Influence of Temperature on Expression of Resistance to Rust in Diploid Alfalfa

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Abstract
Some alfalfa (Medicago sativa L.) plants resistant to the rust fungus (Uromyces striatus Schroet.) in the field have proven susceptible under laboratory conditions. These differences have been attributed to environmental conditions suboptimal for rust development in the field. Our objectives were to study the effects of temperature on resistance to rust in diploid alfalfa. We used a six‐parent diallel cross to measure the inheritance of factors that influence latent period at 20, 25, and 30 °C. Infection‐type data and latent period data were highly correlated (r = 0.97, P < 0.0001). Pustules developed on 37% of all S1 and F1 plants (combined) at 20 °C, on 50% at 25 °C, and on 30% at 30 °C. However, when plants in tests completed at 20 or 30 °C were moved to 25 °C, pustules developed on additional plants until the percentages of plants with pustules were not significantly different (P > 0.05) from those of plants held continuously at 25 °C. The degree of dominance was 0.59, 0.32, and 0.98 at 20, 25, and 30 °C, respectively; thus, resistance appeared most dominant at temperatures suboptimal for disease development. These results indicated that temperature altered the genetic behavior of factors involved in alfalfa resistance to rust, and that the optimum temperature for rust development with the materials we used was about 25 °C.