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, hypersensitive-like reaction preceding a general yellowing or browning of the tissue. Water-soaked lesions were seldom observed, even on plants in a lighted dew chamber at 100% RH. However, by using a plastic disposable 1.0-ml syringe modified to infiltrate rice leaves, water-soaked lesions developed in 5-7 days after infiltration with X. oryzae. Exudate 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 strains 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 (isolated from California rice seeds) in a commercial, lighted dew chamber. Inoculation by the clipping technique usually resulted in bleached or tan-colored lesions regardless of the organism used. Using the infiltration technique, narrow, dark green, water-soaked lesions consistently developed with all strains of X. o. pv. oryzae and X o. pv. oryzicola from Asia, but not with the Texas strains or the unknown bacteria. This simple infiltration technique should be very useful for the accurate determination of the pathogenicity of rice xanthomonads.