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A Test Tube Assay for Estimating Populations of Xanthomonas campestris pv. translucens on Individual Wheat Leaves. E. A. Milus,Assistant professor, Department of Plant Pathology, University of Arkansas, Fayetteville 72701; A. F. Mirlohi, research associate, Department of Plant Pathology, University of Arkansas, Fayetteville 72701. Phytopathology 83:134-139. Accepted for publication 22 October 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-134.

The quantification of bacterial populations associated with individual leaves by most available methods is laborious and expensive. This study was conducted to develop an accurate, inexpensive assay for quantifying a rifampicin-resistant strain (88-14^Rif) of Xanthomonas campestris pv. translucens. Nutrient broth amended per liter with 100 mg of cycloheximide, 2 ml of a 0.25% pimaricin suspension, 100 mg of rifampicin, and 10 mg of cephalexin was dispensed aseptically into test tubes that were 16 × 100 mm. Leaves naturally infested with strain 88-14^Rif were submerged in tubes of amended broth and incubated at 25 C on an orbital shaker at 100 rpm. Tubes were examined 2–4 times per day to establish the time when turbidity resulting from the growth of strain 88-14^Rif first became visible. The relationship between time until initial turbidity and population size of strain 88-14^Rif was expressed in the equation Y = 10.38 – 0.189X + 0.00096X², in which Y = the log₁₀ population size and X = the time of incubation in hours. In addition, the test tube assay determined the population size of strain 88-14^Rif in artificially infected leaves (Y = 9.73 – 0.10X) and the incidence of seed infestation and transmission of seedborne inoculum to seedlings. The test tube assay may be useful for ecological, epidemiological, disease-resistance, or risk-assessment studies requiring detection of and/or estimates of bacterial populations that have selective resistance to antibiotics.