CHARACTERIZATION OF CRYPHONECTRIA PARASITICA POPULATIONS IN SOUTHERN ITALY

Chestnut blight caused by Cryphonectria parasitica is the most serious disease of sweet chestnut in Italy. A survey was made in chestnut stands in two regions of southern Italy, Calabria and Sicily, with the aim of studying both the distribution of vegetative compatibility (vc) groups and the spread of hypovirulence in local populations of C. parasitica. A total of 455 isolates (240 from Calabria and 215 from Sicily) from 19 different locations were characterised. Four vc-groups were found in Calabria (EU1, EU2, EU10, EU12) and three (EU2, EU10, EU12) in Sicily. EU2 and EU12 were the most common vc-groups in Calabria, with frequencies of 42 and 22%, respectively, whereas EU12, with 86% of all isolates, was the dominant vc-group in Sicily. All vc-groups included hypovirulent isolates. However, hypovirulent isolates did not occur in all locations. Conversion rate was tested in vitro and appeared to be dependent on the vc-groups of donor and recipient isolates, respectively. Hypovirulence transmission occurred with 100% frequency between isolates of the same vc-group only. Results indicate that founder effect, selection and genetic drift are most probably the major factors determining the structure of local C. parasitica subpopulations in these two regions. At least in Calabria, gene flow due to sexual reproduction apparently plays a secondary role, Mat1-2 being almost the only mating type present.