Role of Pantoea agglomerans in opportunistic bacterial seed and boll rot of cotton (Gossypium hirsutum) grown in the field.

AIMS To investigate the aetiology of seed and boll rot of cotton grown in South Carolina (SC). METHODS AND RESULTS Bacteria were isolated from diseased locules of cotton bolls collected in a field in SC, USA and tested for the ability to cause comparable disease symptoms in greenhouse grown cotton fruit. Spontaneously generated rifampicin-resistant (Rif(r)) mutants of the isolates were used in confirmatory pathogenicity tests. Resistance to the antibiotic was both stable and effective in differentiating between an inoculated Rif(r) strain, rifampicin-sensitive contaminants and/or endophytes. A series of inoculation methods was tested at various boll developmental stages and at different fruiting nodes on the plant. Field disease symptoms were reproduced by inoculating bolls at 2 weeks postanthesis with bacterial suspensions ranging from 10(3) to 10(6) CFU ml(-1). Pathogenic isolates were categorized as Pantoea agglomerans on the basis of phenotype testing, fatty acid profiling (similarity index = 0.94), and 16s ribosomal DNA sequence analysis (99% nucleotide identity). CONCLUSIONS Pantoea agglomerans isolates from field-collected immature, diseased cotton caused comparable infection symptoms in greenhouse produced cotton fruit. SIGNIFICANCE AND IMPACT OF THE STUDY In 1999, significant yield losses in SC cotton resulted from a previously unobserved seed and boll rot that has since been reported in other southeastern states. This study demonstrated a role of P. agglomerans in producing opportunistic bacterial seed and boll rot of cotton.