Impacts of thinning treatments on dynamics of needle disease caused by Elytroderma deformans (Weir) Darker and interactions with bark beetle-attacks in the northern Rocky Mountains

The fungus Elytroderma deformans causes a serious needle disease of pines in western North America and is considered the most important needle disease of ponderosa pine in Montana. While important, there has been limited evaluation of pre-commercial thinning or other treatments on disease incidence. Our study assessed the efficacy of various silvicultural treatments to reduce impacts from Elytroderma disease within monitoring plots that were established in 2004 and surveyed until 2014. Treatments were randomly assigned through incomplete block design to: (1) thin to 3.7 × 3.7 m spacing; (2) thin to 5.5 × 5.5 m spacing; (3) thin to 3.7 × 3.7 m spacing plus pruning; and (4) control with no thinning or pruning. For assessment, a statistic was developed to represent the average change in Elytroderma presence that occurred from 2006 to 2014 for each plot. This statistic was utilized as the response variable in a model that controlled for initial Elytroderma occurrence within plots to test for treatment effects. Overall, Elytroderma intensified across all plots during the study period and treatments were not effective at preventing new infections. Plots that received the 3.7 m × 3.7 m residual spacing exhibited slightly reduced progression of disease incidence relative to the other treatments. A mountain pine beetle (Dendroctonus ponderosae Hopkins, MPB) eruption occurred during this study. Tree-level MPB and Elytroderma interactions were assessed and no evidence was found to indicate trees infected with Elytroderma had a different likelihood of being attacked by MPB relative to uninfected trees. However, the trees located in plots with high levels of initial Elytroderma disease presence had greater chances of having tree mortality in general, caused by MPB or other factors. MPB did not attack any trees in plots that had the 5.5 × 5.5 m spaced treatment, even where adjacent plots were attacked. Results of this study are presented to inform management of young ponderosa pine forests on impacts of the silvicultural techniques tested on post-treatment Elytroderma dynamics and interactions with MPB-attack.