Distribution and frequency of Cronartium flaccidum on Melampyrum spp. in permanent sample plots in Finland.

In 2006, the distribution and frequency of pine stem rust Cronartium flaccidum was studied on Melampyrum spp. in permanent sample plots of an EU forest health monitoring program and of a BioSoil program in Finland. The frequency of Melampyrum spp. infected with C. flaccidum was low, but the rust was recorded in new locations of eastern Finland. No uredinia of C. flaccidum were observed in samples collected from over 700 plots. Telia were common on Melampyrum sylvaticum but were occasionally also found on Melampyrum pratense and Melampyrum nemorosum. Melampyrum pratense was the most common species growing in plots on mainly dry sites. Melampyrum sylvaticum was seldom detected, suggesting that alternate hosts are absent from northern Finland or that the current sample plot network is too sparse for effective disease monitoring in the north. No relationship between C. flaccidum, Melampyrum spp. and rust incidence in host trees was observed, which implies that the autoecious Peridermium pini is a more likely pathogen than C. flaccidum locally. Because plots containing M. sylvaticum occur almost solely in southern Finland, the plots with M. sylvaticum reflect the best changes in rust epidemics in that area. In the future, nutrient-rich sites containing M. sylvaticum may serve as a source of rust epidemics. It is unlikely that C. flaccidum will spread to dry sites via M. pratense. [ABSTRACT FROM AUTHOR]