Search

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-weekIy stem painting of the hardened bark of young seedlings with 10 g/1 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. [ABSTRACT FROM AUTHOR]

Isolation of clonal cocoa material in screened shade houses for 6 months allows adequate opportunity for detecting vascular-streak dieback caused by Oncobasidium theobromae and in Papua New Guinea a further period of 6 months in intermediate quarantine on an isolated island has been replaced by a similar period in screened houses at a quarantine station in the importing locality. Microscope examination of transverse sections of budwood sticks and pot stalks provides a further very thorough precaution against disease transmission because hyphae of the pathogen are large and easily detected. Hyphae were found within the stalks and placentae of pods from diseased branches but seeds from these germinated normally and there was no evidence of seed transmission. Dipping rubbed seeds in 1 g/l propiconazole plus 5g/l metalxyl caused a small but statistically significant reduction in seeding stem height. However, root length and percentage germination were not affected and this prophylactic seed treatment may be useful in situations where quarantine authorities require additional precautions. [ABSTRACT FROM AUTHOR]