Sulla Powdery Mildew: Phylogeny and Host Range

Abstract

Sulla is a biannual forage legume cultivated throughout the Mediterranean Basin. It can be severely damaged by powdery mildew, but there is little understanding on its causal species or its host range. The taxonomic characterization of Erysiphe species is mainly based on the examination of chasmothecia morphology, or on the molecular analysis of ITS1 and ITS2 sequences. However, a description of chasmothecia morphology or ITS sequences is not always available to clearly assign a given isolate to an existing or novel Erysiphe species. In an attempt to clarify the identity of the powdery mildew infecting sulla crop we studied the morphology and ITS of nine populations collected over years and countries and compared them with available ITS sequences. Phylogenetic analysis showed that the powdery mildews collected on sulla clustered together with isolates collected on Coronilla varia in an independent clade between the E. guarinonii/E. trifoliorum and E. palczewskii clades, strongly suggesting that all these sequences correspond to the same Erysiphe species. Measurements of conidia and conidial foot cells of the sulla-infecting powdery mildews partially overlap those of other Erysiphe species, although they clearly differentiate from them, supporting the assumption that powdery mildews from sulla belong to a distinct species as observed from the phylogenetic analysis. As far as we know, our study is the first to report the molecular characterization of powdery mildew isolated on plants from the Hedysarum clade. Cross-inoculations confirmed a high specialization of the powdery mildew of sulla, with the sulla’s isolates infecting only the S. coronaria accessions, and none of the accessions from the other legume genera studied. All studied S. coronaria accessions were heavily infected by the sulla isolate but not by any other isolate used. All this points to E. hedysari as the causal agent of the S. coronaria powdery mildew.