Four powdery mildew species with catenate conidia infect Galium: molecular and morphological evidence

Abstract

The Erysiphaceae are a group of obligately biotrophic fungi that cause powdery mildew disease of angiosperms. Due to their inability to be cultured on artificial media, the taxonomy of the Erysiphaceae has generally been based on the morphological characteristics of fresh and herbarium specimens. Thus, several morphological species with wide host ranges have long been maintained in this family, even though they clearly consist of several biological species. Erysiphe galii has been known as a powdery mildew of Galium spp. Recently, the former E. galii var. galii has been reassessed as Neoerysiphe galii and E. galii var. riedliana as Golovinomyces riedlianus, along with a taxonomic revision of the generic concept of the Erysiphaceae. The present study was conducted to evaluate the validity of the taxonomic revision of the two varieties of E. galii. During the course of this study, we found that the Galium powdery mildews consist of at least four different species, viz. Neoerysiphe galii, Golovinomyces orontii, G. riedlianus, and an unknown species collected in Argentina. The latter species is described as a new species, Golovinomyces calceolariae. The three species belonging to Golovinomyces are morphologically very similar to each other, i.e. the discrimination between them is rather difficult. The morphological differences of the three Golovinomyces species of Galium are discussed.

Introduction

The Erysiphaceae are a group of obligately biotrophic fungi that cause powdery mildew disease on about 10K angiosperm species (Amano 1986); the group consists of 16 genera and about 650 species (Braun, 1999, Braun and Takamatsu, 2000, Braun et al., 2002, Takamatsu et al., 2005a, Takamatsu et al., 2005b, Liberato et al., 2006). Because they cannot be cultured on artificial media, the taxonomy of the Erysiphaceae has generally been based on the morphological characteristics of fresh and herbarium specimens. Thus, several morphological species with wide host ranges, such as Golovinomyces cichoracearum, Leveillula taurica, Phyllactinia guttata, and Podosphaera fusca, have long been maintained in the Erysiphaceae, even though they clearly represent several distinct biological species (Braun 1987). Taxonomic revisions on generic or higher levels have recently been conducted based on molecular data and anamorphic features (Cook et al., 1997, Braun, 1999, Braun and Takamatsu, 2000, Takamatsu et al., 2005a, Takamatsu et al., 2005b). However, various revisions on species level remain to be performed for this family. Molecular data revealed that more than one species with similar morphology often infect the same host species. For instance, Erysiphe alphitoides (section Microsphaera) occurs on Quercus serrata, an oak tree common in Japan. Molecular and morphological analyses revealed that E. alphitoides is a compound species, consisting of three different Erysiphe species, viz. E. alphitoides s. str., E. hypophylla and E. quercicola; these species often occur simultaneously on the same leaf of Q. serrata (Takamatsu et al. 2007). Two Erysiphe species, viz. E. syringae and E. syringae-japonicae, occur on Syringa spp. (lilacs). These species are clearly discriminated by their rDNA sequences, despite their similar morphology. Biogeographical studies using molecular markers revealed that E. syringae-japonicae was recently introduced into Europe from East Asia (Seko et al. 2008). Therefore, molecular techniques have made it possible to study the distribution and migration of fungal species that are barely indistinguishable from each other by morphological characteristics (Matsuda and Takamatsu, 2003, Takamatsu et al., 2008a, Takamatsu et al., 2008b, Jankovics et al., 2008).

The cosmopolitan genus Galium, belonging to Rubiaceae, consists of about 300 herbaceous plant species. Two powdery mildew species with catenate conidia without fibrosin bodies (tribe Golovinomyceteae), viz. E. galii (Blumer 1933) and E. riedliana (Speer 1969), were known to occur on 36 Galium species (Amano 1986). These two Erysiphe species are characterized by chasmothecia with mycelioid appendages and catenate conidia. Braun (1983) regarded the morphological variation between the two species to be below the species level, and thus E. riedliana was reduced to a variety of E. galii. Variety galii has ascospores that develop after overwintering and lobed hyphal appressoria, whereas var. riedliana has ascospores that develop within the year and nipple-shaped hyphal appressoria. Later, the genus Erysiphe was divided into three genera, viz. Erysiphe s. str., Golovinomyces and Neoerysiphe, based on characteristics of the anamorph, as well as on molecular phylogenetic analyses (Heluta, 1988, Braun, 1999). Erysiphe galii var. galii was reassessed as N. galii (Braun 1999) and E. galii var. riedliana was reapproved as G. riedlianus (Heluta 1988). However, the validity of this taxonomic treatment has not yet been proven using DNA sequence analyses.

The present study was conducted to fill this gap. To this end, we collected Galium powdery mildew specimens from Europe, Asia, and South America, and determined the nucleotide sequences of the rDNA ITS regions, as well as the divergent domains, D1 and D2, of the 28S rDNA of the specimens. During the course of this study, we discovered that the Galium powdery mildews consist of at least four different groups. The results of molecular analyses as well as morphological observations are described in this paper.