Evaluation of Selective Media and Immunoassays for Detection of Xanthomonas albilineans, Causal Agent of Sugarcane Leaf Scald Disease. M. J. DAVIS, Tropical Research and Education Center, University of Florida, IFAS, 18905 S.W. 280 Street, Homestead 33031. P. ROTT, Centre de Cooperation Internationale en Recherche Agronomique pour le Devel-oppement, CIRAD-CA, Station de Roujol, 97170 Petit-Bourg, Guadeloupe, French West Indies; P. BAUDIN, Centre de Cooperation Internationale en Recherche Agronomique pour le Developpement, CIRAD-CA, BP 5035, 34032 Montpellier Cedex, France; J. L. DEAN, Tropical Research and Education Center, University of Florida, IFAS, 18905 S.W. 280 Street, Homestead 33031. Plant Dis. 78:78-82. Accepted for publication 13 September 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0078.

A selective medium (XAS medium) was developed for the isolation of Xanthomonas albilineans (Ashby) Dowson, which causes leaf scald disease of sugarcane. XAS medium supported high plating efficiencies of the pathogen. The growth rate, morphology, and pigmentation of colonies on the medium were useful differential characteristics to identify the pathogen. 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 benomyl, 25 mg of cephalexin, 30 mg of novobiocin, and 50 mg of kasugamycin per liter. The pathogen was isolated on XAS medium with greater than 98% frequency from symptomatic sugarcane in Florida, Guadeloupe, and the Dominican Republic, and was isolated less frequently from asymptomatic sugarcane. Plating efficiencies of 69-127% were obtained on XAS medium for 30 strains of the pathogen collected previously from locations throughout the world. Selective isolation and serological methods for detection were compared. Two methods, dilution plating of sap extracts from stalks and blotting of freshly cut surfaces of stalks, were used to inoculate 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 one 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 approximately 7.4-14.8% less. 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 was found to produce satisfactory results. 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. The selective isolation and serological methods should enable development and implementation of more effective management practices for leaf scald disease.