One crop disease, how many pathogens? Podosphaera xanthii and Erysiphe vignae sp. nov. identified as the two species that cause powdery mildew of mungbean (Vigna radiata) and black gram (V. mungo) in Australia

Abstract

Powdery mildew is a significant threat to mungbean (Vigna radiata) and black gram (V. mungo) production across Australia and overseas. Despite being present in Australia for at least six decades, and being easily recognized in the field, the precise identification of the pathogens causing this disease has remained unclear. Our goal was to identify the powdery mildew species infecting mungbean, black gram, and wild mungbean (V. radiata ssp. sublobata) in Australia. The internal transcribed spacer (ITS) and large subunit (LSU) sequences of the ribosomal DNA (rDNA) and/or morphology of 57 Australian specimens were examined. Mungbean and black gram were infected by two species: Podosphaera xanthii and a newly recognised taxon, Erysiphe vignae sp. nov.. Wild mungbean was infected only with P. xanthii. Mungbean and black gram powdery mildew ITS sequences from China, India and Taiwan revealed the presence of only P. xanthii on these crops despite controversial reports of an Erysiphe species on both crops in India. Sequence analyses indicated that the closest relative of E. vignae is E. diffusa, which infects soybean (Glycine max) and other plants. Erysiphe vignae did not infect soybean in cross-inoculation tests. In turn, E. diffusa from soybean infected black gram and provoked hypersensitive response in mungbean. The recognition of a second species, E. vignae, as another causal agent of mungbean and black gram powdery mildew in Australia, may have implications for existing fungicide control measures and breeding.