No abstract

No abstract

Plant breeders conduct a range of yield trials each year to estimate the yielding potential of sugarcane clones progressing through the breeding program. Only the highest-yielding clones are selected for further testing with very small numbers being released as commercial cultivars. Disease susceptibility varies greatly amongst the tested clones and a number of diseases influence the yield of clones in various stages of the selection process. Disease resistance testing is an important routine aspect of the breeding program. All clones for northern Queensland are screened for disease resistance, while selected clones from other areas are tested for resistance to Pachymetra root rot. Two new terms are introduced: resistance index (RI) and the yield loss resistance index (YLRI). Analyses were conducted relating yielding ability of clones in stage 3 trials to Pachymetra resistance. Pachymetra root rot on average reduced tonnes cane per hectare by 15.8% and tonnes sugar per hectare by 10.2%. There was a slight positive effect on commercial cane sugar. YLRI5 for tonnes cane was 3.5 and for tonnes sugar 5.7. With a RI of 3.7, the current breeding strategy for northern Queensland appears appropriate. The data reported here will be valuable for refining selection strategies to improve breeding efficiencies. These analyses could be undertaken each year using data from all breeding trials throughout Queensland, not only with Pachymetra root rot, but also with other diseases normally endemic in cane fields. The advantage of this technique is that with a minimum of further expenditure, ongoing estimates of disease-induced yield losses can be obtained with the information guiding the selection program.

Plant breeders in the Australian sugar industry conduct yield assessment trials each year to assess the yielding ability of clones in the sugarcane breeding programme. Several endemic diseases impinge on the yield of these clones and the tested clones vary greatly in disease susceptibility. In this study, resistance to the diseases orange rust and yellow spot was assessed in final stage trials in the Northern Queensland programme. Clonal yielding ability was related to disease resistance. The results indicate that both diseases, but particularly yellow spot, influenced the yield (tonnes cane/ha and tonnes sugar/ha) of clones in northern breeding trials in 2000. Yield loss estimates were calculated, as well as the relationship between resistance and yield. There was a high level of resistance to orange rust in clones in these trials but much less resistance to yellow spot; the resistance index or orange rust was 2.2 while for yellow spot it was 5.5. Yield loss resistance index values of 5.0 and above for orange rust suggest there is adequate resistance in clones to minimise losses from this disease. In contrast, the yield loss resistance index for yellow spot (tonnes cane/ha) was below 5.0, therefore, it is concluded that during the 2000 harvest season, there was inadequate resistance to minimise losses. The information gathered from this research will be used to determine the level of leaf disease resistance needed in commercial cultivars to optimise yielding ability. Such decisions should improve the efficiency of selection and the performance of commercial cultivars in the Australian sugar industry.

Summary. The growth of sugarcane in soils from land monocultured with sugarcane, and from land which had either never been cropped with sugarcane, or just recently cropped, was compared under glasshouse conditions. In general, cane growth in new land soils was greater than in monocultured soil (shoot growth 7.4%, root growth 21.4%). Responses to soil pasteurisation were investigated in some soils and were greater in monocultured soils suggesting that root growth constraints were larger in the monocultured soil (210% response in monocultured soils v. 64% in new land soils). Assays for sugarcane root pathogens suggested that Pachymetra chaunorhiza was a major contributor to the old/new land growth responses, but it is unlikely that Pythium spp. were factors in the growth differences. Monitoring of other groups of organisms in soil from one site suggested that sugarcane monoculture may affect populations in the broader biological community.