Evaluation of selective media and immunoassays for detection of Xanthomonas albilineans, causal agent of sugarcane leaf scald disease.
A selective medium (XAS medium), based on Wilbrink's medium supplemented with 5 g KBr, 100 mg cycloheximide, 2 mg benomyl, 25 mg cephalexin, 30 mg novobiocin, and 50 mg kasugamycin per litre, was developed for the isolation of X. albilineans. The medium supported high plating efficiencies (plating efficiencies of 69-127% were obtained for 30 strains of the pathogen collected previously from locations throughout the world). X. albilineans was isolated on XAS medium with >98% frequency from symptomatic sugarcane from Florida, Guadeloupe and the Dominican Republic, and was isolated less frequently from asymptomatic sugarcane. Selective isolation and serological methods for detection were compared. Dilution plating of sap extracts from stalks and blotting of freshly cut surfaces of stalk were used to inoculate the XAS medium. Three enzyme immunoassays (EIA) were used, the tissue-blot EIA, stalk-blot EIA, and dot-blot EIA. The pathogen was detected consistently with each method in all 27 symptomatic stalks, with the exception of 1 stalk with the stalk-blot EIA. The pathogen was detected sporadically in 27 asymptomatic stalks collected from the same 27 plants. Similar detection frequencies were obtained with the dilution-plate and tissue-blot EIA methods. Detection by the other methods was c. 7.4-14.8% less efficient. None or low pathogen population sizes were detected in asymptomatic stalks. Contaminating bacteria interfered with selective isolation, especially when the stalk-blot inoculation method was used and when pathogen population sizes were small. Brief surface disinfestation of stalks with alcohol combined with selective isolation produced satisfactory results. It is concluded that selective 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 pathogen.