Genetic and Pathogenic Relatedness of Pseudoperonospora cubensis and P. humuli

Abstract

The most economically important plant pathogens in the genus Pseudoperonospora (family Peronosporaceae) are Pseudoperonospora 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 characteristics. Nomenclature has many practical implications for pathogen identification and regulatory considerations; therefore, 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, and 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 retaining the two species names P. cubensis and P. humuli until the species boundaries can be resolved unambiguously.

Epidemiology and Population Biology of Pseudoperonospora cubensis: A Model System for Management of Downy Mildews

“…Given that physiological specialization in P. cubensis is manifested among species and genera within the Cucurbitaceae, an intriguing but unexplored area is whether physiological specialization similarly occurs in P. humuli beyond Humulus lupulus. If confirmed, this could in part explain conflicting reports of P. humuli infection of cucurbit species (63,91).…”

“…Although these studies suggest that P. cubensis can potentially infect hop, low levels of compatibility in cross-inoculations suggest that each species is best adapted to its primary host. Further, evidence based on genetic markers does not indicate occurrence of either species on the other host (63). Inference of gene flow between the two species will require a large number of loci and more powerful tools for genetic analysis (38).…”

“…The ITS region offers too little resolution of species boundaries (115) and does not consistently differentiate the two species, particularly among isolates derived from Asia (61,63,94). Several of the morphological traits used by Choi et al (15) are also questionable for phylogenetic inference within the genus, given the inability to disentangle morphological divergence from variation due to the host matrix (88,90).…”

“…Host range studies have been variable when hop and cucurbits are challenged with P. cubensis and P. humuli, respectively (44,63,91). In two studies, P. cubensis was able to cause limited infection on hop under controlled conditions (63,91), but no symptoms or signs of infection were observed on Japanese hop (61). Although these studies suggest that P. cubensis can potentially infect hop, low levels of compatibility in cross-inoculations suggest that each species is best adapted to its primary host.…”

Analysis of microsatellites from the transcriptome of downy mildew pathogens and their application for characterization ofPseudoperonosporapopulations

“…Nonetheless, results from studies using more genetic markers support the species distinction between P. humuli and P. cubensis with nrITS, coxII , and ypt1 (Runge, Choi & Thines, 2011) . Furthermore, the idea of two distinct species was supported through phylogenetic analysis of nrITS, B-tub , and cox2 (Mitchell et al, 2011). However, it was found that in laboratory settings, P. cubensis can infect hop and have light sporulation and that P. humuli can infect cucumber with limited success (Mitchell et al, 2011; Runge & Thines, 2012).…”