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 i noculated with suspensions containing 10(1), 10(5), or 10(8) CFU/ml of X. a lbilineans. The occurrence of symptoms was recorded every 15 days from 45 t o 210 days after inoculation. At the final evaluation date, leaf vascular s ap was plated onto selective medium to detect latent infections. ED50 (log( 10) of the bacterial concentration required to infect 50% of inoculated pla nts) was estimated for each cultivar based on probit analysis of cumulative infection frequency. Frequency of infected plants varied among inoculum do ses 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 agreemen t between experiments was observed for ED50 values of individual cultivars. Differences in ED50 among cultivars agreed with field observations of natu ral 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 10(8)CFU/ml of X. albilineans. Good agree ment between ED50 values and these responses was observed. Greenhouse inocu lation tests using infectivity titration or just one inoculum concentration could provide an alternative to field tests for the assessment of sugarcan e resistance to leaf scald.