Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 6, 2011
Publication Date: July 1, 2011
Citation: Mitchell, M.N., Ocamb, C.M., Grunwald, N.J., Mancino, L.E., Gent, D.H. 2011. Genetic and pathogenic relatedness of Pseudoperonospora cubensis and P. humuli. Phytopathology. 101:805-818. Interpretive Summary: Downy mildew of cucurbits and hop are important plant diseases that cause considerable economic losses. Recent work suggested that these organisms are closely related or perhaps the same. In this study, we conducted genetic studies to clarify how closely related these pathogens were, and also determined if these pathogens can infect the other plant. The genetic data and host specificity experiments indicated that there are biologically relevant characteristics that differentiate these pathogens, and we recommend either retaining the two species names, Pseudoperonspora cubensis and P. humuli, or considering the hop downy mildew pathogen a special form (i.e., a forma specialis) of the cucurbit pathogen.
Technical Abstract: The most economically important plant pathogens in the genus Pseudoperonospora (Peronosporaceae) are P. cubensis and P. humuli, causal agents of downy mildew on cucurbits and hop, respectively. Recently, P. humuli was reduced to a taxonomic synonym of P. cubensis based on internal transcribed spacer (ITS) sequence data and morphological characters. As nomenclature has many practical implications for pathogen identification and regulatory considerations, further clarification of the genetic and pathogenic relatedness of these organisms is needed. Phylogenetic analyses were conducted considering two nuclear and three mitochondrial loci for 21 isolates of P. cubensis and 14 isolates of P. humuli, and all published ITS sequences of the pathogens in GenBank. There was a consistent separation of the majority of the P. humuli isolates and the P. cubensis isolates in nuclear, mitochondrial, and ITS phylogenetic analyses, with the exception of isolates of P. humuli from Humulus japonicus from Korea. The P. cubensis isolates appeared to contain the P. humuli cluster, which may indicate that P. humuli descended from P. cubensis. Host specificity experiments were conducted with two reportedly universally susceptible hosts of P. cubensis and two hop cultivars highly susceptible to P. humuli. P. cubensis consistently infected the hop cultivars at very low rates, with sporangiophores invariably emerged from necrotic or chlorotic hypersensitive-like lesions. Only a single sporangiophore of P. humuli was observed on a cucurbit plant during the course of the studies. Together, molecular data and host specificity indicate that there are biologically relevant characteristics that differentiate P. cubensis and P. humuli that may be obfuscated if P. humuli were reduced to a taxonomic synonym of P. cubensis. Thus, we recommend either retaining the two species names, P. cubensis and P. humuli or considering P. humuli a forma specialis of P. cubensis.