Screening of fungicides, in vitro and in vivo, for the control of witches' broom disease of cacao in the cacao growing region of Bahia, Brazil

摘要

From 36 fungicides tested, in vitro, for the mycelial growth inhibition of Crinipellis perniciosa, the causal agent of witches' broom disease of cacao, eighteen were selected to be evaluated, in vivo, for the control of the disease. The systemic fungicides of the triazole's group: tebuconazole, cyproconazole, difienoconazole, hexaconazole, fluquinconazole, epoxiconazole and propiconazole, had an outstanding performance, showing EC_(50) values on the range of 0.014 to 1.182 mu g of the active ingredient per mL of medium. Among the protective fungicides, cuprous oxide, copper hydroxide and copper oxicloride, were the ones selected based, not only, on their efficacy on the inhibition of mycelial growth of the fungus, but also, on the fact that, theyare already recommended for the control of black pod disease of cacao, caused by three species of Phyiophthora. Tebuconazole was the best of all the fungicides evaluated for the mycelial growth inhibition of C. perniciosa (EC_(50) 0.014 mu g/ mL), and thus the one selected, initially, to be tested on cacao under greenhouse and field conditions. In two greenhouse trials, the fungicide had the same outstanding performance as compared to the results of the in vitro tests. In the first trial, although the concentration used was a little to high, it prevented symptom expression on inoculated seedlings with C. pemrnciosa, even when applied 7 days after inoculation. Even when the fungicide was sprayed 14 and 18 days after the inoculations, it reduced, not only, the number of infected seedlings, but also, stopped or delayed the symptoms expression. In the second trial, once again, the results showed the same efficacy of the fungicide on the control of witches' broom disease of cacao. Even at 250 ppm of the active ingredient, it was able to prevent all the disease symptoms. At the concentration of 500 ppm, even when applied seven days after inoculation, it avoided symptoms expression; and at the concentration of 750 ppm caused, additionally, some phytotoxic effects. Similar results were also observed at 500 ppm, when it was used as a soil drench. Another outstanding effect of the fungicide was in relation to its antisporulant activity. Tebuconazole was able to reduce basidiocarp production by C. perniciosa in 97 and 81 percent when the evaluations were done, respectively, at 45 and 75 days after a single fungicide application.