In comparative trials under a wide range of environmental conditions and management inputs, injection of cocoa trees with partially pH-neutralised phosphonic acid (H3PO3) gave similar, or better, control of pod rot caused by Phytophthora palmivora to that obtained with metalaxyl/cuprous oxide pod sprays. lnjection reduced canker incidence by up to 90% at some sites. Foliar sprays of phosphonic acid had little effect on yield or pod rot incidence at rates up to 24 g a.i./tree/application. Studies to determine the optimum dose and frequency of injection have shown a clear benefit of treatment, with both increased yields and reduced pod rot incidence, but no yield advantage was obtained through high dose rates. Alginate gel paints and direct root uptake were evaluated as alternative application methods, but were less effective than injection. H3PO3 was ineffective against vascular-streak dieback disease of cocoa, caused by Oncobasidium theobromae.