- Swiss needle cast (SNC) is a foliage disease of Douglas-fir caused by Nothophaeocryptopus gaeumannii, an Ascomycete fungus (Mycosphaerellaceae) that causes growth reductions in Douglas-fir plantations across the Pacific Northwest. Epidemiology of the fungus is generally well known in plantation trees, but the relationship between disease expression and foliage nutrition and some climate variables is unclear. While the dynamics of SNC in older trees is also poorly understood.
In Chapter 2, data from the Swiss Needle Cast Cooperative (SNCC) research and monitoring plot network across western Oregon and SW Washington State was utilized to assess the associations between disease severity, needle retention, carbon, and 9 foliage nutrients (N, Na, K, P, Ca, Mg, Mn, Al, and S). Foliage samples were collected from upper, mid and lower crowns of five Douglas-firs from each plot. SNC disease severity was determined from 2-year old needles by multiplying the ratio of occluded stomates by the percentage of needles with fungal reproductive structures (pseudothecia) for the 50 needles. SNC disease severity and needle retention were more highly associated in the mid crown than in upper and lower crown. Mid-crown SNC disease severity and nutrient relationships were determined using linear mixed models. SNC disease severity showed statistically significant positive trends with C (p<0.001), N (p<0.001), Na (p<0.001), K (p=0.004), S (p<0.001), no relationship with Ca, Mg, or Al, and slightly negative trends that were not significant for P and Mn. Although some nutrients were associated with increasing SNC disease severity, more research is required to determine the cause-effect.
In Chapter 3, climatic factors, which strongly influence epidemiology, intensification and impacts of disease on tree growth, are considered. Our study was conducted in 106 systematically placed research plots established by the Swiss Needle Cast Cooperative in 2013-2015. Climate variables tested were monthly and annual precipitation, minimum temperature, maximum temperature, mean temperature, mean dew point temperature, and maximum VPD. We also examined the influence of latitude and longitude on climate variables and severity of SNC. Minimum temperature and dew point temperature were the most significant factors related to SNC disease severity (p<0.001). Oct-Apr mean temperature, Oct-Apr maximum temperature, and Nov-Apr maximum VPD were also associated with SNC disease severity (p<0.001). Monthly precipitation was not associated with mean SNC disease severity during the summer months. Dew point temperature for all months was positively associated with SNC disease severity (p<0.001). We suggest that dew point temperature may be more important in epidemiology of N. gaeumannii than previously thought. Latitude had a strong relationship with SNC disease severity and climate variables, while longitude did not. Analyses of climate relationships within subregions in the study area indicated that relationships between SNC disease severity and climate variables were strongest in the Tillamook region of northwest Oregon.
In Chapter 4, while there is considerable evidence of SNC disease in coastal Douglas-fir plantations, the severity of SNC in mature and old-growth forests is poorly understood. We compared the SNC severity, incidence, needle retention, and foliar nitrogen in tree crowns of mature and old-growth forests and nearby young forests at three locations in the Oregon Coast Range and four locations in the western Cascade Mountains of Oregon. Disease severity, as assessed on 2-year old needles, was greater in younger forests than older forests at all sites. Retention of 1-4 year-old needle cohorts did not differ between young and old trees, but older trees had much larger complements of >4 year-old needles. Incidence of disease was highest for 2-year-old needles in young trees and 3-5 year-old needles in older trees. Total foliar nitrogen concentration did not differ in needles of young and old trees, but at some locations total N differed between canopy positions. Leaf wetness differences were not consistent between young and old tree crowns and did not explain disease severity differences. However, at a study site in the core epidemic area, the younger stand had longer periods of wetness in the upper crowns than a nearby old stand. Leaf wetness and foliar N were hypothesized to play a role in SNC disease severity, but apparently these are not controlling factors. In younger stands, the fungus appeared to block stomates earlier than in older stands and stomatal occlusion was always greater on younger than older trees for 2 year old needles. We speculate that multiple factors may have caused the observed differences, including differences in thermal properties of older and younger stands, needle anatomy, chemical differences, or genetics of old tree and young trees. It is also possible that older trees are less impacted by SNC because they have experienced exposure to the disease over a longer period of time, and this influences host-fungus interactions. Also, four of the sites that we examined were outside the current epidemic area and were not considered diseased, so this may have influenced needle retention. The relationships observed in our study need testing with larger samples to determine if our results are generally applicable to Douglas-fir in western Oregon.
From these studies, we tested nutrient and climate variables, which narrowed down the potential factors associated with SNC for further focused modeling. Also, we provided quantitative and qualitative description of SNC patterns comparing mature and young tress, and suggested more research about SNC in mature and old-growth Douglas-fir forests in needed because of the potential of SNC to influence stands of all ages.