Submitted to: European Journal of Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/8/2006
Publication Date: 8/1/2006
Citation: Widmer, T., Laurent, N. 2006. Plant extracts containing caffeic acid and rosmarinic acid inhibit zoospore germination of phytophthora spp. pathogenic to theobroma cacao. European Journal of Plant Pathology. 115:377-388 Interpretive Summary: Black pod disease caused by a fungal-like organism is one of the most destructive diseases reducing chocolate production in Africa. Chemical control is possible but is costly and dangerous for the local farmer and is not used in its most effective manner. This paper describes a discovery that using certain plant extracts that contain a specific chemical can kill this organism. When the extract was sprayed onto leaves and the fungus-like organism was added, diseased tissue was reduced compared to when no plant extract was applied. This research will greatly benefit the local farmer in providing a safe and cost-effective method to control this important disease.
Technical Abstract: The three most important species of Phytophthora worldwide causing black pod disease of cacao are P. palmivora, P. megakarya, and P. capsici. Chemicals are effective in controlling this disease but more natural methods would be preferred. One alternative is to use natural plant extracts. Rosemary and lavender leaf extracts were found to be effective in reducing germination of P. capsci, P. megakarya, and P. palmivora zoospores when supplemented to agar plates at different dilutions. The extracts displayed the biggest impact on P. megakarya zoospores where it completely inhibited germination at a 25% dilution of the prepared extract. When applied to cacao leaf disks, rosemary extract reduced necrosis caused by P. megakarya zoospores. In a bioassay, pears first treated with lavender extract showed no symptoms of P. megakarya infection compared with the nontreated controls. Based upon HPLC analyses, the active compound in these extracts was determined to be caffeic acid, rosmarinic acid or some simple derivative thereof. When supplemented to agar plates, synthetic caffeic acid and rosmarinic acid completely inhibited germination of P. capsci, P. megakarya, and P. palmivora zoospores at concentrations of 3 and 6 gl-1, respectively. In addition, sage and rice bran extracts, which both contain caffeic acid, were also effective in reducing zoospore germination. In contrast, inhibition of Boytritis cinerea or Trichoderma asperellum conidia germination did not occur, displaying some level of specificity. These extracts could provide an economically safe method for reducing damage caused by black pod disease on cacao until resistant varieties are developed and released.