Virulence Diversity of the Common Bean Rust Pathogen Within and Among Individual Bean Fields and Development of Sampling Strategies.

There is a dearth of information on pathogen variation within an individual field. In this study, virulence diversity of Uromyces appendiculatus, cause of bean rust, within individual fields was investigated. From six bean fields in the United States, Honduras, Dominican Republic, and South Africa, 380 U. appendiculatus isolates were differentiated into 65 virulence phenotypes on bean lines containing Andean- and Middle American-derived rust resistance genes. Race variation among bean rust isolates from different geographic regions was found, and virulence phenotypes found in fields from tropical and subtropical regions were more virulent and diverse than those found in fields from temperate regions. The variance components between fields was greater than the variance within a field based on mean disease score on 12 differentials but the variance components within a field were greater than the variances between fields based on number of virulence phenotypes. This is the first report that multiple site samples are needed to represent the fungal virulence diversity in a diseased field. In developing sampling plans, the entire cost of sampling one field is higher than the cost of taking more samples; therefore, to estimate virulence diversity variation, we recommend selecting fewer fields and collecting more samples per field.