Control of vascular-streak dieback of cocoa with triazole fungicides and the problem of phytotoxicity

The effectiveness of a range of systemic fungicides against vascular-streak dieback of cocoa, caused by Oncobasidium theobromae , was evaluated over 2 years in Papua New Guinea. Monthly foliar sprays of the triazole fungicides hexaconazole, tebuconazole and triadimenol all gave good protection against the disease. The relative efficacy of these chemicals was, in order, tebuconazole (EC₅₀ 0–90 mg a.i. per plant), hexaconazole (EC₅₀ 2.35 mg a.i. per plant), triadimenol (EC₅₀ 4.17 mg a.i. per plant). However, the relative severity of plant growth inhibition induced by these compounds had the same ranking. Growth inhibition was reduced by extending the interval between treatments, but this also reduced the efficacy of disease control. Propiconazole stem paints (EC₅₀ 2502 mg a.i. per plant) were markedly less effective than these sprays. Neutralized phosphorous acid sprays and paints showed no activity against the disease.