Distribution of a foliage disease fungus within canopies of mature Douglas-fir in western Oregon
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.