Studies on the bacterial diseases of Sudan crops

SUMMARY Xanthomonas phaseoli survived for more than 18 months in press‐dried dolichos bean leaves kept in the laboratory. Survival of the organism in pure culture, or in leaf trash mixed with the soil, depends on soil moisture, being longest in air‐dried and shortest in water‐saturated soil. In completely water‐saturated, or in completely air‐dry soil, no effect of soil autoclaving is observed. In moistened soil survival is longer in autoclaved than in untreated soil. In the absence of demonstrable phage activity, disappearance of the organism in water‐saturated soil is attributed to prevailing anaerobic conditions, and in moistened soil to the action of competitive saprophytes. In air‐dry soil the organism is preserved by drying. The bacteria reach the seed, via the placenta, when young pods, less than 2 cm long, become infected by artificial inoculation: they do not invade the vascular tissues. Survival of the bacteria in the seed is considered of a relatively little importance in practice. Seedling infection is characterized by elongated brown lesions on the veins of the simple and first and second trifoliate leaves; those parts of the lamina dependent on affected veins become chlorotic and flaccid. Vein lesions show the pathogen in the vascular bundles, which exhibit no tissue differentiation. A strain of X. phaseoli from Vigna radiata (weed plant) also infects dolichos bean. Its minimal infective dose on Dolichos is considerably higher than that of the dolichos bean strain. The dolichos bean disease can be transmitted by heavy infestation of the whitefly Bemisia tabaci.