VARIETAL RESISTANCE OF SUGARCANE TO NATURAL INFECTION OF SMUT—PRELIMINARY RESULTS

THE CURRENT resistance screening method for sugarcane smut involves dipping setts into a smut spore suspension and inspecting for smut whips in a short plant and ratoon crop cycle. The method is very effective for screening a large number of varieties in a relatively short period of time. Although this method is internationally accepted, it has some drawbacks: i) it does not replicate natural infection and ii) plants are subject to very high disease pressure. Three trials were established at the BSES smut research farm to: i) determine if the ratings obtained by the dip inoculation technique are good at predicting field resistance of varieties and ii) determine the level of bud infection in mature stalks of varieties exposed to natural infection for one year. A natural spread trial was planted in 2007 with 10 replicates of the nine test varieties planted between rows of infected Q205A. The highly susceptible variety Q205A had 44% smut-infected plants in the plant crop but no smut was observed in the other varieties. The average percent smut-infected plants in all varieties at the end of the first ratoon crop was 27%, with 100% infected plants in Q205A, 32% in Q138, 29% in Q188A, 20% in Q190A, 2% in Q232A and 0% in Q151. There was severe stunting in Q205A but other varieties had less severe symptoms. A second trial with 34 varieties was planted in 2008. This trial developed more smut in the plant crop with 19 of the 34 varieties showing some smut and an average of 15% smutinfected plants in all varieties. The highest levels of smut were recorded in Q205A (94% infected plants) and Q157 (88% infected plants). In both trials, there were highly significant (P<0.001) correlations between the ratings derived from the standard dip inoculation technique and the smut incidence and severity in the natural infection trials. Stalks with no smut whips were selected from the plant crop of each variety in the 2007 natural infection trial in October 2008 and planted at the smut research farm. Very little smut was observed in the young plants developing from these stalks, which suggests that there was virtually no latent infection of buds. The data presented in this paper are preliminary and inspection of the trials will continue until the end of the second ratoon.