Abstract
Nothophaeocryptopus gaeumannii is a common native, endophytic fungus of Douglas-fir foliage, which causes Swiss needle cast, an important foliage disease that is considered a threat to Douglas-fir plantations in Oregon. Disease expression is influenced by fungal fruiting bodies (pseudothecia), which plug the stomata and inhibit gas exchange. Trees are impacted when pseudothecia plug stomates on 1-year-old and older needles resulting in early needle abscission. Mature (100 years+) trees appear to be less impacted from disease, and we hypothesize this is due to the greater emergence of pseudothecia on older than younger needles, which allows for more needle retention. We measured the density of pseudothecia occluding stomates across 2- to 5-year-old needles from upper, middle, and lower canopy positions of mature trees at three sites in the Oregon Coast Range and two sites in the western Oregon Cascade Mountains. Binomial generalized linear mixed model (GLMM) was used to test for the effects of canopy position (upper, middle, and lower), sites, needle age (2-5 years old), and years (2016 and 2017), and their interactions on the pseudothecia density. Pseudothecia density varied annually depending on sites, needle age and canopy positions. Pseudothecia density peaked on 3-, and 4-year-old needles, however, needles emerging from the same year, like 2-year-old needles in 2016 and 3-year-old needles in 2017 both emerged in 2014, had consistently similar patterns of pseudothecia density for both years, across site and canopy positions. Canopy position was important for 3-, and 4-year-old needles, showing less pseudothecia in the lower canopy. This research confirms that N. gaeumannii pseudothecia density is greatest in 3- and 4-year old needles in mature trees in contrast to plantations where pseudothecia density usually peaks on 2-year-old needles, and that pseudothecia density (disease severity) is generally lower in mature trees. Something about mature forest canopies and foliage appears to increase the time it takes for pseudothecia to emerge from the needles, in contrast to younger plantations, thus allowing the mature trees to have greater needle retention.
Highlights
Nothophaeocryptopus gaeumannii is a common native, endophytic fungus which occurs only in Douglas-fir (Pseudotsuga menziesii) foliage (Hansen et al, 2000)
We sampled the same mature trees from five of the seven sites in western Oregon that were included in the study by Lan et al (2019); three sites were in the long-term ecological monitoring plot (LTEM) system established by the United States Environmental Protection Agency and two sites were not part of the LTEM system
For less severely diseased stands of mature trees, incidence varied by needle age class and peaked for 3- and 4year old needles rather than for 2-year-old needles
Summary
Nothophaeocryptopus gaeumannii is a common native, endophytic fungus which occurs only in Douglas-fir (Pseudotsuga menziesii) foliage (Hansen et al, 2000). The fungus can cause a foliage disease known as Swiss needle cast, which is currently defoliating and decreasing growth of Douglas-fir along the Pacific Coast in Oregon, Washington, and SW British Columbia (Ritóková et al, 2016; Shaw et al, 2021). Disease is caused when the fruiting bodies of the fungus, known as pseudothecia (Figure 1), emerge from and plug the stomates which causes reduced gas exchange and carbon starvation (Manter et al, 2000) This fungus may be unusual for a pathogen in that newly emerged needles are the predominant substrate for new infection by ascospores (Rohde, 1937; Chastagner and Byther, 1983) and colonization within needles is exclusively intercellular and non-lethal to cells (Stone et al, 2008a). Disease severity on 2-year-old needles may be misleading for mature Douglas-fir trees which typically have greater needle retention of more than 5 years (Lan et al, 2019) and lesser frequency and magnitude of growth losses (Lee et al, 2017)
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