Ratoon stunting disease of sugarcane as influenced by environmental stressors

Field trials were conducted to understand the influence of Leifsonia xyli subsp. xyli, the bacterium responsible for ratoon stunting disease (RSD), on the performance of sugarcane cultivars under irrigated and water stressed conditions. Trials conducted on six cultivars at Townsville showed yield losses of 34%, in terms of tonnes of cane, through the combined effects of water stress and RSD. The combined effects of the stressors were greater than each stressor individually for both the plant and ratoon crops. No significant effects were noted on commercial cane sugar (CCS) in RSD-infected sugarcane whether irrigated or water stressed. However, CCS values were modified by water stress in the healthy plant crop. This crop, when subjected to water stress, produced an average CCS reading of 16.8 which was 15.9% higher than that of the irrigated plots. Mean loss of sugar through the combined influence of water stress and disease was 25%. Sugar yield loss was correlated strongly with estimates of bacterial numbers in the plant crop. The reduction in yield may be related to reduced sap flow. Sap flow was reduced during the day in the disease susceptible, water-stressed cultivar Q120 in contrast to the resistant cultivar H56-752. The latter cultivar showed no obvious differences in flow rates between healthy and diseased plants. The impact of RSD infection was assessed on seven cultivars growing at Tully, Qld. Mean losses, in terms of tonnes of sugar cane per hectare, were 15 per cent for the plant crop and 18 per cent in the ratoon crop. The seven cultivars responded differently, but Q141 appeared to be tolerant and suffered no yield loss despite relatively high bacterial concentrations. Cultivar H56-752 was resistant to RSD, showing low levels of bacteria in both the plant and ratoon crops, and yield losses were insignificant. A strong correlation was found between bacterial numbers recovered from plant crop cultivars and yield losses. CCS values were not influenced by infection, although stalk height was reduced generally. More moderate temperatures in the range 25C-30oC favoured bacterial growth in susceptible cultivars, while bacterial numbers were reduced significantly at 35oC. Bacterial populations in the resistant cultivar H56-752 were not affected by temperature