Involvement of toxins produced by Pseudomonas fuscovaginae in aetiology of rice bacterial sheath brown rot

The incidence of bacterial sheath brown rot of rice was assessed throughout the whole cycle of rice plant cultivation after seedling inoculation, under greenhouse conditions. Typical symptoms of the disease were expressed at the booting. The degree of disease susceptibility could be determined after heading by monitoring panicle emergence rate of diseased plants. Inoculation at the booting stage showed that tolerance could be associated with reduced bacterial growth 4 days after inoculation. Rice plants were sensitive to purified bacterial toxin at all growth stages. The toxins reproduced symptoms observed on diseased plants. In particular, poor panicle emergence, due to inhibition of culm elongation during heading, could be clearly attributed to the detrimental effects of the toxins. For the cultivars investigated in this study, the severity of the toxin damage was related to the degree of cultivar susceptibility to the pathogen, although the phytotoxins are non-host-specific. These phytotoxins can therefore be considered as integral part of the plant pathogen-interaction.