EVALUATION OF SELECTIVE MEDIA AND IMMUNOASSAYS FOR DETECTION OF XANTHOMONAS-ALBILINEANS, CAUSAL AGENT OF SUGARCANE LEAF SCALD DISEASE

A selective medium (XAS medium) was developed for the isolation of Xan thomonas albilineans (Ashby) Dowson, which causes leaf scald disease o f sugarcane, XAS medium supported high plating efficiencies of the pat hogen. The growth rate, morphology, and pigmentation of colonies on th e medium were useful differential characteristics to identify the path ogen. XAS medium consisted of a modification of Wilbrink's medium that was supplemented with 5 g of KBr, 100 mg of cycloheximide, 2 mg of be nomyl, 25 mg of cephalexin, 30 mg of novobiocin, and 50 mg of kasugamy cin per liter. The pathogen was isolated on XAS medium with greater th an 98% frequency from symptomatic sugarcane in Florida, Guadeloupe, an d the Dominican Republic, and was isolated less frequently from asympt omatic sugarcane. Plating efficiencies of 69-127% were obtained on XAS medium for 30 strains of the pathogen collected previously from locat ions throughout the world. Selective isolation and serological methods for detection were compared. Two methods, dilution plating of sap ext racts from stalks and blotting of freshly cut surfaces of stalks, were used to inoculate XAS medium. Three enzyme immunoassays (EIA) were us ed, the tissue-blot EIA, stalk-blot EIA, and dot-blot EIA. The pathoge n was detected consistently with each method in all 27 symptomatic sta lks, with the exception of one stalk with the stalk-blot EIA. The path ogen was detected sporadically in 27 asymptomatic stalks collected fro m the same 27 plants. Similar detection frequencies were obtained with the dilution-plate and tissue-blot EIA methods. Detection by the othe r methods was approximately 7.4-14.8% less. None or low pathogen popul ation sizes were detected in asymptomatic stalks. Contaminating bacter ia interfered with selective isolation, especially when the stalk-blot inoculation method was used and when pathogen population sizes were s mall. Brief surface disinfestation of stalks with alcohol combined wit h selective isolation was found to produce satisfactory results. Selec tive isolation methods provided valid alternatives to immunoassays for detection of X. albilineans. Additionally, XAS medium enhanced plate- count procedures for estimating viable population sizes of the pathoge n. The selective isolation and serological methods should enable devel opment and implementation of more effective management practices for l eaf scald disease.