Transposon mutagenesis and tagging of fluorescent Pseudomonas : Antimycotic production is necessary for control of Dutch elm disease

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Abstract
Antimycotic-producing strains of Pseudomonas syringae are being tested as Dutch elm disease control agents. We examined the role of antimycotic production in disease control. Transposon Tn903 was used to mutagenize the antimycotic-producing strain MSU174. Eighty-one mutants that did not inhibit fungal growth were identified among 15,000 Tn903-containing derivatives. Linkages between Tn903 insertions and defects in antimycotic metabolism were established. Three Tn903-containing strains (two antimycotic producers and one nonproducer) were individually introduced into American elm seedlings. The seedlings were subsequently challenged with Ceratocystis ulmi, the causal agent of Dutch elm disease. Protection of the elms was observed with the two antimycotic-producing strains but not with the nonproducing strain. The introduced strains could be readily recovered from the seedlings after two growing seasons. They were unequivocally identified by the Tn903 insertions they contain.