Effects of water availability on a forestry pathosystem: fungal strain-specific variation in disease severity.

Norway spruce is one of the most important commercial forestry species in Europe, and is commonly infected by the bark beetle-vectored necrotrophic fungus, Endoconidiophora polonica. Spruce trees display a restricted capacity to respond to environmental perturbations, and we hypothesized that water limitation will increase disease severity in this pathosystem. To test this prediction, 737 seedlings were randomized to high (W+) or low (W-) water availability treatment groups, and experimentally inoculated with one of three E. polonica strains or mock-inoculated. Seedling mortality was monitored throughout an annual growing season, and total seedling growth and lesion length indices were measured at the experiment conclusion. Seedling growth was greater in the W+ than W- treatment group, demonstrating limitation due to water availability. For seedlings infected with two of the fungal strains, no differences in disease severity occurred in response to water availability. For the third fungal strain, however, greater disease severity (mortality and lesion lengths) occurred in W- than W+ seedlings. While the co-circulation in nature of multiple E. polonica strains of varying virulence is known, this is the first experimental evidence that water availability can alter strain-specific disease severity.