Concerns have been raised by industry members over lower than expected cane yields associated with high oospore levels in sugarcane cultivars rated to have intermediate resistance to pachymetra root rot. This is a significant issue, as intermediate cultivars represent more than 70% of the sugarcane grown in Australia. It is possible that planting the same intermediate cultivar in successive crop cycles could lead to increased yield losses due to pachymetra root rot. This paper examines the residual soil-borne effect of the current major sugarcane cultivars on the following sugarcane crop in three field trials, located in the Herbert, Central and Southern growing regions. Levels of oospores of Pachymetra chaunomiza and cane yields were assessed in ratoon crops of replicated cultivar-assessment trials and in subsequent crops of intermediate-resistant Q208Φ (planted on the sites of previous cultivar trials). The relationships between Pachymetra oospore levels and cane yield in Q208Φ crops and pre-plant oospore levels were examined. High oospore levels occurred in plots planted to some intermediate cultivars, as well as susceptible cultivars. In the following crop of Q208Φ, which was planted into plots of the previous cultivar trial, Pachymetra oospore levels at harvest were related to oospore levels prior to re-planting at all trial sites. Cane yield (t/ha) of Q208Φ was significantly related to pre-plant oospore levels at a site near Bundaberg (P=0.0049). Yield losses of 21% were incurred at 120 oospores/g soil in Q208Φ. Cultivation of Q208Φ following a crop of Q208Φ did not result in higher Pachymetra oospore populations or yield losses compared with planting Q208Φ after other cultivars of similar resistance rating. In the Herbert and Central field trials, Pachymetra oospore levels were lower and there were no significant relationships between oospore levels and yield in Q208Φ planted at these sites. We demonstrate that significant yield losses in Q208Φ are associated with high Pachymetra oospore levels that occur under intermediate and susceptible cultivars in the previous crop. There was no evidence to support the hypothesis that repeatedly planting the same intermediate cultivar could lead to host-cultivar-specific virulence in P. chaunomiza. Greater emphasis should be placed on breeding and selecting highly resistant cultivars that are suited to soil types conducive to Pachymetra.
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