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.

Abstract:The research on incidence of mistletoe (Viscum album ssp. abietis) on silver fir (Abies alba) was carried out in natural fir stands in Croatia. In the area of Gorski Kotar 32.8 % of the examined dominant fir trees were infected with mistletoe. The mistletoe incidence was presented according to the damage degrees of silver fir from 0 (healthy trees) to 4 (dead trees), rated by the crown defoliation and needle discoloration. With the increase of incidence, mistletoe spreads more onto more vital, i.e. less damaged trees. In the Dinaric Alps 28.6% of the examined fir trees were infected, this percentage amounting to 27.1% for the mountainous regions between the Sava and Drava rivers. The site and stand parameters (exposure, elevation, site quality, forest community and crown closure) were analysed in order to establish whether there was a correlation between these parameters and mistletoe incidence. There was a negative correlation between the elevation and mistletoe incidence. Among the compartments with a closed stands there were considerably less compartments with higher mistletoe incidence than among compartments with a sparsely closed and understocked stands. The other analysed site and stand parameters individually had no significant influence on mistletoe incidence. The correlation of silver fir mortality in 2004 and mistletoe incidence in 2002/03 was analysed, and their strong correlation was established. Mistletoe could be considered as a bioindicator of silver fir decline, and probably a significant contributor to that decline. In the areas where mistletoe incidence is great it can be presumed that silver fir is significantly damaged.

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-The main objective of this research was to assess the genetic diversity of 5 natural fi eld elm populations in Croatia. The study results suggest that the observed populations are characterized by a satisfactory amount of heterozygosity, and that the impact of the Dutch elm disease on the amount of genetic diversity in the sampled populations is currently negligible. However, one population displayed a signifi cant excess of heterozygosity, implying a genetic bottleneck. The existence of a very clear genetic differentiation between the continental and the Mediterranean populations of Ulmus minor in Croatia was noticed.

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