Combined progress in symptoms caused by Hymenoscyphus fraxineus and Armillaria species, and corresponding mortality in young and old ash trees

Abstract

Abstract Ash dieback caused by the fungus Hymenoscyphus fraxineus has been observed in Europe for more than two decades. It was first reported in Denmark in 2002 followed by a rapid decline in the health of Danish ash stands, often showing secondary infections from the pathogen honey fungus. In order to follow the disease and mortality progress a surveillance program was initiated in 2010. The degree of crown dieback as well as the extent and severity of necrosis caused by respectively H. fraxineus and Armillaria sp. was monitored in six common ash (Fraxinus excelsior) stands varying in age, geographical location and genetic origin. Here we present the results from the first decade of disease monitoring using general linear models to examine the symptoms effect on growth and mortality. We used DNA methods to verify the presence of the examined pathogens. Our data showed that by 2010 the disease was widespread, and in following years infection rates and mortality was high and often rapid. Necrosis caused by the two pathogens in question increased in size and severity during the monitoring period. The progress of the disease differed according to age of the stands, with younger stands experiencing higher levels of crown dieback, necrosis and mortality. Although young stands were most affected by the pathogens, old stands gradually also decreased in health, and growth was depressed. After 8\xa0years of steady low mortality rate in the old trees, an increase in mortality was also seen in the last year of monitoring. Metabarcoding analysis suggested the presence of both pathogens in question, and conventional PCR methods confirmed A. gallica as the main honey fungus species associated with ash dieback in Denmark. For the first time in Denmark A. cepistipes was also found to be associated with ash dieback in the youngest stand. A small percentage of trees showed no signs of ash dieback and honey fungus despite a high natural infection pressure in the surveyed stands. It is our expectation that such trees will survive and serve as seed sources and thereby maintain ash in the forests.