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Effect of Sugarcane Cultivars and Location on Inoculum Density of Pachymetra chaunorhiza in Queensland. R. C. MAGAREY, Bureau of Sugar Experiment Stations, P.O. Box 566, Tully, Queensland, Australia. C. M. MEWING, Bureau of Sugar Experiment Stations, P.O. Box 566, Tully, Queensland, Australia. Plant Dis. 78:1193-1196. Accepted for publication 20 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1193.

The influence of sugarcane cultivars on the inoculum density of Pachymetra chaunorhiza was assessed by monitoring the changes in numbers of oospores of P. chaunorhiza in the soil of field plots by a direct count assay. Cultivars with different levels of disease resistance were planted at each of six sites in a district where Pachymetra root rot was known to occur. Inoculum densities were quantified before planting and after either 2 or 3 yr of cropping. The effects of cultivar, site, and the interaction between site and cultivar were examined by analysis of variance. All three parameters were significant, suggesting that cultivars have a measurable effect on inoculum density and that this effect varies with location. Field ratings were assigned on the basis of changes in soil inoculum density and were compared with glasshouse ratings for each cultivar. The correlation between glasshouse and field ratings was high (r2 = 0.59, and P< 0.001). Monitoring changes in soil inoculum density in fallow plots enabled the natural rale of attrition of oospores in the absence of the host to be assessed. Oospores survived in fallow plots for at least 5 yr.