Pathotype characterization of Aphanomyces euteiches isolates collected from pea breeding nurseries

Introduction

Aphanomyces euteiches Drechsler is an oomycete pathogen that affects legume crops, causing root rot, a severe disease of peas (Pisum sativum L.) worldwide. While significant research progress has been made in breeding pea-resistant varieties, there is still a need for a deeper understanding of the diversity of pathogen populations present in breeding nurseries located in various legume-growing regions around the world.

Methods

We analysed the diversity of 51 pea-infecting isolates of A. euteiches, which were recovered from four American (Athena, OR; Le Sueur, MN; Mount Vernon, WA; Pullman, WA) and three French (Riec-sur-Belon, Templeux-le-Guérard, Dijon) resistance screening nurseries. Our study focused on evaluating their aggressiveness on two sets of differential hosts, comprising six pea lines and five Medicago truncatula accessions.

Results

The isolates clustered into three groups based on their aggressiveness on the whole pea set, confirming the presence of pathotypes I and III. Pathotype I was exclusive to French isolates and American isolates from Athena and Pullman, while all isolates from Le Sueur belonged to pathotype III. Isolates from both pathotypes were found in Mount Vernon. The M. truncatula set clustered the isolates into three groups based on their aggressiveness on different genotypes within the set, revealing the presence of five pathotypes. All the isolates from the French nurseries shared the same Fr pathotype, showing higher aggressiveness on one particular genotype. In contrast, nearly all-American isolates were assigned to four other pathotypes (Us1, Us2, Us3, Us4), differing in their higher aggressiveness on two to five genotypes. Most of American isolates exhibited higher aggressiveness than French isolates within the M. truncatula set, but showed lower aggressiveness than French isolates within the P. sativum set.

Discussion

These results provide valuable insights into A. euteiches pathotypes, against which the QTL and sources of resistance identified in these nurseries displayed effectiveness. They also suggest a greater adaptation of American isolates to alfalfa, a more widely cultivated host in the United States.

Inoculum dose-disease response relationships for the pea root rot pathogen, Aphanomyces euteiches, are dependent on soil type and other pathogens

The oomycete pathogen, Aphanomyces euteiches, was implicated for the first time in pea and lentil root rot in Saskatchewan and Alberta in 2012 and 2013. Subsequent surveys from 2014 to 2017 revealed that Aphanomyces root rot (ARR) was widespread across the Canadian prairies. The absence of effective chemical, biological, and cultural controls and lack of genetic resistance leave only one management option: avoidance. The objectives of this study were to relate oospore levels in autoclaved and non-autoclaved soils to ARR severity across soil types from the vast prairie landscape and to determine the relationship of measured DNA quantity of A. euteiches using droplet digital PCR or quantitative PCR to the initial oospore inoculum dose in soils. These objectives support a future end goal of creating a rapid assessment method capable of categorizing root rot risk in field soil samples to aid producers with pulse crop field selection decisions. The ARR severity to oospore dose relationship was statistically significantly affected by the soil type and location from which soils were collected and did not show a linear relationship. For most soil types, ARR did not develop at oospore levels below 100/g soil, but severity rose above this level, confirming a threshold level of 100 oospores/g soil for disease development. For most soil types, ARR severity was significantly higher in non-autoclaved compared to autoclaved treatments, demonstrating the role that other pathogens play in increasing disease severity. There was a significant linear relationship between DNA concentrations measured in soil and oospore inoculum concentration, although the strength of the relationship was better for some soil types, and in some soil types, DNA measurement results underestimated the number of oospores. This research is important for developing a root rot risk assessment system for the Canadian prairies based on soil inoculum quantification, following field validation of soil quantification and relationship to root rot disease severity.

Aggressiveness of Diverse French Aphanomyces euteiches Isolates on Pea Near Isogenic Lines Differing in Resistance Quantitative Trait Loci

Aphanomyces root rot is a major disease in many pea growing regions worldwide. Development of resistant varieties is necessary to manage the disease. Near isogenic lines (NILs) carrying resistance alleles at main quantitative trait loci (QTLs) were developed by marker-assisted backcrossing. This study aimed to evaluate the aggressiveness of diverse French isolates of Aphanomyces euteiches on NILs carrying different resistance QTLs. Forty-three A. euteiches isolates from different French pea growing regions were tested for aggressiveness on eight NILs carrying single or combinations of resistance QTLs and two susceptible or resistant control lines, in controlled conditions. Three clusters of isolates, unrelated to geographical origin, were identified, including 37, 56, and 7% of isolates with high, moderate, and low average levels of aggressiveness, respectively. Three groups of pea lines were also identified. The first group consisted of a pea resistant control line, moderately to highly resistant to all of the isolates. The second group included five NILs carrying a major-effect resistance allele at QTL Ae-Ps7.6, with a medium to broad range of effects on the isolates. The third group consisted of three NILs carrying minor-effect resistance alleles, with a narrow range of effects on the isolates. The results suggest that highly aggressive isolates occur naturally, which may be selected by future partially resistant pea varieties carrying QTLs and increase the risk of erosion of QTL effect. QTL pyramiding strategies for a higher level and a broader range of effect of quantitative resistance on A. euteiches populations will be required for breeding for durable pea resistant varieties.

Genetic and Pathogenicity Diversity of Aphanomyces euteiches Populations From Pea-Growing Regions in France

Aphanomyces euteiches is an oomycete pathogen with a broad host-range on legumes that causes devastating root rot disease in many pea-growing countries and especially in France. Genetic resistance is a promising way to manage the disease since consistent QTL controlling partial resistance have been identified in near isogenic lines of pea. However, there are still no resistant pea varieties cultivated in France. This study aimed to evaluate the phenotypic and genetic diversity of A. euteiches populations from the major pea-growing regions in France. A collection of 205 isolates, from soil samples collected in infested pea fields located in five French regions, was established and genotyped using 20 SSR markers. Thirteen multilocus genotypes were found among the 205 isolates which displayed a low genotypic richness (ranged from 0 to 0.333). Two main clusters of isolates were identified using PCoA and STRUCTURE, including a predominant group comprising 88% of isolates and another group representing 12% of isolates mainly from the Bourgogne region. A subset of 34 isolates, representative of the fields sampled, was phenotyped for aggressiveness on a set of resistant and susceptible varieties of four legume hosts (pea, faba bean, vetch, alfalfa). Significant differences in disease severity were found among isolates and three groups of aggressiveness comprising 16, 17, and 2 isolates, respectively, were identified using HCA analysis. A higher diversity in pathogen aggressiveness was observed among isolates from Bourgogne, which included different legumes in its crop history. Little relationship was observed between genetic clusters and pathogenicity in the subset of 34 isolates, as expected using neutral markers. This study provides useful knowledge on the current state of low to moderate diversity among A. euteiches populations before resistant pea varieties are grown in France. New insights and hypotheses about the major factors shaping the diversity and evolution of A. euteiches are also discussed.

Inoculation with Nonpathogenic Fusarium solani Increases Severity of Pea Root Rot Caused by Aphanomyces euteiches

Aphanomyces euteiches is an important root-rotting pathogen of pea. When recovering isolates of A. euteiches from infested soils in Wisconsin using pea as a bait host, isolates of Fusarium solani often were recovered. Experiments were established to compare disease symptoms of pea seedlings inoculated with isolates of A. euteiches and F. solani alone or in combination. Inoculation of pea seedlings with either of two isolates of A. euteiches produced typical root rot symptoms. However, inoculation of pea seedlings with an isolate of F. solani resulted in no disease symptoms, indicating that the isolate was nonpathogenic to pea. Co-inoculation of pea seedlings with A. euteiches and the nonpathogenic isolate of F. solani resulted in significantly (P = 0.05) greater disease severity than inoculation with A. euteiches alone. Both A. euteiches and F. solani could be reisolated, individually or together, from pea seedlings following individual or co-inoculations, respectively. Although the mechanisms of interaction between these two species are unknown, the synergism documented in this study indicates that the interactions of pathogens with nonpathogens may affect development of disease symptoms.