Comparison of Biochemical, Molecular, and Visual Methods to Quantify Phaeocryptopus gaeumannii in Douglas-Fir Foliage

A recent epidemic of Swiss needle cast along the Oregon coast has prompted efforts to quantify foliar infection and colonization of the causal agent Phaeocryptopus gaeumannii. In this paper, we compare four methods to quantify colonization of Douglas-fir foliage by P. gaeumannii: fruiting body abundance, ergosterol content, dot blot analysis, and TaqMan based real-time quantitative polymerase chain reaction (PCR). Results from the four techniques were all significantly correlated. Fruiting body density and quantitative PCR are two methods least affected by the presence of other needle fungi and had the highest correlation. The methods also were used to compare foliage colonization in nine field sites exhibiting a range of disease severity. All four methods provided evidence that sites differed in the degree of fungal colonization, but only quantitative PCR consistently separated sites with moderate to severe levels of disease from sites with low disease estimated by foliage color, canopy density, and growth measurements.