Citation:

Lan, Y., D. Shaw, E. Lee, AND P. Beedlow. Distribution of a foliage disease fungus within canopies of mature Douglas-fir in western Oregon. Frontiers in Forests and Global Change. Frontiers, Lausanne, Switzerland, 5:743039, (2022). https://doi.org/10.3389/ffgc.2022.743039

Impact/Purpose:

Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) is an ecologically and economically important tree species in the Pacific Northwest (PNW) that is infected by the foliar fungus Nothophaeocryptopus gaeumannii (Rhode) Petrak which causes Swiss needle cast (SNC) disease. The recent SNC epidemic in coastal Oregon has expanded from 131,000 acres infected in 1996 to 590,000 acres in 2015 and has resulted in a substantial economic loss to the timber industry. SNC is a particular concern to the timber industry because disease severity is greater in young Douglas-fir plantations than in mature stands but reasons for this remain unknown. PESD scientists in collaboration with forest health specialists at the Department of Forestry, Oregon State University (OSU) investigated SNC disease patterns in mature forests across an elevation and precipitation gradient in western Oregon, USA. PESD and OSU scientists show that: 1) disease severity is much greater in young stands than mature stands; 2) the density of fruiting bodies (pseudothecia) is highest for two-year-old needles in young trees and 3-4-year-old needles in mature trees; 3) pseudothecia density is higher in the upper than lower canopy which is the opposite of expected. Disease severity and impact are less in mature than plantation trees because mature trees have greater needle retention associated with a longer time for pseudothecia to emerge and occlude the stomates. We speculate that multiple factors including needle anatomy, canopy architecture, chemical and thermal properties, and micro-climate may have caused the tree age differences in disease severity. Our work is important to the timber industry and other stakeholders in the Pacific Northwest where N. gaeumannii is ubiquitous and increasing in range and magnitude under a changing climate. SNC in combination with climate stress are predicted to decrease productivity, forest health and condition in both young and old Douglas-fir forests in western Oregon and elsewhere.

Description:

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 one-year old and older needles resulting in early needle abscission. Trees with less than three years of needle retention show growth losses. Mature (100 yr +) 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 two- to five-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. Multivariate analysis of variance (MANOVA) was used to test for the main effects of canopy position (upper, middle, and lower), sites, and years (2016 and 2017), and their interactions on the pseudothecia density. Pseudothecia density varied annually depending on needle age and canopy position. Pseudothecia density peaked on three-, and four-year-old needles for both 2016 and 2017, however, needles emerging from the same year, as two-year-old needles in 2016 and three-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 three-, and four-year-old needles, showing less pseudothecia in the lower canopy. Site was important for only two-year old needles where the coastal site at Cascade Head had the significantly greatest infection. This research confirms that N. gaeumannii pseudothecia density is greatest in three-, and four-year old needles in mature trees in contrast to nearby plantations (Lan et al. 2019) where pseudothecia density was much greater on two-year old needles. In addition, the density of pseudothecia overall was less on three- and four-year old needles of mature trees, than on two-year old needles in plantations. 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.

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