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The growth of Xanthomonas oryzae pv. oryzae (Xoo), causal agent of bacterial leaf blight and typical vascular pathogen, was investigated in tissues and guttation fluids of rice leaves to elucidate the mechanisms of susceptibility in the host-parasite interaction. Light and scanning electron-micros observation showed that, in the compatible cultivar-bacterial strain combination, bacterial masses were abundant in the lumen of xylem vessels, but were absent in other vascular elements. The inner surface of spiral and ring vessels where the bacterium multiplied appeared to be digested by the causal organism. Histochemical tests also revealed that the xylem vessel walls were degraded after infection with Xoo. Based on the histological study, the growth of Xoo was examined in guttation fluids collected from rice leaves. Unexpectedly, the growth rate of Xoo was very low in an aqueous solution of freeze-dr guttation fluid. When 0.5% sucrose was added, however, the bacterium could grow extensively in a solution containing freeze-dried guttation fluids. In addition, the growth of Xoo was also promoted by substances in the pectic fraction extracted from cell walls of rice leaves. The results suggested that nutritional factors such as pectic substances from affected xylem vessel walls were involved in the multiplication of Xoo in the lumen of xylem vessels.

One hundred and twenty-nine monoconidial isolates of Pyricularia grisea from rice were collected in Vietnam, mainly in the Mekong River Delta (MRD), in 1995-1996 to characterize their virulence. These isolates were classified into 12 pathogenic groups (races) based on their virulence to 12 Japanese differential rice varieties (modified Kiyosawa's differentials). Race 002.4 was predominant and was found in 10 provinces of MRD, followed by races 106.4, 006.4, 102.4 and 002.0. Differential varieties Aichi Asahi (which carries resistance gene Pia) and K59 (Pit) were susceptible to 93.8% and 86

A survey of geographical distribution of pathogenic races of Xanthomonas oryzae pv. oryzae in Japan was conducted.

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