Further Contributions to the Development of a Differential Set of Pea Cultivars (Pisum sativum) to Investigate the Virulence of Isolates of Aphanomyces euteiches

Common root rot (Aphanomyces euteiches Drechs.) has become a very destructive disease in the French pea crops since 1993. For an accurate investigation of the virulence variability among French A. euteiches populations and between French and foreign populations, a new set of differential pea genotypes was developed. Thirty-three American and European pea lines, displaying different levels of resistance, were screened in a growth chamber against two French isolates. Symptoms (disease severity from 0 to 5, evaluating symptom surface on roots and epicotyl) and percentage of top fresh weight (inoculated/uninoculated top fresh weight ratio) were measured. From this screening 12 relatively resistant lines, from various genetic backgrounds, were identified along with a highly susceptible control. This set of 13 genotypes was inoculated under controlled conditions with 14 isolates from France, Sweden, USA, Canada and New Zealand, to investigate genotype–isolate interactions. Root symptoms were rated (disease severity), and a susceptibility/resistance threshold was established at disease severity = 1. Significant quantitative interactions were observed, and five 'resistance patterns' were identified, leading to a set of six pea genotypes: Baccara (susceptible), Capella, MN313, 902131, 552 and PI180693. Fields trials of this set in 1999 and 2000 gave the same resistance rankings than in growth chamber conditions. This set will allow more accurate assessments of the variability in virulence/aggressiveness of A. euteiches isolates from France and foreign countries, and further investigations of the epidemiological and genetic basis of pea–A. euteiches interactions.