Jill E. Demers,
Jo Anne Crouch, and
Lisa A. Castlebury, Systematic Mycology and Microbiology Laboratory, United States Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705
Puccinia horiana, the cause of chrysanthemum white rust, is a regulated fungal plant pathogen in the United States, while P. chrysanthemi, the cause of chrysanthemum brown rust, is a widespread but less destructive pathogen. Accurate identification is essential to enforce quarantine measures, but the two species cannot be differentiated visually in the absence of mature spores or symptoms. A multiplex real-time PCR assay was developed to detect and discriminate between P. chrysanthemi and P. horiana. Species-specific hydrolysis probes labeled with different fluorescent dyes were designed based on the rDNA internal transcribed spacer region. Seven fresh samples and 270 herbarium specimens of chrysanthemum rust were tested with the assay with results confirmed using spore morphology. P. horiana and P. chrysanthemi were accurately detected from all fresh samples, and as little as 1 pg of template DNA was reproducibly detected. Of the herbarium specimens, 99% were positive for at least one species using the multiplex assay with 7% positive for both species. This multiplex assay can discriminate between P. chrysanthemi and P. horiana and provides an additional tool for identification of P. horiana to ensure appropriate application of quarantine measures.