FUNGICIDES FOR CONTROL OF ORANGE RUST IN THE 2001 QUEENSLAND CROP

Orange rust, a disease causing only minor effects in Australian cane crops for over 100 years, had a very severe effect on Queensland sugar yields in 2000. This was especially so in central district crops of the now highly susceptible variety Q124. The disease had slightly lesser, but still significant effects, on the 2001 crop. This paper reports on an experimental program aimed at developing an economic fungicide control strategy for the disease. Four fungicides were trialed: Bumper (propiconazole), Folicur (tebuconazole), Alto (cyproconazole) and Dithane (mancozeb). A range of treatments was applied in varying numbers of applications to provide a range of disease control strategies at two trial sites. This provided data enabling the best spray schedules to be selected. Of the spray schedules, repeated applications (nine) of the fungicide Alto (cyproconazole) gave the highest yields at one site while multiple applications of Dithane (mancozeb) gave the highest yields at another. A combination of Folicur (tebuconazole) and Dithane (mancozeb) also gave good control. An economic analysis suggested fungicide control of orange rust provided a cost benefit when all costs were taken into consideration. Surprisingly low disease levels were found to result in yield losses. Applying fungicide when disease levels are low early in the season was found to provide a better economic return compared to applying fungicide later in the season. Further research is needed to clarify yield loss disease thresholds. Farmers now have a combination of control measures for orange rust control. The long-term strategy remains the cropping of resistant varieties.