Studies on the false smut of rice

Abstract

1) The chlamydospores on the yellow, yellowish-green, greenish-yellow and green smut balls, respectively, which were collected in the preceding year and stored for eight months in the refrigerator and under the laboratory conditions, were employed for inoculation to the coleoptiles of rice variety Norin-mochi No. 1. The germinated seeds of which the coleoptiles developed at 1-3mm in height, were inoculated on the coleoptiles by means of spraying to the germinated seeds with the chlamydospore suspensions (80-100 spores per one optical field (X 900)) or by dipping them in the suspension. The inoculated coleoptiles were kept for 3-5 days at 20°C in 1958 and at 25°C in 1959 and 1960 experiments, then the germinated seeds thus inoculated were planted in fields and grown till the mature stage of rice plant.2) The infection occurred most readily when inoculation was made on the seedlings at 1-10mm in height with the yellow spores, while no infection occurred at 12mm or more. The greenish-yellow spores were also virulent to the coleoptiles of 3-6mm in height, but not infections to those of 8mm or more. The percentage of infection was much higher when the coleoptiles were inoculated at very early stages of growth. The pathogenicity of chlamydospores was the highest in the yellowish-green spores, considerable in the greenish-yellow and the yellow spores, but very limited in the green spores. So far as the yellow spores were concerned, the percentage of infection was the highest when inoculated with the concentration of 400 spores per one optical field (X 900); it diminished with the decrease of spore concentration, then no infection was secured with that of 40 spores. As to the temperature condition at the time of inoculation, 25°C proved most favourable. Inoculation by means of the dressing resulted in lower percentage of infection in comparison to that resultant from the spraying or the dipping.3) The percentage of infection due to seedling inoculation on the respective stems of a plant was the highest in the ears which emerged from the mother stems. Smut balls in the infected ears were most numerous in the somewhat upper part above the base, next in the basal part, but very few in the part from middle to apical. Above 50% in the affected ears only a single smut ball per ear, a considerable number in the ears bore 2-4 smut balls, and a few more than 5, but seldom 15, 16, or 19. The number of cluster of smut balls in an infected ear increased with increase of smut balls per ear. The percentage of sclerotia-bearing smut balls was 23.3%. From the above-noted results on infection due to the inoculation, the percentage of infection by the order of tillering, the distribution and occurrence of cluster of smut balls in an ear was seen to be the same as those due to natural infection in upland rice.