Effect of Quantitative Resistance in Wheat on the Development of Puccinia striiformis During Early Stages of Infection. L. H. M.Broers, International Maize and Wheat Improvement Center, Lisboa 27, Apdo Postal 6-641, 06600 Mexico DF, Mexico. R. M. Lopez-Atilano, International Maize and Wheat Improvement Center, Lisboa 27, Apdo Postal 6-641, 06600 Mexico DF, Mexico. Plant Dis. 80:1265-1268. Accepted for publication 18 July 1996. Copy right 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1265.

Flag leaves of three quantitatively resistant wheat cultivars and one highly susceptible wheat cultivar were inoculated with urediospores of Puccinia striiformis to study the effect of quantitative resistance on development of the fungus during the first 6 days of the infection cycle. The results indicated that the most important mechanism of resistance is reduction in the frequency of formation of appressoria. On resistant cultivars, twice as many germ tubes failed to produce appressoria and grew over stomata than on the susceptible cultivar. Of the infection structures that oriented themselves on stomata, most were able to form substomatal vesicles. In resistant cultivars, the number of infection sites that developed substomatal vesicles decreased over time. This result suggests a resistance mechanism that disintegrates substomatal vesicles. In addition, the formation of primary infection hyphae is delayed considerably in resistant cultivars. Although large cultivar differences for quantitative resistance have been observed at the cellular level, these cannot fully explain differences al the plant level or field level. Therefore, events that happen after the first 6 days of fungal development, like reduced growth and late abortion, may further explain differences in level of resistance among the quantitatively resistant wheat genotypes.