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Interaction Between Flooding Stress and Phytophthora Root Rot Among Alfalfa Cultivars. A. L. Barta, Professor, Department of Agronomy, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691. A. F. Schmitthenner, Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691. Plant Dis. 70:310-313. Accepted for publication 18 October 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-310.

The association between resistance to Phytophthora megasperma f. sp. medicaginis (P. m. medicaginis) and flooding injury was determined in greenhouse studies using 18 alfalfa cultivars and genotypes with various levels of resistance to P. m. medicaginis. Flooding stress applied for 3 days after zoospore inoculation resulted in as much plant damage as flooding stress applied for 6 days before inoculation. Increased secondary production of zoospores during postinoculation flooding may contribute to this response. Relative responses of cultivars to P. m. medicaginis were very similar when inoculated at either the vegetative (3-wk-old) or flowering (10-wk-old) stage of growth. Significant differences were found in both flood tolerance and P. m. medicaginis resistance among the cultivars. Cultivars resistant to P. m. medicaginis were generally more resistant to flooding stress in the absence of P. m. medicaginis; however, not all flood-tolerant cultivars displayed increased P. m. medicaginis resistance. Because the interaction among genotypes between P. m. medicaginis resistance and flood tolerance was significant, the two stress responses probably are not closely coupled. Ten-week-old plants with shoots removed just before treatment showed significantly more flooding injury and Phytophthora root rot damage than 3-wk-old plants. The stress of shoot removal may predispose the plant to injury from either physical or biological stress.

Keyword(s): Medicago sativa, waterlogging injury.