Abstract
Stem canker and Phytophthora pod rot (PPR) or black pod caused by Phytophthora palmivora are serious diseases of cocoa (Theobroma cacao L.) in Sulawesi, Indonesia, causing high yield losses for smallholders, possibly exceededonlybylossesduetothecocoapodborer(CPB),Conopomorphacramerella.Potassiumphosphonate(phosphite) applied by trunk injection has been demonstrated to effectively control canker and PPR in Papua New Guinea. The method was tested in a field trial in south-east Sulawesi. Fifty trees were injected with phosphonate, 50 with water and 50 were left untreated. Phosphonate was applied at a rate of ~16 g active ingredient per tree per year, depending on the size of each tree. Treeswereevaluatedeachmonthforcankerseverity,forPPRincidenceandforCPBincidenceandseverity.From4months aftertheinitialinjection,treestreatedwithphosphonatehadnegligiblelevelsofcanker.Overa2.5-yearperiod,phosphonate significantly decreased PPR incidence. Cycles of PPR infection occurred in the wet season with PPR incidence fluctuating fromlessthan30%togreaterthan75%.These fluctuationsmighthavebeenduetovariationsinrainfallcausingnaturalcycles of sporulation and infection. CPB incidence did not differ significantly between treatments. Since trunk injection of phosphonate effectively controls stem canker and decreases PPR in the long term it provides a valuable option for the management of these diseases for cocoa smallholders.
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