Morphology of uredinia and urediniospores of the fungus Melampsora larici-epitea Kleb. a damaging pathogen of common osier (Salix viminalis L.) in Poland

Rust (Melampsora spp.) is a damaging disease of willows (Salix spp.), including common osier (S. viminalis L.). So far, the pathogens of this species found in Europe were identified as M. larici-epitea Kleb. or M. ribesii-viminalis Kleb. Moreover, a stem infecting form (SIF), deprived of a sexual stage in its life cycle was reported. The aim of this study was to find out which species of the rust fungi cause disease symptoms on common osier in Poland. The isolates from common osier were compared to the ones originating from its putative hybrids with trembling aspen (Populus tremula L.) and Simon's poplar (P. simonii Carr.). Fungal isolates were obtained in 2008-2010 from 15 different genotypes of willows, including seven varieties of common osier (4 Swedish and 3 Polish), two landraces of common osier and six putative hybrids with poplars. Fungal isolates originated from three experiment sites, including west (Wielkopolska and Lubuskie) and north-east (Warmia) regions of Poland. To ensure the genetic uniformity, the isolates were derived from single uredinia, obtained from natural infection conditions. In all collected samples the position of uredinia was always hypophyllous. The diameter of uredinia was measured by Sigma Scan Pro software, after inoculation of four standard genotypes, including two common osier and two willow hybrids. The studies proved that the main cause of common osier rust is Melampsora larici-epitea f. typica. All studied isolates, including the ones obtained on putative hybrids, were very similar according to the size of uredinia and the size and morphology of urediniospores. The average size of a uredinium was 1.1 mm diameter and slightly differed between the isolates (from 0.9 to 1.3 mm), depending on willow genotype, the quality of plant material used for artificial inoculations and infection conditions. The average size of a typical urediniospore was 12.4 x 10.5 µm, but the sizes varied from 9.8 to 13.2 µm. Urediniospores from common osier were slightly bigger and more oval (12.5 x 10.4 µm) as compared to spores from the putative hybrids Salix x Populus (12.3 x 10.6 µm), but the differences were statistically insignificant. The spores were ovoid, globoid or angular, evenly echinulated. The size of uredinia, as well as the size and morphology of urediniospores were in full agreement with literature data for M. larici-epitea f. typica