- 36 strains of Burkholderia spp. isolated from sugarcane were evaluated for biological control of leaf and pseudobulb necrosis of orchid caused by B. gladioli. 29 of the sugarcane strains suppressed the disease in greenhouse assays. We generated a draft genomic sequence of one suppressive strain, B. seminalis TC3.4.2R3. The genome mapped to three replicons, two chromosomes and the plasmid pC3, with an estimated size of 7.67 Mb, and includes gene clusters for the biosynthesis of the antibiotic pyrrolnitrin, a rhamnolipid, the siderophores ornibactin and pyochelin, and IAA, as well as ethylene degradation via ACC deaminase. To gain insight into the mechanisms by which TC3.4.2R3 suppresses orchid necrosis, we generated a library of transposon mutants and screened 3,840 mutants for suppression of leaf necrosis. 12 mutants that lacked biocontrol activity were selected and the transposon insertions were mapped to eight genes, all of which are on chromosome 1 of the genome. Of the eight genes essential to biocontrol that were identified in this genetic screen, one is in a wcb cluster that is related to synthesis of the cell capsule, a key determinant in bacterial-host interactions in other systems, and the other seven are highly conserved among Burkholderia spp.