Artificial Production of Kernel Bunt of Rice, Caused by Neovossia horrida, Due to Boot Inoculations

Abstract

In Japan, kernel bunt of rice, caused by the fungus Neovossia horrida (Tak.) Padw. & Khan, is usually of minor importance with negligible losses. Occassionally, however, an unusual outbreak of the disease has been noticed locally, where almost all spikes were affected showing very high incidence of the diseased grains on them. Such a sporadic, unusual occurrence of the disease enabled us to assume that some unknown infection mechanism, other than floral infection during anthesis, might operate for the development of the disease. In 1989 and 1990, a series of boot inoculations were made of rice plants (Oryza sativa L.) to see whether or not the spikes in boot stage or just in heading were liable to infection with the fungus. Two ml each of inoculum (culture filtrates of the fungus which contained 2×10⁵ conidia (secondary sporidia)/ml) were injected with a hypodermic syringe into a boot (sheath) at different growth stages (from panicle-formation stage to heading time). The rice plants were grown in pots and kept in a glasshouse throughout the experiments. Inoculations during the period of late boot stage (2 days before heading) to heading time resulted in high rates of infection (9 spikes out of 20 inoculated in 1989 and all 9 spikes inoculated in 1990). Percent diseased grains on each infected spikes were 27.8-60.0 (av. 40.8) and 4.4-55.1 (av. 25.2) respectively in 1989 and 1990. Diseased grains of various symptom types were produced, among which grains showing no apparent fungus structure outside but filled inside with teliospore masses were most numerous (36.3% and 63.2% respectively in 1989 and 1990). On the other hand, inoculations at or before early boot stage (6 days before heading time) were all unsuccessful. None of the uninoculated control plants was infected. The success of boot inoculations would make a proof of the previously reported effectiveness of chemicals sprayed on rice plants at boot stage in controlling the disease.