Comparison of ITS sequences from UK and North American sugar-beet powdery mildews and the designation of Erysiphe betae

Powdery mildew of sugar beet, a disease of major economic significance, was first described at the beginning of the 20th century, and since then there has been some confusion over the correct taxonomic identity of the causal agent. In Europe, the fungus was initially classified as the novel species Microsphaera betae, later re-named Erysiphe betae, whilst in America it was identified as E. polygoni, despite sugar-beet isolates from both regions having a host range restricted to Beta species. It is possible that more than one fungus causes the disease, as published descriptions of conidiogenesis have differed. In this study, isolates of the fungus collected from sugar beet in the UK and USA were investigated for polymorphisms in the rDNA ITS region to determine if the same species caused the disease in both countries, whether there was any justification for the retention of the name E. polygoni in the USA, and to search for evidence of a second species infecting sugar beet. From a total of 18 isolates examined, 23 ITS sequences were obtained. Fifteen of these, which included the UK and USA isolates, were identical and the remainder had single-base substitutions, indicating that the fungi were conspecific. Dendrogram analysis of Erysiphales ITS regions revealed that the UK and North American isolates were more closely related to E. heraclei than to E. polygoni. It is proposed that the species name Erysiphe betae be used for the powdery mildew fungus that infects sugar beet. No evidence was found in this study for a second sugar-beet powdery mildew species.