A rapid method of screening sugarcane clones for resistance to red rot

Red rot, caused by the fungus Colletotrichum falcatum, is an important sugarcane disease. It is the most damaging disease in some South Asian countries causing significant yield and crop losses. In the Sugar Research Australia (SRA) variety-development program, advanced clones in the final stage of selection are screened for resistance to red rot before being released for commercial production. The conventional method of red rot rating in the field using 12-month-old mature cane is time-consuming, labour-intensive and is impacted by the weather and crop lodging. A rapid method of screening for resistance to red rot was developed (two-eye-setts method) and compared to historical ratings obtained from a conventional method. In addition, two other methods, controlled condition testing (CCT), a method used in India, and a leaf midrib inoculation method were tested. In the two-eye-setts method, six clones with known ratings were inoculated with red rot culture through holes made in the middle of the two-eye-setts and incubated at 30°C and 90% relative humidity for 2 weeks. Inoculated setts were split longitudinally and visually assessed for symptoms using standard disease indices and photographed. The images were analysed using the machine-learning algorithm Classification and Regression Tree to estimate the percentage of symptomatic pixels as image cover. Symptom expression was poor in the setts inoculated using the CCT method. The leaf midrib method showed no differences among the inoculated leaves. The two-eye-setts method produced excellent symptoms in all inoculated setts, and visual indices of disease showed strong correlation (r=0.99) with the historical ratings of clones. Image cover correlated strongly with disease indices (r=0.93) and historical red rot rating (r=0.88). The two-eye-setts method along with image analysis can substantially shorten the time required for screening for red rot s from over a year to about 3weeks. This approach will be implemented to screen for resistance to red rot in the SRA variety-development program.