Xanthomonas albilineans produces albicidin, a unique and specific toxin that causes foliar symptoms of sugarcane leaf scald disease. In the related species Xanthomonas campestris pv. campestris, a cluster of genes called rpf (for regulation of pathogenicity factors) is involved in control of various cellular processes. Marker exchange mutants of the rpfF gene in strain XaFL07-1 of X. albilineans from Florida were generated, and the mutants were verified by PCR analysis and by use of an X. campestris diffusible signal factor (DSF) reporter strain. Both albicidin and protease production by these mutants appeared significantly reduced compared to XaFL07-1, suggesting that DSF is involved in regulation of albicidin biosynthesis by X. albilineans. However, sugarcane cultivar CP80-1743, moderately susceptible to leaf scald, exhibited pencil line symptoms indicative of albicidin production on emerging leaves after inoculation of stalks by the decapitation method. Preliminary experiments indicated that at least some rpfF mutants colonized sugarcane stalks less efficiently, both spatially and in intensity, than wild type X. albilineans. Additional inoculation experiments are in progress to assess disease severity caused by rpfF mutants, and to study the role of DSF production in sugarcane stalk colonization by X. albilineans.