Bacterial endophytes: Bacillus spp. from annual crops as potential biological control agents of black pod rot of cacao

Diseases are the most important factors limiting production of Theobroma cacao in South America. Because of high disease pressure and environmental concerns, biological control is a pertinent area of research for cacao disease management. In this work, we evaluated the ability of four Bacillus spp. isolated from vegetable crops, for their ability to colonize T. cacao seedlings and reduce the severity of black pod rot (Phytophthora capsici). Of the Bacillus spp. tested, application of B. cereus isolates BT8 (from tomato) or BP24 (from potato) together with the polysilicon surfactant Silwet L-77 (0.24% vol/vol) resulted in long-term (>68 days) stable colonization of cacao leaves. Further investigation revealed that foliar colonization by BT8 and BP24 was primarily epiphytic, with endophytic populations typically representing 5-15% of total foliar bacteria. Significant reductions of disease severity (P ≤ 0.05) on cacao leaf disks challenged with P. capsici were recorded from after day 26, and through 68 days following colonization with BT8. No bacterial colonists were observed in or on leaves that developed after bacteria application, suggesting that the bacteria were not capable of systemic movement through vascular tissues. These newly developed, non-colonized leaves from colonized plants exhibited disease suppression, which supports a probable disease suppression mechanism of induced systemic resistance for the BT8 isolate.