Assessment of Swiss Needle Cast Disease: Temporal and Spatial Investigations of Fungal Colonization and Symptom Severity

Abstract

Increasing severity of Swiss needle cast (SNC), a foliar disease of Douglas‐fir caused by the fungus Phaeocryptopus gaeumannii, has become a matter of concern in forest plantations throughout coastal Oregon and Washington. This study monitored SNC disease in three Oregon Douglas‐fir plantations bi‐annually in 1998–1999, and compared differences in fungal colonization and symptom development in trees from north‐ and south‐facing plots at each plantation. Fungal colonization as quantified by ergosterol content, pseudothecia density and quantitative PCR was significantly correlated with symptom severity (needle retention and needle cholorosis). All three measures of fungal colonization were highly correlated with each other; and only the ergosterol–pseudothecia relationship differed between plots, presumably due to the non‐species specific nature of ergosterol measurements. Differences in symptom severity and fungal colonization between north‐ and south‐aspect plots were consistent with climate differences. At low to moderate levels of infection, trees growing on warmer (i.e. south slopes in the western, and north slopes in the eastern Coast Range) slopes had higher levels of colonization, particularly during the winter months. Plots with southern exposures, which received greater amounts of solar radiation, had greater amounts of needle abscission compared to north‐aspect plots with similar amounts of fungal colonization. As a result, greater fungal abundance and symptom expression developed on south‐aspect slopes within the Oregon Coast Range.

Response of Douglas-fir leaf area index and litterfall dynamics to Swiss needle cast in north coastal Oregon, USA

“…However, foliage litterfall decreased as aspect became more northeasterly despite an expectation of lower water stress and more rapid growth. In this study, however, aspect is confounded with SNC severity as indicated by higher fungal colonization and more severe SNC symptoms reported by Manter et al [40] for south slopes.…”

“…Litterfall SLA increased as plots became steeper and more southerly (p < 0.0001), while it decreased with distance from the coast (p < 0.0001). SNC severity and its interaction with distance from the coast were not significant, most likely because SNC severity has been shown to increase on southern aspects [40] and decrease with distance from the coast [50]; hence, the marginal effect of SNC was not significant. Litterfall SLA changes systematically with Julian date (p = 0.0002), with a minimum in late March/early April and a maximum in December.…”

“…The disease has reduced tree vigor and caused premature needle abscission [40], significantly altering several crown structural attributes [65] and leading to average volume growth losses as high as 52% in severely affected plantations [38]. Growth losses presumably result from both reduced LAI [65] and disruption of gas exchange in surviving needles [39].…”

Assessments of Population Structure, Diversity, and Phylogeography of the Swiss Needle Cast Fungus (Phaeocryptopus gaeumannii) in the U.S. Pacific Northwest

“…Later studies of SNC disease physiology revealed that these symptoms are caused by reduced gas exchange in the needles due to the occlusion of the stomata by ascocarps (pseudothecia) produced by this fungus. This blockage of the stomata leads to a reduction in carbon assimilation proportional to the percentage of total stomates occupied by pseudothecia [3] Abscission occurs when a needle ceases to function as a carbon source [4]. Regional SNC aerial surveys conducted annually since 1996 have documented gradual expansions in the area affected by the disease and in…”

The role of weather and topography in the development of Dothistroma septosporum.

“…Manter et al (2003) also noted that plots with southern exposures, which received greater amounts of solar radiation, had greater amounts of needle abscission compared to northaspect plots with similar amounts of P. gaeumannii colonization. Any influence of aspect on severity of Dothistroma infection has yet to be documented.…”