Studies on the chemical control of Phytophthora palmivora (Butl.) Butl. on Theobroma cacao L. in Nigeria.

In laboratory trials, phenyl mercury nitrate at 0·02 p.p.m. and fentin acetate at 0·2 p.p.m. severely retarded growth of four isolates of Phytophthora palmivora on cassava agar. These two chemicals, with captan, maneb and a dithiocarbamate-copper chelate, were also highly toxic to encysted zoospores of a ‘rubber’ group isolate of P. palmivora.

Deposits of captan on pods were readily removed by artificial rain, but some improvement in tenacity was obtained by the addition of a sticker. In other laboratory trials, the deposit from low-volume sprays of cuprous oxide dried more quickly on pods than that from high-volume sprays but showed no advantage in subsequent resistance to weathering.

In a field trial in 1961, seven fungicide treatments were applied three-weekly and compared with an unsprayed control. The lowest percentage black-pod infection followed treatment with fentin acetate: Bordeaux mixture and carbide Bordeaux both gave good control. The captan treatments were completely ineffective. More black pods were harvested from close-spaced trees than from those wide-spaced.

Weekly applications of 0·15% fentin acetate to seedlings induced no significant damage.

In a field trial made in 1962 very heavy rainfall provided a severe test of the fungicides, the most effective being Bordeaux mixture and carbide Bordeaux mixture applied three-weekly, carbide Bordeaux mixture applied four-weekly and fentin acetate applied two-weekly. Captan with added sticker was again no better than the control. There was no marked effect of spacing.

Comparisons of Bordeaux and carbide Bordeaux mixtures made at two other sites in 1962 showed no difference in disease control. It is suggested that carbide Bordeaux mixture could be replaced by the cheaper preparation made with lime.