No abstract

Sugarcane is an important Australian crop earning over $1.8 billion in export revenue annually. The crop is grown as a monoculture, and much of the production area has been continually cropped for over 60 years. Increasing production trends plateaued in the 1970s and soil based constraints now reduce industry income by $200~1 annually. The condition, termed sugarcane yield decline (YD), is similar to replant diseases in other crops. Large growth responses to soil fumigation, soil solarisation, and the application of fungicides, suggest that soil microbiology is intimately involved. Research has identified a previously unclassified oomycete, Pachymetra chaunorhiza, as a new sugarcane root pathogen. Additional root pathogens identified include Pythium arrhenomanes and various nematode species. Other organisms which appear to be involved in YD, are discussed.

The presence of vesicular-arbuscular mycorrhizal (VAM) fungi in long-term cane-growing fields associated with yield decline led to the supposition that VAM fungi may be responsible for the poor yields. A glasshouse trial was established to test the effectiveness of a species of VAM fungi, Glomus clarum, extracted from one of these North Queensland fields on the growth of sugarcane (Saccharum interspecific hybrid), maize (Zea mays), and soybean (Glycine max) for 6 phosphorus (P) rates (0, 2.7, 8.2, 25, 74, 222 mg/kg). For maize and soybean plants that received VAM (+VAM), root colonisation was associated with enhanced P uptake, improved dry weight (DW) production, and higher index tissue-P concentrations than those without VAM (–VAM). By comparing DW responses of maize and soybean for different P rates, savings in fertiliser P of up to 160 and 213 kg/ha, respectively, were realised. Sugarcane plants were generally less responsive. Apart from a 30% DW increase with VAM when 2.7 mg P/kg was added, DW of +VAM plants was equivalent to, or worse than in the case of 222 mg P/kg, DW of –VAM plants. For all 3 host species, colonisation was least at the highest P application, presumably from excessive P within the plant tissue. Critical P concentrations for the 3 host species were below those reported elsewhere, and for soybean and sugarcane, the critical concentration for +VAM plants was lower than that of –VAM plants. There are 3 implications that arise from this study. First, VAM fungi present in cane-growing soils can promote the growth of maize and soybean, which are potential rotation crops, over a range of P levels. Second, the mycorrhizal strain taken from this site did not generally contribute to a yield decline in sugarcane plants. Third, application of P fertiliser is not necessary for sugarcane when acid-extractable P is <30 mg/kg if sufficient VAM propagules are present, or mp;lt;47 mg/kg if a mycorrhizal response is not anticipated.

Sugarcane smut caused by the fungus Sporisorium scitamineum is one of the major diseases of sugarcane worldwide, causing significant losses in productivity and profitability of this perennial crop. Teliospores of this fungus are air-borne, can travel long distances and remain viable in hot and dry conditions for more than six months. The disease is easily recognised by its long ‘whip’-like sorus produced on the apex or side shoots of sugarcane stalks. Each sorus can release up to 100 million teliospores in a day; the spores are relatively small (≤7.5 µ), light and can survive in harsh environmental conditions. The air-borne teliospores are the primary mode of smut spread around the world and across cane-growing regions. The most effective method of managing this disease is via resistant varieties. Due to the complex genomic makeup of sugarcane, selection for resistant traits is difficult in sugarcane breeding programs. In recent times, the application of molecular markers as a rapid tool of discarding susceptible genotypes early in the selection program has been investigated. Large effect resistance loci have been identified and have the potential to be utilised for marker-assisted selection to increase the frequency of resistant breeding lines in breeding programs. Recent developments in “omics” technologies (genomics, transcriptomics, proteomics, and metabolomics) have contributed to our understanding and provided insights into the mechanism of resistance and susceptibility. This knowledge will further our understanding of smut and its interactions with sugarcane genotypes, and aid in the development of durable resistant varieties.

No abstract