Chemical control of vascular‐streak dieback disease of cocoa in Papua New Guinea

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Abstract
High‐volume foliage sprays of 18 compounds including 13 systemic fungicides in field trials at 4‐week intervals did not control vascular‐streak dieback (VSD) of cocoa caused by Oncobasidium theobromae in Papua New Guinea. Soil drenching with carbendazim and other systemics was also ineffective, possibly because insufficient carbendazim may have been taken up into the xylem to inhibit the fungus. Stem paints with four systemic fungicides in aqueous formulations were also ineffective, but 4‐weekly stem painting of the hardened bark of young seedlings with 10 g/l propiconazole in 25% white oil caused a highly significant reduction in VSD incidence under field conditions. The treatment had no curative effect. Some phytotoxicity was observed but this was not apparent when the concentration of propiconazole was doubled and the treatment interval lengthened to 8 weeks. The ineffectiveness of systemic fungicides as soil drenches and foliage applications was probably due to failure of these compounds to accumulate at the shoot apex where infection occurs. A stable and non‐toxic formulation containing surfactants was developed which may improve control by aiding bark penetration by propiconazole or related compounds.

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