Fertilization impacts on Swiss needle cast disease severity in western Oregon

The influence of fertilization on disease severity is unknown in most forest pathosystems. Fertilization treatments were randomly applied to 0.01ha plots centered on individual dominant or co-dominant Douglas-fir trees in ten Douglas-fir stands from coastal Oregon to the foothills of the Oregon Cascade Range, USA. This region is affected by Swiss needle cast, caused by the fungal pathogen Phaeocryptopus gaeumannii. Selected stands represented a range of Swiss needle cast disease severity, and 10 replications of each fertilization treatment were applied in each stand. The six treatments included nitrogen (urea), calcium as lime (calcium carbonate), calcium as calcium chloride, phosphorus (monosodium phosphate), a site-specific blend (Kinsey) and an unfertilized control. Fertilization took place from February–April 2007, and single branches were collected from treated trees for disease severity assessment of foliage in May 2010. Disease severity of 1- and 2-year-old needles was evaluated by counting the frequency of infected needles and the density of P. gaeumannii fruiting bodies (pseudothecia) on a random subset of needles from each tree and needle age class. Fertilization treatment effects on infection index (mean fruiting body density) were tested by mixed-effects models that accounted for site as a blocking factor. Treatment effects on infection index at each of the study locations were also tested by ten separate ANOVAs. Across and within sites, fertilization treatment did not significantly affect infection index of 1- or 2-year old needles (p>0.05). Small differences in mean fruiting body density (⩽3%) between fertilization and control treatments across sites were not statistically significant, nor are they believed to be biologically or economically significant. Decisions regarding fertilization should be based on site-specific attributes, such as soil chemistry. There is no evidence that fertilization directly ameliorates or exacerbates Swiss needle cast severity in western Oregon.