Morphology of germlings of urediniospores and its value for the identification and classification of grass rust fungi

The identification and classification of grass rust fungi is often difficult since most traditionally used morphological characters are quantitative and subjective. Besides, when using the host range as a taxonomic criterion, it is important to realize that a rust fungus may have jumped to a new host species and that host range may also be affected by the variability and age of the host plant, and inoculation conditions. The present study was initiated to assess the taxonomic value of the germling morphology of the urediniospores of grass rust fungi.The germling morphology of grass rust fungi was observed after inoculation on the barley line L94. Since the rate of development and the morphology of the germlings was similar in host and non-host plants until formation of the first haustorium, rust fungi collected from barleys were studied at about 20 h after inoculation and from other grasses at about 40 h.Germling morphology was proven to be a reliable and useful criterion for identification and classification of grass rust fungi. It enabled an easy discrimination of species complexes that are very similar in traditionally used morphological characters, e.g. Puccinia hordei and P.recondita. Species complexes which are distinct on the basis of these traditionally used morphological characters were also distinct in germling morphology, viz. Puccinia coronata, P. graminis and P.brachypodii. Besides, germling morphology differed greatly between taxa subsumed under the P. recondita and P. brachypodii complexes. In other taxa the differences were mostly quantitative.The differences and similarities in germling morphology observed within and between species complexes were in general correlated with differences in isozyme banding patterns, nuclear DNA contents, and literature data on several other molecular, biochemical, and hybridization experiments.The results from the studies on germling morphology and isozyme banding patterns suggest to treat the species included in the P. brachypodii and P. recondita complexes as separate species, to recognize varieties within P. coronata and P.striiformis (including the newly described var. poae), to unite P.hordei and Uromyces rusts from barleys in one species, and not to assign any taxonomic rank yet to taxa subsumed under P.graminis.