Analysis of the Italian dutch elm disease fungal population

Sixty-two Ophiostoma ulmi sensu lato strains have been collected from symptomatic trees in seven areas of Central Italy. Isolates were compared with 10 reference strains, belonging to the species O. ulmi and to the two subspecies of O. novo-ulmi, in order to establish the genetic variability within the Italian population of this fungal pathogen. The structure of the population has been analysed by means of morpho-physiological features and of the direct amplification of minisatellite-region DNA polymerase chain reaction (DAMD-PCR) by using the M13 core sequence. The DNA profiles have been compared with taxonomic parameters (growth rate, culture aspect and fertility barriers). Taxa could thus be well separated. None of the isolates collected was recognized as O. ulmi. Isolates assigned to the two subspecies of O. novo-ulmi (novo ulmi and americana) by means of the fertility test, showed short genetic distances with the respective reference strains and they constituted subgroups according to their geographical origin. The high level of variation detected indicates a postepidemic situation in Italy. Some inconsistency was found within the subspecies clusters. Several isolates, assigned to subspecies americana using fertility test, were in the novo-ulmi cluster and vice versa. A possible explanation is that these isolates are americana–novo-ulmi hybrids.