FiveStemphyliumspp. Pathogenic to Alfalfa: Occurrence in the United States and Time Requirements for Ascospore Production

Abstract

Stemphylium isolates from alfalfa from 10 states in the U.S. were identified according to new taxonomic criteria. Seventeen of the 22 isolates were S. alfalfae/Pleospora alfalfae from California, Washington, Idaho, Utah, Kansas, Wisconsin, and New York. The other five isolates were S. botryosum/P. tarda from Minnesota, Wisconsin, Pennsylvania, and New York. All isolates produced similar symptoms on excised alfalfa leaves in tests to confirm pathogenicity. The time required for ascospore production, which has become more important in recent taxonomic revisions of Stemphylium/Pleospora, was determined under standardized conditions for isolates of five Stemphylium spp. obtained from E. G. Simmons. Culture plates of 0.1 strength potato-dextrose agar were seeded with conidia from 14-day-old cultures, sealed with Parafilm, and incubated under an 8-h photoperiod of 40-65μE.m −2 .s −1 of cool-white fluorescent lighting at 15±2 C. The numbers of days after dishes were seeded with conidia until mature ascospores were produced were: 12.7±3.3 for S. alfalfae/P. alfalfae, 26.4±3.3 for S. globuliferum/Pleospora sp., 27.3±2.8 for S. vesicarium/Pleospora sp., 28.9±2.5 for S. herbarum/P. herbarum, and 62.6±2.1 for S. botryosun/P. tarda. The time requirements for ascospore production determined under these standardized conditions complemented morphological taxonomic features in distinguishing some of the Pleospora pathogens of alfalfa, especially P. alfalfae and P. tarda, the two most common species in the U.S. These conditions also reduced ascospore production time for P. tarda to 2 mo