Effects of plant age on symptom development produced by Pyricularia oryzae toxin

Main objective of this experiment was to determine suitable plant age for toxin inoculation on detached leaves and whole plant conditions. For detached leaves inoculation, it was found that 20 day old plants were the most sensitive to both crude extract toxin (CET) derived from culture filtrate of P. oryzae and tenuazonic acid. The degree of sensitivity was manifested by yellowing produced around the lesions of inoculated leaves. Forty day old plants were found to be the most appropriate age for CET inoculation as they produced typical blast symptom while tenuazonic acid could produce similar typical blast symptoms on leaves at all plant ages. The reactions to CET were varied to the different plant ages while the reactions to tenuazonic acid were not significantly different among them. The experimental result indicated that the lesion sizes produced on blast susceptible and resistant varieties upon CET and tenuazonic acid were not significantly different. In general, leaves exhibited higher sensitivity to CET than tenuazonic acid. It probably might be due to the concentration of the former higher than the latter. CET at concentration of 25 percent was the most appropriate for spray inoculation on 2-3 week-old seedlings.