AGRICULTURE AND BIOLOGY JOURNAL OF NORTH AMERICA ISSN Print: 2151-7517, ISSN Online: 2151-7525 © 2010, ScienceHuβ, http://www.scihub.org/ABJNA Screening three Aspergillus species for antagonistic activities against the cocoa black pod organism (Phytophthora palmivora) Adebola, M.O. and Amadi, J.E* Department of Plant Biology, University of Ilorin, PMB 1515, Ilorin, Nigeria ABSTRACT In vitro screening using the dual culture technique was undertaken to assess the potential of three Aspergillus species, A. fumigatus, A. repens and A. niger as biological control agents against Phytophthora palmivora, the pathogen of cocoa black pod disease. The test organisms were isolated from the same cocoa farm where the disease occurred. Results revealed that all the test antagonists effectively checked the growth of the pathogen. The test antagonists grew faster than the pathogen and produced inhibition zones thereby limiting the growth of the pathogen. In solid medium, A. repens was the most antagonistic organism under the conditions of this study. The culture filtrates of the test fungi also inhibited the growth of P. palmivora with A. niger showing the highest percentage inhibition (54%) and A. repens the least (44.5%). Keywords: In vitro, antagonism, Aspergillus, inhibition. INTRODUCTION Cocoa (Theobroma cacao L.) family Sterculiaceae is native to the rainforest of tropical America. It is the basic raw material for the production of cocoa butter, cocoa bread, cocoa cake, cocoa oil or pomade, wine, shoe polish, cocoa powder, organic fertilizer, nematicides, alkali and native soap. Many diseases plaque the cocoa tree the most popular being the black pod disease caused by a fungus, Phytophthora palmivora. Chemical control of this disease is not cost-effective coupled with the obvious environmental hazards. According to Purdy and Schmid (1996), high volume spraying of chemical caused pod injury, which leads to superficial blackening of the pod surface due to death of epidermal cells. Burying pod husks has little effect on viability of the pathogen and only increase their population in the soil. Frequent harvesting and removal of infected pods will generally reduce black pod but these methods are not often adopted by farmers (Tondje et al., 1993; Nduombe- Nkeng et al., 2004). Biological control, being relatively cheaper, less laborious and environmentally friendly, therefore, becomes an attractive option. According to Okigbo (2000), biological control has proved to be durable on its effect and has the advantage of not requiring repeated periodic application as in case of chemical fungicides. The objective of this study, therefore, is to investigate the potential of three Aspergillus species isolated from the cocoa rhizosphere and rhizoplane as biological control agents of P. palmivora. MATERIALS AND METHODS Isolation of the Pathogen and Test organisms: Native potential fungal antagonists were isolated from cocoa rhizosphere and rhizoplane in cocoa farms at Aba-Ijesha in Atakunmosa LGA Osun State Nigeria. Cocoa leaves, stems and roots from healthy trees were collected and surface-sterilized with sodium hypochlorite and then rinsed in three changes of sterile distilled water. Isolations were made from these cocoa parts in water agar (WA) at room temperature (28 ± 2oC) for 7 days, to allow for the growth of all organisms (Tondje et al., 2006). Isolations were also made from the soil and infected plants. Transfers were made unto potato dextrose agar (PDA) plates (Odigie and Ikotun, 1982). Stock cultures of the isolates were maintained at 4o C in bottle slants for subsequent studies. Screening the isolates for antagonism a. Dual-culture technique: Inhibition of pathogen growth by the test antagonists was carried on PDA using the dual culture technique. Five millimeter- diameter mycelial plugs of each test antagonist were placed at the periphery of three different culture plates and incubated for 2 days at 28 ± 2oC (Evans et al., 2003; Holmes et al.,2006). After two days each plate was doubly-inoculated with another 5-mm- diameter mycelial plug of the pathogen placed 5 cm from the test antagonist. The dual culture plates were incubated for additional 9 days at 28 ± 2oC. In the control experiment, the test antagonists were replaced with sterile agar plugs. The growth of the pathogen in both the test and control experiments was recorded. Data were obtained for the percentage inhibition of radial growth (100 x (R1 - R2)/R1 - where