SOME BIOLOGICAL PARAMETERS OF THE SUGARCANE SMUT FUNGUS, USTILAGO SCITAMINEA

SUGARCANE smut, caused by Ustilago scitaminea, is an important disease of sugarcane worldwide. Spores of the smut fungus infect sugarcane plants through buds. Germination and infectivity of spores are influenced by various biological and physical factors. Factors such as age of the spores, infection environment, moisture, temperature and time are important parameters in smut biology. It is important to understand the influence of these biological parameters to develop a successful management strategy. While about 60% of the spores collected from the top section of the whip were viable, only 31% of those from the bottom quarter of the whip (immature section within the leaf sheath) germinated. An in vitro study was conducted to determine the effect of water source (bore, town and deionised water) and substrate (water agar and glass slide) on spore germination. On water agar, spore germination with deionised water was significantly (P>0.05) higher (90%) than with bore water (68%) and town water (69%). Significantly lower germination (<3%) was observed on glass slides compared to water agar regardless of water source. Soluble minerals or chemicals in the bore and town water might have affected the viability of spores. Spores started to germinate after two hours of incubation at 20, 23, 25, 30 and 36°C on water agar, and reached a maximum of 60% to 80% after seven hours. At lower temperature (12°C), spores started to germinate after four hours of incubation, and reached a maximum of 40% after seven hours. At 40°C, only 3% of spores germinated after seven hours of incubation. Four sugarcane varieties, two susceptible (Q205A, Q170A) and two intermediate (Q155 and Q190A), were inoculated by dipping one-eye setts in smut spore suspensions at 31oC or ambient temperature (13oC) for 10 min. All setts were then incubated at 31oC for ten days. The overall disease level was significantly (P<0.05) higher on clones inoculated in the warm suspension (34%) compared to those inoculated at ambient temperature (25%). These results suggest that the fungal spores actively attach or infect the buds during the short 10 min dip inoculation and that warm water is more favourable for the infection process.