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Effect of Plant Age and Leaf Maturity on the Quantitative Resistance of Rice Cultivars to Xanthomonas campestris pv. oryzae. M. F. Koch, Department of Plant Breeding, Agricultural University, P.O. Box 386, Wageningen, Netherlands, and T. W. Mew, Division of Plant Pathology, International Rice Research Institute, P.O. Box 933, Manila, Philippines. Plant Dis. 75:901-904. Accepted for publication 4 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0901.

Resistance of rice (Oryza sativa) cultivars to bacterial blight, caused by Xanthomonas campestris pv. oryzae, as assessed by lesion length, increased considerably with plant age. The fastest increase occurred between 30 and 50 days after sowing. Quantitative resistance was evident at all growth stages as a reduction in lesion length relative to susceptible cultivars. Lesions of all cultivars decreased about equally in absolute length with plant age, but the relative decrease in lesion length was greater with moderately resistant cultivars than with highly susceptible cultivars. The change in quantitative resistance with increasing plant age is different from that of adult plant resistance to X. c. oryzae caused by the Xa-3 gene for resistance. Immature leaves that were still extending were more susceptible than adjacent mature, fully extended leaves, but immature leaves of moderately resistant cultivars were less susceptible than those of highly susceptible cultivars. To compare entries of different maturation dates, lesion length should be assessed when plants are between maximum tillering and flowering.