In order to improve understanding of its diversity, 338 isolates of Cryphonectria parasitica , the causal agent of chestnut blight, were sampled from 10 chestnut populations throughout chestnut-growing coastal and continental areas of Croatia. Eighteen vegetative compatibility (VC) types were identified. The VC type EU-1 was the most widespread, comprising 42·9% of the isolates, followed by EU-2 (21%) and EU-12 (14·2%). In respect to the occurrence of the main VC types, the C. parasitica populations in Croatia combined features of both northwestern and southeastern European populations. Perithecia and mating-type ratios of approximately 1 : 1 were found in all populations, suggesting that sexual reproduction of the fungus is common in Croatia. Natural hypovirulence was also evident in all populations, with incidence of hypovirus-infected isolates ranging from 12·7% in Istria-Buje to 66·6% in the continental part of the country. A total of 36 hypovirus-infected isolates sampled throughout Croatia were analysed in ORF-A and ORF-B by RT-PCR/RFLP analysis. All viral isolates belonged to the Italian subtype of Cryphonectria hypovirus 1 (CHV-1) and were closely related to the isolates found in other European countries. The RFLP patterns found were also identical or similar to the patterns of three isolates collected in Croatia 22 years ago, suggesting a slow evolution of the hypovirus.

Cryphonectria parasitica, the causal agent of chestnut blight, has been present in Slovenia since at least 1950. To improve understanding of its diversity, 254 isolates of the fungus from 11 Slovenian populations were sampled. Fifteen vegetative compatibility (vc) types were identified. The dominant vc type was EU-13, comprising 40AE1% of all isolates tested, followed by EU-1 (19AE7%), EU-2 (12AE2%) and EU-12 (9%). The vc type diversity in the most diverse population sampled in Slovenia was higher than in the populations found previously in northern Italy and Croatia. Both mating types and perithecia were observed in surveyed populations. Natural hypovirulence was found in six out of seven populations tested, with frequencies ranging from 72AE2% in the population sampled near the Croatian border to 11AE1% in the population sampled near the Austrian border. All identified hypoviral isolates (21) belonged to the Italian subtype of Cryphonectria hypovirus 1 and were closely related to the hypoviruses found in other European countries. Despite the high vc type diversity, incidence of hypovirulence was also high, indicating widespread natural biological control of the disease.

Candidatus (Ca.) Phytoplasma ulmi', the causal agent of elm yellows, was found widely distributed across elm populations in Croatia, infecting Ulmus laevis and Ulmus minor. Especially high prevalence of the infection, approximately 75%, was detected in U. laevis, but more than half of the trees were symptomless. 'Candidatus Phytoplasma solani' and 'Ca. Phytoplasma asteris' were also detected. The latter could possibly represent a new 16SrI subgroup, most closely related to 16SrI-B. Diversity of 'Ca. Phytoplasma ulmi' in Croatia was determined by sequencing of the 16S rRNA gene, ribosomal protein genes rpl22 and rps3, secY and secY-map genes, in 62 phytoplasma isolates. Phylogenetic analysis indicated that Croatian isolates share a common origin and are closely related to strains of 'Ca. Phytoplasma ulmi' from southeastern Europe. However, comparative sequence analysis revealed mutations at positions where variability has never been detected before, including positions within sequences unique to 'Ca. Phytoplasma ulmi' in the 16S rRNA gene, rpl22, rps3 and secY. New genotypes were identified based on the sequenced genes. This study points to a significantly higher genetic diversity than previously reported, and a necessity to revise the formal description of this phytoplasma species and to include newly discovered characteristics.