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

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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.