In previous work, UEA1 and UCSC1, two geographically distinct, powdery mildew isolates, were recognized for their ability to infect Arabidopsis thaliana. We have clarified the identity of these isolates by determining their host ranges, reexamining their morphology, and comparing their DNA sequences for the 5.8S ribosomal RNA and two flanking internal transcribed spacer sequences. These experiments confirm that UEA1 is a member of Erysiphe cruciferarum and that UCSC1 belongs to E. cichoracearum. Interactions of the two Erysiphe isolates with 360 A. thaliana accessions were examined to provide a comprehensive profile of naturally occurring powdery mildew resistance in this weedy species. The majority of A. thaliana accessions (213) were susceptible to both isolates. Among the accessions exhibiting some degree of resistance, most (84) responded differentially to UEA1 and UCSC1 and the remainder were resistant to both isolates. Notably, resistance to UCSC1 cosegregated with RPW7, a locus previously demonstrated to confer resistance to UEA1 in Ms-0 x Landsberg (erecta) crosses. With this large collection of resistant accessions, questions about species specificity, genetic diversity and the evolution of resistance to powdery mildews can be addressed.
Phylogenetic relationships were inferred for 45 powdery mildews plus two outgroup species from internal transcribed spacer sequence data. Phylogenetic analyses revealed that the powdery mildews formed six evolutionary lineages, which corresponded well to recently revised mitosporic taxa. Clade 1 consisted of Erysiphe, Microsphaera, and Uncinula, all of which have an Oidium subgenus Pseudoidium mitosporic state. Clade 2 consisted of Erysiphe galeopsidis and Erysiphe cumminsiana, which have an Oidium subgenus Striatoidium mitosporic state. Clade 3 consisted of Erysiphe species with an Oidium subgenus Reticuloidium mitosporic state. Clade 4 consisted of Leveillula and Phyllactinia, which have Oidiopsis and Ovulariopsis mitosporic states, respectively. Clade 5 consisted of Sphaerotheca, Podosphaera, and Cystotheca, which have Oidium subgenera Fibroidium and Setoidium mitosporic states. Clade 6 consisted of Blumeria graminis, which has an Oidium subgenus Oidium mitosporic state. Seventeen morphological characters were analysed and found not to conflict with the molecular data, so the morphological and molecular data were combined. The combined data increased the overall support for the six clades.Key words: Ascomycete, Blumeria, Erysiphe, fungi, molecular evolution, plant pathogen.
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