Swiss needle cast (SNC) is an endemic disease of Douglas-fir caused by Nothophaeocryptopus gaeumannii. The fungus infects newly emerged needles between May and August. As the fungus develops, the fruiting bodies (pseudothecia) block the stomata and inhibit gas exchange, therefore reduce photosynthesis and growth rate. Epidemiology of N. gaeumannii is generally well known in plantation trees, but the fungus dynamics in older trees is poorly understood. The objective of this research is 1) to verify if anecdotal observations that SNC is more severe in young trees than in mature trees is true, with comparisons of needle retention, foliar nitrogen, and leaf wetness; 2) to understand the epidemiology of mature trees by examining the pseudothecia density across different needle ages.
Seven study sites were selected in 2016 and 2017 in western Oregon. At each site, 3 young trees (20-30 years old) and 3 mature trees (120- 450 years old) were climbed to sample needles from lower, middle, and upper canopy positions. Severity of SNC was measured by stomata occlusion by pseudothecia on 2-yr-old needles. Disease incidence, needle retention, foliar nitrogen, and leaf wetness were also measured. In addition, pseudothecia density and incidence of 2-6-year old needles on mature trees were measured.
Results/Conclusions
Disease severity on 2-year old needles was significantly greater in younger forests than older forests at all sites (p<0.0001). Foliar nitrogen concentration differed among years, canopy positions, and sites, but was not different between mature and young trees. Leaf wetness and foliar nitrogen were hypothesized to play a controlling role in SNC severity difference between mature and young trees, but neither of them were consistent between mature and young trees in our study. Retention of the four-year needle cohorts were greater in older than younger trees. Pseudothecia incidence was highest on 2-year-old needles in young trees and on 3-, 4- and 5- year-old needles in mature trees, showing that young and mature trees might have different pathogen patterns. In mature Douglas-fir of western Oregon, pseudothecia density varied year to year depending on needle age and canopy position. Needles emerged from the same year had similar patterns of pseudothecia density. Pseudothecia density in upper and middle crown are higher than in lower crown.
This study provides increased knowledge of SNC disease ecology in mature Pacific Northwest Douglas-fir forests. Knowledge of mature forests is necessary for management of older stands for endangered wildlife like the spotted owl and marbled murrelet.