Ustilago spp. pathogenic on Aegilops. II. Ustilago tritici

Abstract

Examination of 53 herbarium specimens of Ustilago spp. collected on Aegilops L. showed that U. passerinii, U. schumanniana, U. ehrenbergiana, and U. aegilopidis (sic) are synonyms of U. tritici. From published descriptions it was concluded that U. ugamica, too, is synonymous with U. tritici. Morphology and hybridization studies showed that eight field collections of Ustilago from Aegilops cylindrica, A. kotschyi, A. lorentii, A. tauschii, and A. triuncialis from Iran, Iraq, and the U.S.S.R. are U. tritici. The large proportion of U. tritici among herbarium specimens and field collections indicates that it is the most common Ustilago sp. on Aegilops. Thirty species of Aegilops and Triticum were inoculated with five collections from Aegilops and two races from Triticum. Aegilops bicornis was susceptible, and three Aegilops and six Triticum species were resistant to all inocula. On the remaining hosts the pathogen showed specialization at the species and genus level; specialization at the genus level depended in part on the karyotype of the host. This specialization and the low compatibility between certain collections or races as determined by crosses probably are due to evolution during long-term association with isolated host populations. The use of a trinomial system for U. tritici is rejected because of the multitude of possible formae speciales and the overlapping of pathogenicity by races on several host species.