Health of elms and Dutch elm disease in Estonia.
During three years, 2014-2016, Dutch elm disease (further DED) was investigated on 1225 elm trees at 4 different sampling sites and 2 sub-sites in Estonia. For the first time, both subspecies of the invasive pathogen Ophiostoma novo-ulmi: O. novo-ulmi subsp. novo-ulmi, and O. novo-ulmi subsp. americana, were detected by col1 and cu genes in Estonia and north-eastern Europe. Ophiostoma novo-ulmi subsp. americana was identified only at one site in northern Estonia, in Tallinn. In addition, during our assessments, the health of elms there appeared worse than at other sampling sites: O. novo-ulmi subsp. americana demonstrated higher aggressiveness. Simultaneous occurrence of both subspecies and their hybrids was not detected. A repeat survey of 109 elms in 2014 and 2016 demonstrated ca. 22% probability of mortality within 24 months, irrespective of urban vs. rural habitat. In sub-site A1 in Tallinn, O. novo-ulmi subsp. americana has been found since 2013. DED signs were noted on 39% of all 1225 surveyed trees. Among the assessed elm species, Ulmus laevis showed higher resistance than U. glabra: 82% and 66% of trees, respectively, showed high vitality. In addition, no U. laevis trees were found dead, compared to 18% of the U. glabra.