Severity of Swiss needle cast in young and mature Douglas-fir forests in western Oregon, USA

Abstract

Swiss needle cast (SNC), caused by Nothophaeocryptopus gaeumannii, is an important foliage disease of Douglas-fir (Pseudotsuga menziesii) forests of the Pacific Northwest. The fungus lives endophytically within the foliage, until forming reproductive structures (pseudothecia) that plug stomates and cause carbon starvation. When pseudothecia appear on one-and two-year-old foliage, significant needle abscission can occur, which reduces productivity of the tree. 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 tree crowns of mature and old-growth conifer forests and nearby young forests at three locations in the Oregon Coast Range and four locations in the western Cascade Range of Oregon. We assessed disease severity for N. gaeumannii on two-year-old foliage, incidence by presence of N. gaeumannii on all foliage, foliage retention for the first four years, and foliar nitrogen of one-year-old foliage. We also compared leaf wetness at three heights in one mature and one young tree at five of the seven sites. Disease severity was greater in young forests than mature forests at all sites except for high elevation Cascade Range areas. Incidence of disease was highest for two-year-old needles in young trees and 3-5 year-old needles in mature trees, except for one coastal site. Retention of 1-4 year-old needle cohorts differed between young and mature trees, and mature trees had much larger complements of > four-year-old needles. Total foliar nitrogen (TN) concentration did not differ in needles of young and mature trees, but at some locations total N differed between canopy positions. Leaf wetness differences were not consistent between young and mature tree crowns. However, at one 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 they do not explain differences in adjacent young and mature trees. Although the fungus is present in old and young trees, the likelihood of disease expression and lower foliage retention appears to be greater in younger plantation trees than mature and older trees in western Oregon Douglas-fir forests.

Distribution of a Foliage Disease Fungus Within Canopies of Mature Douglas-Fir in Western Oregon

“…This manuscript extends our previous epidemiological study (Lan et al, 2019) to examine the differences in disease severity between needle age classes and canopy position of mature Douglas-fir trees based on within-leaf measurements of the density of pseudothecia occluding stomates. There is no published data on pseudothecia density of multiple needle age classes in mature trees.…”

“…Recent epidemiological evidence suggests that disease severity, as measured by incidence times the percentage of pseudothecia occluding stomates (Mulvey et al, 2013), is greatest in 2-year-old needles for young trees, and 3-5 year-old needles for older trees (Lan et al, 2019). The reason for this difference remains elusive, as foliar nitrogen and leaf wetness did not explain the difference.…”

“…Evidence from dendrochronological studies indicate that Swiss needle cast impacts in Oregon were least severe in the first half of the 20th century and increased in frequency, severity and range after ∼1980 due to increasing winter temperature associated with climate change (Black et al, 2010;Lee et al, 2013Lee et al, , 2017. The Swiss needle cast epidemic in the most recent decades is a primary economic and ecological concern to the timber industry in the Pacific Northwest because disease severity is greater in young coastal Douglas-fir plantations than in mature stands but reasons for this remains elusive (Lan et al, 2019;Mildrexler et al, 2019). Coastal young Douglas-fir trees typically retain up to 4 years of needles but may have only current and 1-year-old foliage in severely infected plantations (Hansen et al, 2000;Maguire et al, 2002;Zhao et al, 2011) due to early needle abscission when between 25 and 50% of the stomates are occluded (Hansen et al, 2000;Manter et al, 2003;Stone et al, 2008a).…”

“…Needle retention and fungal fruiting body presence and abundance on 1-and 2-year-old foliage have routinely been used as indices of disease severity in Douglas-fir plantations (Hood, 1982;Michaels and Chastagner, 1984;Hansen et al, 2000). However, 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).…”

Applications of a conceptual framework to assess climate controls of forest tree diseases

“…Susceptibility of Douglas-fir foliage to fungal colonization is not affected by stand species composition or density, but tolerance to N. gaeumannii varies across individual trees (Wilhelmi et al, 2017). Stand age is influential, as trees in older stands (120-400 years) experience consistently lower disease severity than nearby younger (20-30 years) plantation trees (Lan et al, 2019).…”