NEW METHODS FOR SCREENING SUGARCANE CLONES FOR RESISTANCE TO LEAF SCALD

LEAF SCALD OF sugarcane occurs in all sugarcane growing regions in Australia and resistance to the disease is an important trait in the Sugar Research Australia plant breeding program. The disease is caused by the bacterium Xanthomonas albilineans, and can cause death of plants of susceptible varieties. The method for rating varieties for resistance to leaf scald was developed in the 1960s and involves decapitating every shoot on 4–5 month old plants above the growing point and spraying or painting the freshly cut surface with juice extracted from an infected stalk. The plants are then allowed to grow for a further 8–9 months before they are scored for the incidence and severity of infection. The severity of leaf scald is strongly influenced by moisture stress and the reliability of trials fluctuates from year to year. Another problem with the current trial method is that results are available only after the end of the planting season and therefore breeders have to make decisions on propagation of clones without the latest data, which can lead to a delay in release of varieties. This paper describes a new method for conducting leaf scald resistance trials that attempts to speed up the screening for leaf scald and to reduce the labour inputs in conducting the trials. Two-eye setts were inoculated with a suspension of bacteria by spraying the suspension onto the blades of lopping secateurs and the cut surface of the sett with an automatic spray system. Four experiments were conducted over four years from 2012 to 2015. The disease severity was recorded in the first two experiments at one to two monthly intervals for 12–13 months after planting and in another two trials at 6–7 months. The disease severity recorded for the varieties with the new sett inoculation method was significantly correlated with the severity recorded in trials conducted in the same years with the decapitation method in all years. In 2015, the sett inoculation method had lower levels of infection than the decapitation method and took longer to develop severe symptoms. The new sett inoculation method in most years would give acceptable ratings for clones after 6–7 months and will save labour in conducting trials.