Infectivity Titrations of Clavibacter xylisubsp.xyli and Sugarcane Cultivars Differing in Susceptibility to Ratoon Stunting Disease. N. A. Harrison, Ft. Lauderdale Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, 3205 College Avenue, Ft. Lauderdale 33314. M. J. Davis, Ft. Lauderdale Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, 3205 College Avenue, Ft. Lauderdale 33314, and J. L. Dean, USDA, ARS, U.S. Sugarcane Field Station, Canal Point, FL 33438. Plant Dis. 70:556-558. Accepted for publication 25 November 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-556.

Single-bud cuttings from healthy stalks of eight sugarcane cultivars differing in susceptibility to ratoon stunting disease were inoculated with 10- or 1,000-fold serial dilutions of the F-1 strain of Clavibacter xyli subsp. xyli. Inoculum levels ranged from 10¹ to 10⁷ cfu/ml. Sap from mature stalks of plants grown from cuttings was examined for C. x. subsp. xyli with phase-contrast microscopy and/or a fluorescent-antibody staining procedure. Probit analyses of quantal responses of sugarcane cultivars to C. x. subsp. xyli gave log₁₀ ED₅₀ values (dosage per milliliter required to achieve infection of 50% of the population) from 2.87 for cultivar CP 53-1 to 12.3 for H 60-6909. ED₅₀ values were inversely correlated with yield-loss estimates for the cultivars.