Abstract

Villosiclava virens (anamorph: Ustilaginoidea virens) is the causal agent of rice false smut, a fungal disease that occurs worldwide; however, little information on the basic biology of the fungus is available. In our study, we examined the effect of the culture medium, carbon and nitrogen source, temperature, pH, water potential and light on mycelial growth and the effects of temperature and wetness duration on conidial germination, and identified the conidial lethal temperature. Potato sucrose agar (PSA) was the best medium for fast mycelial growth, and wakimoto toceshi (XBZ) and potato dextrose agar also favored mycelial growth, whereas Czapek agar was not suitable. Sucrose (2.6 mm d^-1) and starch (2.2 mm d^-1) were the best carbon sources, and ammonium nitrate (2.1 mm d^-1), ammonium sulfate (2.2 mm d^-1) and ammonium chloride (2.2 mm d^-1) were the most suitable nitrogen sources for mycelial growth. The fungus was able to grow at temperatures from 12 to 32°C, with the optimal mycelial growth occurring between 28 and 30°C. The average radial growth rate on PSA was 2.5 mm d^-1at 28°C and 2.4 mm d^-1 at 30°C. Active mycelial growth was observed at pH between 4.5 and 11, and optimal growth was observed at pH 7 to 8. The fungus was able to grow at a water potential as low as -6 Mpa but did not grow at -8 Mpa. The mycelial growth rate was significantly higher in the dark than in 12 h alternating cycles of dark and fluorescent light, and fluorescent light inhibited mycelial growth. Conidial germination occurred at temperatures from 12 to 34°C, and the optimal temperature for germination appeared to be 28 and 30°C for which a minimum wetness period of 2 h was required. The lethal temperature of the conidia was 50°C. These results provided a better understanding of the biology and ecology of V. virens and the environmental conditions required for the infection of rice.

Key words: Ustilaginoidea virens, mycelial growth, conidial germination, wetness duration, lethal temperature