The efficacy of three Aspergillus isolates for the Biological control of cocoa black pod disease pathogen (Phytophthora palmivora) on the field.

Abstract

Theobroma cacao ( cocoa tree), is a small (48m tall)evergreen tree in the family Sterculiaceae (synonym Malvaceae). Its seeds are used to make cocoa powder and chocolate. Phytophthora palmivora, the cocoa black pod disease pathogen cause the greatest damage to cocoa tree. Previous effort to combat this menace of cocoa such as agronomic and horticultural practices and the use of chemical were inadequate. This study investigated the efficacy of three Aspergillus species (Aspergillus fumigatus, Aspergillus repens, and Aspergillus niger) isolated from the cocoa rhizosphere and rhizoplane as biological control agent of phytophthora palmivora. The suspension of each of the three fungi was prepared by blending solid rice substrate with distilled water, 20% pectin and ammonium diphosphate (2:1 v/v) at pH5.5 for ten seconds in a blender and filtered through cheesecloth. The final concentration of each of the suspensions was adjusted to 10ˆ8 propagules/ml. The experimental plots were set up in three cocoa farms untreated for many years at Aba Ijesa, 45Km from Ile Ife, Osun State of Nigeria. The efficacy of the three fungal isolates were compared to the fungicide ( Metalaxyl-m) while water was used as control. The effects of the test antagonists on the pod infection differed significantly (P˂0.05). The percentage infection of pods by the pathogen was very low and found not to be significantly different (P˂0.05) in plots treated with chemical (11%) and A. repens (11%). The three antagonists effectively reduced the pressure of disease over time in this experiment. Percentage infection rose steadily in the control till the end of the experiment. The application of the three biocontrol agents effectively hampered the establishment of the pathogen on the pods, enhanced flowering, and increased cocoa pod production in the field.