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Rate of Lesion Expansion in Leaves as a Parameter of Resistance to Xanthomonas campestris pv. oryzae in Rice. M. F. Koch, Department of Plant Breeding, Agricultural University, P.O. Box 386, Wageningen, Netherlands, and T. W. Mew, Department of Plant Pathology, International Rice Research Institute, P.O. Box 933, Manila, Philippines. Plant Dis. 75:897-900. Accepted for publication 4 March 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0897.

Lesions on five rice (Oryza sativa) cultivars clip inoculated with three strains of Xanthomonas campestris pv. oryzae at two plant ages appeared 45 days after inoculation and continued expanding until leaf senescence. A quadratic model accounted for 99% of the variation in the measured values. Mean lesion length and mean daily rate of lesion expansion were highly correlated. Younger plants and susceptible cultivars had higher rates of increase than older plants and moderately susceptible cultivars, respectively. The rate of expansion decreased somewhat with time for strains PXO71 and PXO99 and increased or remained the same for strain PXO86. There were no major differences among cultivars for the change in the rate of lesion expansion over time. Because of this, there were no major changes in the relative levels of resistance of the six cultivars over time. Therefore, multiple scorings are not expected to give extra information about this factor. The observed differences between strains for the change in the rate of lesion development over time may indicate a characteristic of strains of X. c. oryzae worth further study.