinophila is not the primary cause of canopy dieback. Growth rates of ash trees with P. fraxinophila sporocarps declined as the number of sporocarps increased. The incidence of P. fraxinophila is associated with a decline in mean annual precipitation across Nebraska and eastern Montana. These results suggest that this disease may contribute to the decline of ash woodlands, especially where drought stress is common.