| Abstract | Yellow spot is a leaf disease caused by the fungus Mycovellosiella koepkei. Although known in Queensland since 1950, some facets of the disease have never been adequately researched. Yellow spot principally, but not exclusively, affects cane in the wet tropical coastal areas of Queensland. The disease is favoured by warm wet conditions, usually being observed during and shortly after the wet season (March-May). Up to 50,000 ha may be affected in Queensland in years of above average rainfall. Little yield-loss research had previously been conducted and there was a need to review the current breeding strategy to ensure adequate resistance in commercial varieties.Research associated with this project aimed to:1. determine yield losses resulting from the disease;2. assess the current levels of resistance in the Australian germplasm;3. make recommendations regarding the current breeding strategy.Research outputs impinged on each of these areas. Resistance was assessed on the basis of leaf areas diseased at the peak of the disease epidemic (April-May). Assessment of parent canes and commercial cultivars suggested there is less resistance to yellow spot than to the other important diseases in northern Queensland, including Pachymetra root rot and orange rust. This is a cause for concern and warrants consideration by breeders and pathologists. There were only minor differences in the proportion of resistant, intermediate and susceptible parents sourced from each district (Northern, Herbert, Burdekin, Central and Southern).Yield-loss research with fungicides showed that yellow spot may reduce the CCS of susceptible clones by up to 2 units early in the harvest season. In resistant clones, as expected, losses were negligible. Selection trials in the breeding program (FATs) were also utilised to assess yield losses and the influence of clonal resistance on losses. This approach again showed that yellow spot may reduce yields significantly, especially in the wetter years. In the 2001 season, yellow spot was of greater significance than either orange rust or Pachymetra root rot. Losses of up to 20% in cane and sugar yields were observed.An analysis of the conditions required for spore germination showed that high relative humidity (>99%) and relatively high temperatures (20-30?C) were needed for spores of M. koepkei to germinate. This correlates with disease incidence, since it is most severe on the wet tropical coast. An analysis of Bureau of Meteorology data for Cairns, Townsville, Mackay and Brisbane indicated that Cairns was the most favourable site out of these three; again consistent with disease observations. Further studies on environmental conditions favouring the disease, and the analysis of longer-term weather data may facilitate an assessment of the probability of severe disease outbreaks in cane-growing areas. By linking this with yield loss research, it may be possible to accurately identify the level of resistance needed in commercial canes to minimise commercial losses. |
