MOLECULAR EVIDENCE OF DISTINCT INTRODUCTIONS OF THE EUROPEAN RACE OF GREMMENIELLA-ABIETINA INTO NORTH-AMERICA

The presence of the European (EU) race of Gremmeniella abietina var. a bietina, the causal agent of Scleroderris canker of conifers, was firs t reported in North America in 1975 in the northeastern United States and subsequently in southern Quebec and Newfoundland during the late 1 970s, where it quickly became established. We analyzed DNA profiles in samples from a historic collection of G. abietina var. abietina that included some of the first isolates of the EU race reported in the Uni ted States to test hypotheses concerning the G. abietina var, abietina epidemic in North America. Genetic diversity was partitioned by an an alysis of molecular variance with haplotype frequencies and distances. Genetic differentiation was high between populations in continental N orth America and Newfoundland (between region differentiation, Phi(d) = 0.665, P < 0.001). This result was not consistent with the hypothesi s of a single introduction of the pathogen into the northeastern Unite d States followed by secondary spread into northeastern Canada. In con trast, small levels of genetic differentiation were observed among con tinental North American populations (Phi(d) = 0.047, P = 0.079), sugge sting gene flow among these populations. A single haplotype of G, abie tina var, abietina dominated the continental populations (80% of the i solates) but was absent from Newfoundland and Europe. Five haplotypes were found in the Newfoundland population, all of which were either ab sent or very rare on the continent. Populations from continental North America clustered together and were distinct from a second cluster co mposed of European and Newfoundland populations. A phylogenetic analys is of the haplotypes indicated that some of the rare haplotypes may ha ve derived from somatic mutations, whereas others probably occurred as the result of new introductions. The results are consistent with a sc enario of distinct primary introductions of this pathogen into Newfoun dland and continental eastern North America followed by secondary asex ual propagation.