Load of the ash dieback pathogen hymenoscyphus fraxineus differs in soil

Abstract

The ascomycete Hymenoscyphus fraxineus causes the devastating ash dieback disease of European ash (Fraxinus excelsior L.). Spore traps are often used to measure the amount of ascospores in the environment, but the pathogen-load of the soil in ash stands has not been recorded so far. This is of particular interest with regard to the occurrence of ash stem necrosis, a decisive factor for the severe course of the disease. In order to gain a more differentiated insight into the pathogen-load in ash stands, we analysed soil samples from four ash tree sites in southern Germany, covering a clone plantation, two seed orchards and a forest. The pathogen-load was determined using a quantitative TaqMan real-time PCR assay for ten to twenty plots per stand. Results obtained by the species-specific assay highlighted that the pathogen-load is heterogeneously distributed in the ash stands. H. fraxineus DNA targets were detected in 17% of the soil samples. The pathogen-load differed according to soil depth, with the highest pathogen abundance in the top 5 cm, followed by 5–10 cm and finally 10–15 cm. Pathogen-load and thereby infection pressure were found to be highly variable for the individual trees in one stand. Overall, the study discovered detectable levels of H. fraxineus in the soil of all four study sites, which supports the hypothesis that H. fraxineus can be found in the soil of ash stands. The qPCR approach was found to be an effective method for monitoring the load of H. fraxineus in soil and for demonstrating the successful application of the method on the sample type of custom-made spore traps. Results suggest the implication of site-specific pathogen-load determination in future H. fraxineus-monitoring and selection of less susceptible ash trees for breeding and seed production.