Geographical Variation in Alfalfa Accessions for Resistance to Two Isolates of Peronospora trifoliorum
- Daniel Z. Skinner and
- Donald L. Stuteville
We evaluated the resistance of 631 plant introduction accessions (PIs) of alfalfa (Medicago sativa L.) from 50 countries, to two isolates (I-7 and I-8) of Peronospora trifoliorum de Bary, causal fungus of alfalfa downy mildew. The objectives were to investigate the association of alfalfa subspecies, basic germplasm source, fall dormancy, and regions of origin, with levels of resistance to P. trifoliorum. Differences between mean percentages of resistance to the fungus isolates were tested for statistical significance with t-prime tests, or LSD tests, adjusted in accordance with Bonferroni's inequalities. Five M. sativa subspecies were included: falcata, × varia, sativa, × hemicycla, and caerulea. The mean proportions of plants resistant to one isolate were not significantly different from the proportions resistance to the other isolate within any subspecies. Hence, genetic ancestry of the plants apparently was unrelated to isolate-specific resistance. Mean percentages of resistant plants ranged from 26.8 (subsp. falcata), to 6.3 (subsp. caerulea). Subspecies sativa accessions averaged 15.4% resistant plants. Significant differences between isolates within individual accessions were found in 160 accessions. The PIs with significantly more plants resistant to I-7 than to I-8 were from South and Central America, the Mediterranean area, and Australia. Plant introductions with significantly more plants resistant to isolate I-8 than to I-7 were from northern Europe and North America. This isolate-specific resistance did not appear to be associated with the geographic centers of origin of the basic types of alfalfa germplasms.
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