Impact of the foliar pathogen Swiss needle cast on wood quality of Douglas-fir

Many stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) near coastal areas of Oregon and Washington are heavily infected with the foliar pathogen causing Swiss needle cast (SNC) disease, and yet there is very little research on the resulting wood quality. Modulus of elasticity (MOE), modulus of rupture (MOR), microfibril angle (MFA), wood density, latewood proportion, and sapwood moisture content were examined in 20- to 28-year-old trees from 15 stands that were infected with varying intensities of SNC. SNC severity was quantified by measuring needle retention, the number of needle cohorts retained at three crown levels. Correlations between disease severity and wood properties were examined at both the stand and within-stand levels. Trees from heavily infected stands (needle retention <2 years) had higher MOE, wood density, and latewood proportion and lower sapwood moisture content than trees from healthier stands. Breast-height age (BHage) was also correlated with these properties, but age alone did not explain all of the increases. MFA was not associated with SNC severity. Within stands, needle retention was not associated with MOE or MOR. The increase in latewood proportion in diseased stands appears to be the driving factor behind their increase in stiffness (MOE). Ring width decreased with decreased needle retention, and the examined wood properties generally showed stronger correlations with ring width than with needle retention.