Low genetic diversity of the Ophiostoma novo-ulmi population in North America

A sample of 112 Ophiostoma novo-ulmi isolates across the range of Dutch elm disease in North America from California to Nova Scotia revealed only 16 different vegetative compatibility (vc) types. One vc type, the North American vc supergroup (AMSG) was predominant at 58% of the sample. Another vc type, the American second vc supergroup (AM2SG), which was genetically related to the AMSG, represented 20% of the sample. A third vc type, the European vc supergroup (EUSG), represented 10%. The remaining 13 isolates constituted a small but highly heterogeneous component of 13 unique vc types. No differences were detected in the mean growth rates or pathogenicities of the AMSG, AM2SC, EUSG or heterogeneous component. Only 2 of the 112 isolates were found to be infected with deleterious cytoplasmically transmissible mycoviruses ('d-factors'). This low level of genetic diversity among North American O. novo-ulmi isolates contrasts markedly with the very high diversity in current European populations. The combination of low d-infection frequency, low vc type diversity and the presence of a less efficient Dutch elm disease vector (Scolytus multistrtatus) in North America also suggests that North American O. novo-ulmi populations might be potential targets for attempted biological control of Dutch elm disease via the release of d-factors. Two recent O. novo-ulmi outbreak areas, the Willamette Valley, Oregon and Washington DC, were shown to exhibit a clonal (single vc type, single sexual compatibility type) population free from d-factors. They might therefore be favorable locations for an experimental d-factor release.