PROGRESS OF BACTERIAL-BLIGHT ON RICE CULTIVARS CARRYING DIFFERENT XA GENES FOR RESISTANCE IN THE FIELD

Progress of bacterial blight of rice, caused by Xanthomonas oryzae pv. oryzae, was studied in the Philippines by planting six rice cultivars (IR24, IR36, IR54, IR60, IR64, and IR1695) carrying different Xa genes for resistance during the 1988 and 1990 seasons. Four epidemiological parameters, 1) disease incidence, 2) disease severity, 3) area under the disease progress curve (AUDPC), and 4) apparent infection rate (r), were used to rank rice cultivars' resistance to X. o. oryzae. The susceptible check IR24 and resistant check IR1695 differed greatly in reaction to X. o. oryzae in the field during the 2-yr study. In the field, IR36, IR54, IR60, and IR64 showed high levels of resistance to X. o. oryzae compared with IR24. The progress of bacterial blight on these cultivars was significantly lower than that on IR24 as measured by AUDPC. The differences in rate of disease progress between rice cultivars had an effect on grain yield, and the reductions in yields were positively correlated with AUDPC values. High disease severity values were observed in 1988 when rainfall was high. For most rice cultivars, AUDPC values were positively correlated with rainfall, indicating that this factor was influential to development of bacterial blight of rice.