Infectivity Titration for Assessing Resistance to Leaf Scald Among Sugarcane Cultivars

Greenhouse experiments were conducted to determine potential of infectivity titration to evaluate resistance of sugarcane to leaf scald disease caused by Xanthomonas albilineans. In two experiments, single-bud cuttings were inoculated with suspensions containing 10¹, 10⁵, or 10⁸ CFU/ml of X. albilineans. The occurrence of symptoms was recorded every 15 days from 45 to 210 days after inoculation. At the final evaluation date, leaf vascular sap was plated onto selective medium to detect latent infections. ED₅₀ (log₁₀ of the bacterial concentration required to infect 50% of inoculated plants) was estimated for each cultivar based on probit analysis of cumulative infection frequency. Frequency of infected plants varied among inoculum doses and cultivars and resulted in ED₅₀ values ranging from 3.0 to 12.3 and 3.1 to 9.8 in the first and second experiments, respectively. Good agreement between experiments was observed for ED₅₀ values of individual cultivars. Differences in ED₅₀ among cultivars agreed with field observations of natural disease incidence. Cultivar responses to leaf scald were compared based on the cumulative frequencies of death and recovery in symptomatic plants, and the frequencies of symptomatic plants observed at different evaluation dates for plants inoculated with 10⁸ CFU/ml of X. albilineans. Good agreement between ED₅₀ values and these responses was observed. Greenhouse inoculation tests using infectivity titration or just one inoculum concentration could provide an alternative to field tests for the assessment of sugarcane resistance to leaf scald.