Aspects of quantitative resistance to Xanthomonas campestris pv. oryzae in rice

Quantitative resistance (QR) to Xanthomonas campestris pv. oryzae (Xco), the causal organism of bacterial blight in rice, is evident as reduced lesion length and slowed lesion development relative to lesions in susceptible cultivars. QR was evident at all growth stages and leaf ages as a reduction in lesion length following clip inoculation with virulent isolates of the pathogen. QR affects the ability of bacterial cells to establish and multiply within a leaf. No confirmed evidence of cultivar x isolate interactions of QR factors was found. Varying levels of incomplete race-specific major gene resistance were found and were difficult to distinguish from differences in QR. Differences between cultivars for lesion number following spray inoculation varied from the differences in lesion length following clip inoculation, indicating the presence of an extra component of resistance limiting bacterial entry into intact leaves.Multiple QR resistance factors were shown to be present in four moderately resistant cultivars. A minimum of five factors was found in varying combinations in six rice cultivars. The residual effect of the Xa-4 gene for resistance, or a closely linked gene, also imparted some level of QR. Selection using one isolate was effective in improving the level of QR to another isolate.