Section outline
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First, Ophiostoma ulmi killed elms in most of the European countries (Peace 1960). The northernmost findings of O. ulmi are known in Norway (Solheim et al. 2011), in Sweden (Menkis et al. 2016b; ‘EPPO Global Database’ 2019), Finland (Hintikka 1974), Estonia (Lepik 1940). But then the pathogen spread declined, apparently due to fungal viruses (Mitchell and Brasier 1994).
Thereafter, step by step, O. ulmi was replaced by a more aggressive new pathogen Ophiostoma novo-ulmi (Brasier and Buck 2001) causing the second pandemic.
The second pandemic of DED, the current one, had begun already in the 1940s at two different locations: the Moldova–Ukraine region in Eastern Europe and the Southern Great Lakes area in North America (Brasier 1990, 1996b). In Estonia the spread and some outbreaks of DED had been observed for the first time in the last decades of the last century. At that time the disease was considered insignificant. The outbreaks of DED have been increasing since the second decade of the new century.
There is no evidence that DED is still present in Finland (Hantula 2021).
The geographical ranges of the two O. novo-ulmi subspecies are overlapped in several parts of Europe (Brasier and Buck 2001) which also has induced their hybridisation (Konrad et al. 2002; Santini et al. 2005b; Martín et al. 2010), because the gene flow between them lacks strong barriers (Brasier and Buck 2001).
Reports on hybrid fungi were quite rare until the 1990s (Brasier 1995; Brasier and Buck 2001). In the eastern part of Europe, hybrids between the two subspecies of pathogen had already been detected in Hungary (Brasier et al. 2004), Poland (Brasier and Kirk 2010), the Czech Republic (Dvořák et al. 2007), Lithuania (Motiejūnaitė et al. 2016) and Latvia (Matisone et al. 2020), Estonia (Jürisoo 2021a) and Sankt Petersburg (Jürisoo 2021b).
DED was first reported in the United States in 1928, with the beetles believed to have arrived in a shipment of logs from the Netherlands. By the beginning of the 21st century hundreds of millions in North America (Brasier 2001, Brasier and Buck 2001).
Dutch elm disease has reached New Zealand. It was found in Napier where it was eradicated and was also found in the Auckland Region in 1989 (Ganley 2016).