Evidence that DL-3-aminobutyric acid and acibenzolar-S-methyl induce resistance against bacterial head rot disease of broccoli.

Head rot of broccoli caused by Pseudomonas marginalis (Brown) Stevens and P. fluorescens Migula is a major disease in Brittany (France). To date, no accession with a satisfactory field resistance has been identified, and available pesticides are not effective in controlling the disease. The aim of this study was to test whether acibenzolar-S-methyl (ASM), DL-3-aminobutyric acid (BABA) and potassium phosphonate (K(2)HPO(3)), known to induce resistance against various diseases, can help protect broccoli against head rot. The susceptible broccoli F1 hybrids Marathon and Shogun were grown in a greenhouse until head formation. They were then sprayed with ASM (0.23 mM AI), BABA (20 mM AI) or potassium phosphonate (37.41 mM AI) until runoff. In one experiment, heads from treated plants were excised, inoculated (10(4) cfu ml(-1)) and incubated in Magenta GA7 vessels. In another experiment, heads were inoculated on treated living plants. Disease ratings were made 5 days after inoculation. Antibiotic- and water-treated plants served as controls. Results obtained showed that, on excised treated heads, potassium phosphonate was not protective and disease scores were comparable with those of the water control. BABA- and ASM-treated excised heads were poorly, but significantly, protected. On whole plants with heads attached, the latter two compounds were much more effective. ASM-induced resistance increased in effectiveness over 8 days after inoculation, whilst that induced by BABA decreased. This result suggests that testing disease resistance inducers on excised broccoli heads is not accurate. ASM and BABA may offer alternative methods for controlling head rot of broccoli. Copyright 2005 Society of Chemical Industry.