The Genetic Structure of Pseudoperonospora cubensis Populations

Abstract

Pseudoperonospora cubensis is a destructive foliar pathogen of economically important cucurbitaceous crops in the United States and worldwide. In this study, we investigated the genetic structure of 465 P. cubensis isolates from three continents, 13 countries, 19 states of the United States, and five host species using five nuclear and two mitochondrial loci. Bayesian clustering resolved six genetic clusters and suggested some population structure by geographic origin and host, because some clusters occurred more or less frequently in particular categories. All of the genetic clusters were present in the sampling from North America and Europe. Differences in cluster occurrence were observed by country and state. Isolates from cucumber had different cluster composition and lower genetic diversity than isolates from other cucurbits. Because genetic structuring was detected, isolates that represent the genetic variation in P. cubensis should be used when developing diagnostic tools, fungicides, and resistant host varieties. Although this study provides an initial map of global population structure of P. cubensis, future genotyping of isolates could reveal population structure within specific geographic regions, across a wider range of hosts, or during different time points during the growing season.

Field Response of Cucurbit Hosts toPseudoperonospora cubensisin Michigan

“…'M R-1' was among the most resistant melon cultivars in this study and in a study in South Carolina (11). Differences found in this study in disease response among melon cultigens in Michigan compared with South Carolina could be due to differences in pathogen populations or environmental conditions (21). Our data suggest that there are differences in virulence and patho genicity between Michigan and South Carolina, consistent with hy potheses that these populations are not from the southern United States.…”

“…A previous study found similarities in the genetic cluster composition year to year (2007 to 2009) of P. cubensis populations in the United States and in Michigan, which supports these results (21). It has been proposed that Michigan downy mildew primary pop ulations originate either from the Georgia, South Carolina, and North Carolina areas or from greenhouses located along the border with Canada (18,21).…”

“…In the eastern United States, inocula likely arrive from fields located in Florida or Mexico (10,17). Michigan is the number one producer of pickling cucumber in the United States and it is possible that primary inocula could also come from cucumber production greenhouses in surrounding areas (21).…”

“…Hence, genetic resistance is of paramount importance for economically viable disease management. Due to reported differences in the genetic structure of the pathogen popula tions within and among countries (21), cultigen (cultivars and plant introductions [Pis]) screening on a regional level for downy mildew resistance is imperative. The objectives o f this study were to deter mine the disease response of 41 cultigens to P. cubensis populations naturally present in Michigan between 2010 and 2011, and identify changes in pathogen pathotypes between the 2 years.…”

A genomics perspective on cucurbit-oomycete interactions

“…cubensis, several isolates have been identified, and recent data suggests that the pathogen is continually evolving; this is evidenced by different genotypes of Ps. cubensis observed in various geographic regions of the United States (Lebeda and Widrlechner 2003;Quesada-Ocampo et al 2012). At present, the host range for Ps.…”