Abstract:
Severe destruction of cocoa pods in fields by Black Pod Disease (BPD) caused by Phytophthoramegakarya (Pm) is a major challenge to cocoa growing farmers. The inability of cocoa farmers in Nigeria to predict BPD outbreak encourages indiscriminate use of environmentally toxic fungicides. Prediction of time of BPD occurrence can provide information on the possible Areas Under Severe BPD Attack (AUSBPDA), minimise fungicide misuse and increase control accuracy. However, there is a dearth of information on BPD outbreak in Nigeria. Therefore, this study was designed to investigate and develop a modified model for BPD outbreak in Southwestern Nigeria.
Structured questionnaires were administered to 12 cocoa farmers and 12 commercial cocoa farms in Adaàgbà, Dáagi-Lógbà, Iyánfowórogi, Mòyè, Olórò and Wáàsimi, with two locations in Ǫbáfémi-Owódé, Òwenà, and Owódé-Igàngán. Farms were visited from May, 2015 to May, 2016 to determine the frequency of fungicide application as well as monitor BPD occurrence and severity. Infected cocoa pods (504) and topsoil (156) samples were collected for laboratory analysis. Isolation and identification of Pm were done using standard procedures. The Multiple Regression Model (MRM): Y=α+β1X1+β2X2+…+βnXn where Y is Nx1 matrix of response variable, X1,X2,…Xn are NxK matrices of regressors, and β1,β2,…βn regression coefficients was used in model development. Four existing models (MRM1,...MRM4) and the modified MRM5 (ETAPOD) were fitted from real life BPD data. The performance of the models were ascertained using Standard Error of Regression (SER), Root Mean Square Error of Prediction (RMSEPred.) and Adjusted R2 (R-SqAdj.). Prediction(s) made by the best fitted model was compared to field observations of monthly BPD occurrence (MBO), Total Annual Occurrence (TAO), and Average Annual Occurrence (AAO) from 2015/2016 season.
All the cocoa farmers used fungicides for BPD control and 64% applied it monthly. The percentage BPD occurrence for Adaàgbà, Dáagi-Lógbà, Iyánfowórogi, Wáàsimi, Ǫbáfémi-Owódé, Òwenà, and Owódé-Igàngán were 16, 15, 16, 23, 22, 30, and 9, respectively. The percentage BPD severity in Òwenà and Ǫbáfémi-Owódé was 100, while Adaàgbà, Dáagi-Lógbà, Iyánfowórogi, Owódé-Igàngán and Wáàsimi were 95.1, 84.3, 90.9, 71.1, and 96.7, respectively. Severity in Mòyè and Olórò was undetermined. Photomicrograph of Pm showed hyaline, septate and heterogeneously branched sporangiophores with uniflagillated, ellipsoidal zoospores. The mycelia appeared cotton-white with secretion of lemon-yellowish metabolite. The preferred model was ETAPOD followed by MRM4, MRM1, MRM2, and MRM3 in terms of SER (0.22, 0.39, 0.45, 0.45 and 0.45), RMSEPred.(0.30, 039, 0.46, 0.46 and 0.46) and R-SqAdj.(0.67, 0.49, 0.32, 0.32, and 0.31), respectively. Predictions made with ETAPOD showed that MBO, TAO and AAO forÒwenà and Wáàsimi were 9.05, 72.3 and 6.0% compared with observed BPD values of 9.5, 70.0, and 5.8%, respectively. Adaàgbà, Iyánfowórogi, and Owódé-Igàngán had 9.43, 77.8, and 6.5% as their predicted BPD values compared with the observed values of 9.0, 53.5, and 4.46%, respectively. ETAPOD performed better than other models and its predicted values were within the range of real life occurrence.
The modified model (ETAPOD) was effective in the prediction of black pod disease occurrence. This will minimise problems of fungicide misuse and improve cocoa production in Nigeria.
