AN IMPROVED INFILTRATION TECHNIQUE TO TEST THE PATHOGENICITY OF XANTHOMONAS-ORYZAE PV ORYZAE IN RICE SEEDLINGS

When testing the pathogenicity of Xanthomonas oryzae pv. oryzae by the standard leaf clipping technique, we normally observed a bleached, hy persensitive-like reaction preceding a general yellowing or browning o f the tissue. Water-soaked lesions were seldom observed, even on plant s in a lighted dew chamber at 100% RH. However, by using a plastic dis posable 1.0-ml syringe modified to infiltrate rice leaves, water-soake d lesions developed in 5-7 days after infiltration with X. oryzae. Exu date was often evident after 8-10 days. The infiltration technique was compared to the leaf clipping technique by inoculating plants of rice cv IR-24 with six strains of X. o. pv. oryzae from Asia, three strain s of X. o. pv. oryzae (X1-5, X1-8, and X37-2) from Texas, four strains of X. o. pv. oryzicola, and two unknown yellow-pigmented bacteria (is olated from California rice seeds) in a commercial, lighted dew chambe r. Inoculation by the clipping technique usually resulted in bleached or tan-colored lesions regardless of the organism used. Using the infi ltration technique, narrow, dark green, water-soaked lesions consisten tly developed with all strains of X. o. pv. oryzae and X. o. pv. oryzi cola from Asia, but not with the Texas strains or the unknown bacteria . This simple infiltration technique should be very useful for the acc urate determination of the pathogenicity of rice xanthomonads.