Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Kenneth E. Damann


A PCR method with primers for t-RNA gene spacers was used for detection of Xanthomonas albilineans (Xa), the causal agent of leaf scald disease of sugarcane. A DNA segment of approximately 170 bp was amplified for Xa strains, but also for X. campestris pv. campestris, X. campestris pv. translucens and two sugarcane bacterial endophytes. Good agreement between PCR and isolation to detect Xa from sugarcane sap was observed. PCR was faster allowing pathogen detection in less than one day. Both methods resulted in low detection frequencies from asymptomatic plants, which may contain Xa below detection limits. The lowest limit for PCR was 26 cfus per reaction or $1.3\times10\sp4$ cfu/ml. Adoption of procedures to concentrate and lyse a larger number of bacterial cells could increase the PCR sensitivity. DNA from 42 Xa isolates from four countries and seven other bacterial species was analyzed by rep-PCR with BOX and ERIC primers. Generated fingerprints of Xa were distinct from the other bacterial species. Within Xa, seven, nine and twelve groups were revealed by BOX, ERIC, and BOX and ERIC combined, respectively. Good agreement between group clustering and geographic origin and serovars was observed. Fingerprints were reproduced with cell lysate. Rep-PCR was faster and more reliable than Biolog or pathogenicity assay and could be used to identify and study diversity in Xa. Greenhouse experiments were conducted to determine the potential of infectivity titration to assess sugarcane cultivar resistance to leaf scald. Single-bud-cuttings were inoculated with three Xa inoculum concentrations. An ED$\sb{50}$ for each cultivar was estimated based on probit analysis of cumulative infection frequencies. Different ED$\sb{50}$ values allowed cultivar grouping into three classes which were in agreement with field observations. Good agreement between ED$\sb{50}$ values and 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$\sp8$ cfu/ml of Xa, also was observed. Greenhouse assays, using infectivity titration or just one inoculum concentration, could provide an alternative to field tests for assessing sugarcane resistance to leaf scald.