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Crown gall is an economically important and widespread plant disease caused by tumorigenic bacteria that are commonly affiliated within the genera Agrobacterium, Allorhizobium, and Rhizobium. Although crown gall disease was reported to occur on rhododendron, literature data regarding this disease are limited. In this study, an atypical group of tumorigenic agrobacteria belonging to the genus Rhizobium was identified as a causative agent of crown gall on rhododendron. Genome analysis suggested that tumorigenic bacteria isolated from rhododendron tumors are most closely related to Rhizobium tumorigenes, a new tumorigenic bacterium discovered recently on blackberry in Serbia. However, R. tumorigenes and novel rhododendron strains belong to separate species and form a homogenous clade within the genus Rhizobium, which we named the “tumorigenes” clade. Moreover, tumorigenic bacteria isolated from rhododendron seem to carry a distinct tumor-inducing (Ti) plasmid, compared with those carried by R. tumorigenes strains and Ti plasmids described thus far. To facilitate rapid identification of bacteria belonging to the “tumorigenes” clade, regardless of whether they are pathogenic or not, a conventional PCR method targeting putative chromosomal gene-encoding flagellin protein FlaA was developed in this study. Finally, our results suggested that this novel group of tumorigenic agrobacteria occurs on blueberry but it cannot be excluded that it is distributed more widely.

Pseudoidium kalanchoes, retrieved from powdery mildew on Kalanchoe spp. in Asia, has been re-examined and reassessed based on phylogenetic analyses of the ITS and 28S rDNA regions. The results confirmed that sequences retrieved from Asian Kalanchoe powdery mildew have 100% congruence with sequences obtained from Erysiphe sedi on Sedum spp. A German collection of P. kalanchoes on Kalanchoe blossfeldiana was morphologically and genetically examined and analyzed and is designated as neotype with an ex-neotype reference sequence. Morphologically, the asexual morph on Kalanchoe spp. agrees with the asexual morph of E. sedi. Consequently, P. kalanchoes is reduced to synonymy with E. sedi.

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