Integrated control of Phytophthora megakarya in Cameroon

Control of Phytophthora megakarya, the causal agent of cocoa black pod in Cameroon is commonly obtained by fungicide spraying of either copper compounds or a mixture of metalaxyl-M with copper compounds. Research on biocontrol of P. megakarya has identified an isolate of Trichoderma asperellum, PR11 as a promising biocontrol agent (BCA) of P. megakarya. The objective of this study was to test rational pesticide and integrated control approaches using fungicide and fungicide-BCA combinations. Field trials were carried out over a period of two years (2007-08). Two trial sites were selected for both years. At each site a randomized complete block design was used with three replicate plots per treatment. Plots were sprayed in three-week intervals using motorized mistblowers. Treatments were i) control (water only), ii) T. asperellum PR11 formulated with starch, iii) T. asperellum followed by copper hydroxide (Kocide), and iv) Ridomil gold, a mixture of metalaxyl-M and cuprous-oxide, followed by Kocide. The second year a fifth treatment was added, v) Banko plus (Chlorothalonil with Carbendazim) followed by Kocide, whereas the Kocide/Ridomil treatment was changed to a Ridomil only treatment and the Kocide-T. asperellum treatment was changed to T. asperellum formulated with clay. The number of rotten and ripe pods was recorded weekly. There were marked and significant differences between sites (P<0.0001) but not between years with regard to total percentage pod rot. Overall, the Ridomil only and the Ridomil-Kocide treatments were significantly (P<0.0001) better than all other treatments except the PR11-Kocide treatment which was similar to the Ridomil-Kocide treatment. Reductions in pod rot compared with the absolute control were between 24 and 73% for the Ridomil only and between 15 and 68% for the Ridomil-Kocide treatments. Depending on the site, PR11 was able to reduce percentage pod rot between 13 and 34% whereas the combination of PR11 and Kocide reduced pod rot by 48% in one instant. This study demonstrates the potential for integrated control using fungicide and fungicide-BCA combinations. Such integrated approaches can reduce the amount of fungicide applied (thus reducing costs) and may help to mitigate the risk of fungicide resistance - especially since metalaxyl is currently so widely used for the control of P. megakarya. (Texte integral)