Virulence and Genetic Variability Among Isolates of Mycosphaerella pinodes

Mycosphaerella blight, caused by Mycosphaerella pinodes (Berk. & Bloxam) Vestergr., is the most important foliar disease of field pea (Pisum sativum L.) worldwide (2,7,16,17,23,30,31). In Canada, average dry seed yield losses caused by this disease have been estimated at 10%, and losses of over 50% have been reported in field trials (28,34). Management for Mycosphaerella blight is limited to fungicidal seed treatment, crop rotation (18), and chemical sprays on a preventive and systematic schedule (2). However, each method has deficiencies. The use of resistant cultivars would be the most effective and economical method. Unfortunately, cultivars with high levels of resistance to Mycosphaerella blight are not commercially available. Some promising resistance sources have been identified from pea cultivars and accessions tested in Canada (32,34), New Zealand (17), and the United Kingdom (7). Studies on genetic analysis of resistance to M. pinodes showed that resistance is conferred by one, two, or multiple genes (7,25). A number of pathotypes of M. pinodes have been reported based on their reactions on differential hosts in different countries. For example, 22 pathotypes were identified in Canada (33), 6 in West Germany (22), 6 in Poland (10), and 15 in Australia (1). In these studies, resistance or susceptibility of differential cultivars were determined by host reactions, via a disease severity scale. These contrasting results are most likely the results of not using uniform differential hosts, inoculum concentrations, environmental conditions, inoculation methods, or disease assessment scales. Molecular markers, in combination with pathogenicity tests, have been used to identify pathogen biotypes or races (5,8,24). Pathogen population dynamics, evolutionary relationships between pathogen races (6), geographical distribution and genetic diversity of pathogens (11), and differentiation of pathogen species with similar phenotypic characters (12,14,21) also have been studied using molecular markers. There are many molecular techniques that can be used for assessment of molecular variation in plants and pathogens. Amplified fragment length polymorphism (AFLP) DNA is more efficient in rapidly regenerating genotype data for large numbers of individuals, including plants (13), bacteria (15), and fungi (24). González et al. (11) demonstrated that four AFLP combinations could generate band numbers four times higher than 10 primers of random amplified polymorphic DNA (RAPD), and the number of polymorphic bands was three times higher than that produced by the RAPD primers. Molecular markers have been used in the studies of Ascochyta blight complex on field pea caused by three related species, Ascochyta pisi Lib., M. pinodes, and Phoma medicaginis var. pinodella (Jones) Boerema (23). A. pisi was clearly distinguished from M. pinodes and P. medicaginis var. pinodella using RAPD markers (3) and restriction fragment length polymorphism (RFLP) of rDNA spacers (9). Onfroy et al. (23) distinguished M. pinodes from P. medicaginis var. pinodella using RAPD markers and demonstrated that the two species had low intraspecific genetic variability. Information concerning pathogenicity and genetic variation among isolates of M. pinodes is lacking. Therefore, the objectives of this study were to (i) test virulence of M. pinodes isolates on six differential hosts to determine pathotypes of the isolates tested and (ii) analyze the relationship between virulence and genetic variation among isolates of M. pinodes.