INCIDENCE VERSUS SEVERITY—WHAT DIFFERENCE WOULD IT MAKE TO SMUT SCREENING

SMUT IS AN important disease of sugarcane and was first reported in Australia in 1998. Since then, BSES Limited has been conducting smut screening programs in Indonesia, the Ord River Irrigation Area (ORIA) and in Bundaberg. In smut screening trials, stalks of clones from various stages of breeding programs are dip-inoculated with a smut spore suspension (106 spores/mL water) and planted in the field. Two methods of rating smut resistance screening trials were compared in a series of eight trials in 2008 and seven trials in 2009 in Bundaberg. Disease incidence (percent of infected stools) and disease severity (visual assessment of the severity of stunting and smut whip production) were measured in the plant and ratoon crops. A set of standard varieties with known field resistance was included in all trials. Regressions of the log transformed percent smut incidence and disease severity and the long term rating of the standard varieties were highly significant for both characters in both years. The resistance ratings for 2090 clones in 2008 series of trials and 2268 clones in 2009 series were calculated from the regression equations of the log transformed incidence or severity and the long term rating of the standard varieties. The ratings based on incidence would have discarded 4.7% of clones in 2008 and 6% of clones in 2009 that would have been retained if the disease severity rating had been used. Only 0.6% of clones in 2008 and 1.8% of clones in 2009 would have been discarded by the disease severity method that would have been retained if the incidence ratings had been used. Overall, the disease severity method would retain more clones but the differences between the two methods were small. Further research is required to better understand the relationship between the artificial smut resistance screening methods and field reaction of varieties in the different regions of Australia. The implications of the results on current and future screening trials are discussed.