Distinct Taphrina strains from the phyllosphere of birch exhibiting a range of witches' broom disease symptoms

The phyllosphere is an important microbial habitat and reservoir of organisms that modify plant health. Taphrina betulina is the causal agent of birch witches' broom disease. Taphrina species are dimorphic, infecting hosts in the filamentous form and residing in the host phyllosphere as non-infectious yeast. As such, they are expected to be found as resident yeasts on their hosts, even on healthy tissues; however, there is little experimental data supporting this supposition. With the aim of exploring the local infection ecology of T. betulina, we isolated yeasts from the phyllosphere of birch leaves, using three sample classes; infected leaves inside symptom-bearing branches, healthy leaves from symptom-free branches on symptom-bearing trees and leaves from symptom-free branches on symptom-free trees. Isolations yielded 224 yeast strains, representing 11 taxa, including T. betulina, which was the most common isolate and was found in all sample classes, including symptom-free samples. Genotyping revealed genetic diversity among these T. betulina isolates, with seven distinct genotypes differentiated by the markers used. Twenty-two representative T. betulina strains were selected for further study, revealing further phenotypic differences. These findings support that T. betulina is ubiquitous on birch and that individual trees host a diversity of T. betulina strains.