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A New Selective Medium for Pseudomonas solanacearum. G. A. Granada, Former Graduate Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706. Luis Sequeira, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 67:1084-1088. Accepted for publication 4 April 1983. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-1084.

A medium (SM-1) with good plating efficiency and high selectivity was developed and used to isolate Pseudomonas solanacearum from artificially and naturally infested soils. The medium was prepared by adding to Kelman’s tetrazolium chloride (TZC) medium the following: crystal violet (50 μg/ml), thimerosal (5 μg/ml), polymyxin B sulfate (100 μg/ml), tyrothrycin (20 μg/ml), and chloromycetin (5 μg/ml). Most fungi did not grow on this medium. With some soil samples, however, adding either cycloheximide (50 μg/ml) or chlorothalonil (80 μg/ml) to the medium eliminated the problem of fungal contamination. After 2–3 days of growth at 30 C, colonies of P. solanacearum on this medium were round, pulvinate, fluidal, and milky white. For 19 of 20 strains tested, plating efficiency of SM-1 ranged from 81 to 138% of that of TZC. These strains included representatives of the three races of the pathogen; only one strain of race 3 (119, from Costa Rica) grew poorly on SM-1 (41% plating efficiency). Growth of most soil bacteria was inhibited (more than 99% in 11 soil samples containing 1.02 ×107–1.03 × 108 cfu/g dry wt) on SM-1. In one soil sample from Costa Rica, however, only 67.4% of the bacterial population was inhibited. The good plating efficiency and marked reduction of background bacteria obtained with SM-1 allowed recovery of P. solanacearum from soils containing as few as 2.2 × 102 cfu/g. The bacterium was isolated easily from naturally infested soils obtained from various locations in the United States and Costa Rica.