Fraxinus excelsior tolerant to ash dieback on the island of Gotland determined using phenotypic and genotypic traits Running headline: Ash dieback-tolerant Fraxinus excelsior on Gotland

Ash dieback caused by the invasive fungal pathogen Hymenoscyphus fraxineus is responsible for massive dieback of European ash (Fraxinus excelsior L.) in Europe. On Gotland island of Sweden, ash dieback invaded in 2001-2002 causing high mortality and rapid decline of numerous F. excelsior trees. However, a small proportion (up to 5%) of F. excelsior individuals growing in vicinity to dead and/or declining trees were healthy-looking and without disease symptoms, indicating possible tolerance to the disease. The aim of the present study was to map healthy-looking individuals of F. excelsior on Gotland and to monitor their tolerance to ash dieback disease using phenotypic and genotypic traits. Search and mapping of healthy-looking F. excelsior was carried out in 2013 and 2014, and a total of 135 trees were mapped. Health status of all mapped F. excelsior trees was monitored ones in 2017 i.e. 3 or 4 years after the mapping. In addition, analysis of genetic marker for disease tolerance from 50 mapped and 10 control trees was carried out. Monitoring the health status of mapped F. excelsior showed that 99.3% of trees had 0-10% crown damage i.e. similar as at the time of mapping, and a single (0.7%) tree had 10-20% crown damage. The presence of dead tops, wilting foliage or cancers was not observed on any of the mapped trees. Molecular analysis of leave tissue materials from the mapped trees showed the presence of cSNP for disease tolerance in 85.0% of trees, while it was missing in the remaining six (15.0%) trees. Analysis of data from control trees showed the presence of cSNP for disease tolerance in 3 (37.5%) trees, but not in 5 (62.5%) trees. Thus, the results demonstrated that search, assessment and mapping of healthy-looking F. excelsior can be an appropriate approach for the selection of ash dieback-tolerant individuals of local origin. By contrast, genetic marker used in the present study possessed limited capacity to discriminate reliably among tolerant and susceptible individuals of F. excelsior.