Trunk injection with phosphorous acid to control Phytophthora black pod of cocoa

Black pod of cocoa (Theobroma cacao L.) caused by Phytophthora palmivora (Butl.) Butl. is a serious disease in all major cocoa growing countries. Application of copper fungicides using conventional methods often gives unsatisfactory control because of wet weather conditions and heavy infection. We evaluated the efficacy of phosphorous acid (PA) against the pathogen in the laboratory, and against the disease in the field using trunk injection technique. In vitro studies indicated PA at low concentrations significantly inhibited mycelial growth; the ED50 value was estimated at 12.4 ug/ml. Lesion size and degree of sporangial production on inoculated pods collected from trees injected with PA were significantly reduced compared to those from untreated trees. The inhibitory effect was evident one week after treatment and persisted for at least eight weeks. Field evaluation of PA was initiated in Jerangau, an area with high black pod incidence and heavy rainfall. Two, four and eight gm of PA per tree were injected through the main trunk using a plunger-type trunk injector (COLT 75, TIS Enterprise, PA, USA). Significant reduction in the incidence of black pod was recorded 1-3 months after treatment and it became even more pronounced after 3-5 months. There was no significant difference in disease incidence among trees injected with various dosages of PA. It is estimated that in a cocoa planting with an average of 800 kg/ha and 40percent black pod incidence, application of 2 gm PA/tree/season could give a nett saving of $200-$400/ha