A New Selective Medium forPseudomonas solanacearum

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Abstract
A medium (SM-1) with good plating efficiency and high selectivity was developed and used to isolate P. 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 .mu.g/ml), thimerosal (5 .mu.g/ml), polymyxin B sulfate (100 .mu.g/ml), tyrothrycin (20 .mu.g/ml), and chloromycetin (5 .mu.g/ml). Most fungi did not grow on this medium. With some soil samples, however, adding either cycloheximide (50 .mu.g/ml) or chlorothalonil (80 .mu.g/ml) to the medium eliminated the problem of fungal contamination. After 2-3 days of growth at 30.degree. 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 81-138% of that of TZC. These strains included representatives of the 3 races of the pathogen; only 1 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 .times. 107-1.03 .times. 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 .times. 102 cfu/g. The bacterium was isolated easily from naturally infested soils obtained from various locations in the USA and Costa Rica.