How do current ratings of sugarcane varieties for resistance to smut relate to natural infection
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SUGARCANE SMUT, CAUSED by a fungus Sporisorium scitamineum, is an important disease of sugarcane in Australia. Sugarcane smut can be managed effectively through the propagation of resistant varieties. In Sugar Research Australia’s (SRA) smut screening experiments, stalks of varieties from various stages of breeding programs are cut into one-eye setts and then dipped into a smut spore suspension (5 × 106 spores/mL water) for 10 min at 31 °C. After germination, the plants are transplanted to the field and disease incidence is measured in the plant crop and first and second ratoon crops. This method is effective for screening of a large number of varieties in a relatively short period (10–12 months) and is used in other countries. Although this method is widely accepted, it has some drawbacks: i) test plants are subject to very high disease pressure; and ii) it does not replicate natural infection. Three experiments were established in 2007, 2008 and 2009, to determine if the ratings obtained by artificial inoculation technique predict field resistance of varieties. All experiments were planted with 10 or 5 replicates of the test varieties planted between rows of infected Q205A, and maintained until second ratoon. Highly susceptible varieties Q205A and Q157 had >40% infected plants in plant crops whereas little smut was observed in intermediate and resistant varieties. Average % of smut infected plants increased in all experiments from the plant crop (5–12%) to first ratoon (21–46%) and second ratoon (26–59%) crops. The correlation coefficient values between smut incidence in the natural infection experiments and the historical ratings obtained using dip-inoculation methods ranged from r = 0.82 to 0.72, indicating a good agreement between natural infection trials and dip inoculation ratings.