Current status of the Dutch elm disease pathogen populations affecting Ulmus minor in Greece

Summary One hundred and eight Ophiostoma ulmi sensu lato isolates were collected from field elm trees with symptoms in 14 Prefectures of Greece. The purpose of this study was to assign Greek isolates to species and afterwards subspecies of the DED fungi and to analyse the genetic variability within the Greek populations of these pathogens. Isolates were compared with six reference strains belonging to O. ulmi and the two subspecies of O. novo-ulmi. The structure of the population has been analysed by means of morpho-physiological features (growth rates, colony morphology and fertility responses) and by DNA sequencing and PCR-RFLP amplification of the cerato-ulmin (cu) and the colony type (col1) gene regions. According to fertility tests, both subspecies of O. novo-ulmi were detected in Greece, but none of the isolates collected was identified as O. ulmi. O. novo-ulmi ssp. novo-ulmi occurred more frequently than ssp. americana (73 and 35 isolates, respectively) and their ranges overlapped. All isolates that behaved as ssp. novo-ulmi in the fertility tests had the cu, as well as the col1 profile of ssp. novo-ulmi. Surprisingly, all isolates that behaved as O. novo-ulmi ssp. americana in the fertility test had the cu, as well as the col1 (with one exception) profile of O. novo-ulmi ssp. novo-ulmi. A possible explanation for this inconsistency could be the occurrence of hybridization between the two subspecies in Greece.