Effects of Freeze Damage on Soybean Seed Mycoflora and Germination. J. A. Osorio, Department of Plant Pathology and Seed Science Center, Iowa State University, Ames 50011. D. C. McGee, Department of Plant Pathology and Seed Science Center, Iowa State University, Ames 50011. Plant Dis. 76:879-882. Accepted for publication 23 February 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0879.

Pods of soybean (Glycine max) plants grown in three environments (pots under greenhouse conditions, pots exposed to weather, or field plantings) were inoculated at growth stage R5 with isolates of Fusarium graminearum, Alternaria alternata, or Phomopsis longicolla. The applied fungus became the dominant colonist of the pods in all environments. Plants grown in pots were exposed to freezing at –4.5 or –2.5 C at growth stage R6 under controlled-environment conditions, and the latest of three field plantings experienced a natural frost of –2.0 C at the same growth stage. On inoculated plants, freezing was associated with increased seed infection by F. graminearum and A. alternata and with reduced seed infection by P. longicolla. Pod inoculations by F. graminearum and A. alternata caused greater reductions in seed germination than that which occurred in noninoculated plants when plants were subjected to freezing at –4.5 C. These fungi, however, had little effect on seed germination in plants not subjected to freezing. Inoculum of P. longicolla reduced seed germination for both frozen and nonfrozen plants in all environments; however, there was no evidence that damage from this fungus was exacerbated by freezing. Survey data indicated that Fusarium, Alternaria, and Phomopsis spp. are common residents of soybean pods in Iowa.