Cocoa pod as substrate for cultureing phytophthora palmivora (Butl.) Butl., causal organism of black pod disease, and development of fungicidal control for the disease.

The U.S.T. isolate of Phytophihora palmivora (Butl.) Butl. which causes the black pod disease of cocoa was characterised. Oats, maize and cocoa husk were chemically analysed and used to prepare different growth media using the methods of water decoction and solvent (ether) extraction. Growth rate, sporulation and sporangial length/breath ratio of the isolate were determined on the media. Oats, which had relatively high levels of carbohydrates (68.29%) and protein (15.54%) was the best growth substrate recording the highest level of sporulaton of 26.20 x 10 ml and sporangial length/breath ratio of 1.56. Cold sterilized cocoa media proved superior to the heat-sterilized media. Sporangial germination of the isolate was inhibited in vitro by acids partially neutralised acids, copper compounds and mixtures of acids and Copper compounds. Copper Sulphate, Bordeaux mixture and Kocide 101 proved highly inhibitory and recorded ED50 values of less than 50ppm (0.005%) whiles the acids and partially neutralised acids had ED50 values of less than l50ppm (0.015%). Copper Sulphate had the lowest value of 16. l6ppm and partially neutralised Propionic acid had the highest of 103.62ppm. Mixtures of acids and copper compounds had ED50 values of less than l00ppm (0.0 1%) with Copper Sulphate plus Phosphrous acid mixtures and Bordeaux mixture plus Phosphorous acid mixtures recording the lowest ED50 value ranges of 9.13 to 19.4lppm and 10.93 to 23.77ppm respectively. Vegetative growth of the isolate was inhibited in vitro by acids, partially neutralised acids, copper compounds and mixtures of acids and Copper compounds with ED50 values of less than 400ppm (0.04%). Phosphorous acid and partially neutralised Phosphorous acid recorded the lowest values of 20.8 lppm and 38.3 lppm whiles Kocide 101 and Bordeaux mixture had the highest values of 249.36ppm and 345.5 lppm respectively. Mixtures of acids and copper compounds had ED50 values of less than 300ppm (0.03%) with Copper Sulphate plus Phosphorous acid mixtures and Bordeaux mixture plus Phosphorous acid mixtures recording the lowest ranges of 1.70 to l1.29ppm and 105.50 to 213.09ppm respectively. Strong positive correlation existed between vegetative growth inhibition and concentration of test chemicals or fungicides. In detached pod bioassays, trunk injections of partially neutralised acids significantly (5%) reduced lesion areas and sporulation of pods inoculated with the isolate. Injection rates of 2.5gltree, 7.5gltree and 15.0ltree of 20% partially neutralised Phosphorous acid gave the best treatment effect. 20% partially neutralised Ethanoic and Propionic acids at 5.0 g/tree and 7.5g/tree did not affect lesion area and sporulation significantly (5%). Foliar spray of both 1% and 4% partially neutralised Phosphorous, Ethanoic and Propionic acids at rates of 5.0g/tree and 20.0g!tree respectively as well as 8% partially neutralised Phosphorous acid at 40g/tree did not significantly (5%) reduce lesion areas of cocoa pods but both 1% and 4% Kocide 101 and Bordeaux mixture at 5.0g/tree and 20.0g/tree respectively did. 4% partially neutralised Phosphorous acid, Ethanoic acid, Kocide and Bordeaux mixture at 20g/tree as well as 1% Kocide 101 at 5.0gltree all reduced sporulation significantly (5%) but 1% partially neutralised Phosphorous acid, Ethanoic acid, Propionic acid and Bordeaux mixture did not. Although location did not affect the overall capacity of treatment effect of trunk injections and foliar spray, it brought about transient differences in the performance of treatments from one location to another. Treatment effects of trunk injections of partially neutralised acids were detected as early as 2 weeks after treatment applications. Whiles treatment effects of foliar spray of partially neutralised acids were not detected after 2 weeks, that of Kocide 101 and Bordeaux mixture were detected Trunk injections of partially neutralised Phosphorous acid still had significant (5%) effects 12 weeks after treatment applications but foliar spray did not. High doses of up to 20g/tree of 4% Kocide 101 and Bordeaux mixture did not offer reliable treatment effects after 12 weeks. The interaction of time and treatment effect was attributed to mode of treatment application and the weathering action of rainfall at the locations. Trunk injection was observed to be less weather dependent.

Source: KNUST