EFFECT OF SUGARCANE CULTIVARS AND LOCATION ON INOCULUM DENSITY OF PACHYMETRA-CHAUNORHIZA IN QUEENSLAND

The influence of sugarcane cultivars on the inoculum density of Pachym etra chaunorhiza was assessed by monitoring the changes in numbers of oospores of P. chaunorhiza in the soil of field plots by a direct coun t assay. Cultivars with different levels of disease resistance were pl anted at each of six sites in a district where Pachymetra root rot was known to occur. Inoculum densities were quantified before planting an d after either 2 or 3 yr of cropping. The effects of cultivar, site, a nd the interaction between site and cultivar were examined by analysis of variance. All three parameters were significant, suggesting that c ultivars have a measurable effect on inoculum density and that this ef fect varies with location. Field ratings were assigned on the basis of changes in soil inoculum density and were compared with glasshouse ra tings for each cultivar. The correlation between glasshouse and field ratings was high (r(2) = 0.59, and P < 0.001). Monitoring changes in s oil inoculum density in fallow plots enabled the natural rate of attri tion of oospores in the absence of the host to be assessed. Oospores s urvived in fallow plots for at least 5 yr.