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

Author
item MELNICK, RACHEL - PENN STATE UNIVERSITY
item ZIDACK, NINA - PENN STATE UNIVERSITY
item Bailey, Bryan
item MAXIMOVA, SIELA - PENN STATE UNIVERSITY
item GUITINAN, MARK - PENN STATE UNIVERSITY
item BACKMAN, PAUL - PENN STATE UNIVERSITY

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2008
Publication Date: 6/1/2008
Citation: Melnick, R.L., Zidack, N.K., Bailey, B.A., Maximova, S.N., Guitinan, M., Backman, P.A. 2008. Bacterial endophytes: Bacillus spp. from vegetable crops as potential biological control agents of black pod rot of cacao. Biological Control. 46:46-56.

Interpretive Summary: Cacao (Theobroma cacao), the source of chocolate, is a small tree grown on small farm in tropical regions around the world. In order to produce the crop, farmers must over come many plant diseases. We studied the ability of beneficial bacteria, bacillus species, to control the cacao disease black pod, caused by Phytophthora capsici. One of the bacteria studied was able to grow on and in cacao leaves without causing damage while at the same time limiting disease. It may be possible to protect cacao from black pod disease by using beneficial bacteria. New disease control strategies would benefit farmers by stabilizing crop yields and profitability. In addition, United States farmers who produce commodities used in chocolate production and the United States chocolate industry would benefit by the stabilization of global cocoa bean supplies and markets.

Technical Abstract: Diseases are the most important factors limiting the 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 to colonize Theobroma 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) and BP24 (from potato) together with the polysilicon surfactant Silwet L-77 (0.24% v/v) resulted in long-term (68-70+ days) stable colonization of cacao leaves. Further investigation revealed that foliar colonization by BT8 and BP24 was primarily epiphytic, with endophytic populations representing only 5-15% of total foliar bacteria. Significant reduction of disease severity (P<0.05) on leaf disks challenged with P. capsici was recorded after 32 days of colonization with BT8. No bacterial colonists were observed in the leaves that were developed after bacterial application, suggesting that the bacteria were not capable of systemic movement through vascular tissues. Additionally newly developed, non-colonized leaves from the colonized plants exhibited disease suppression, which presents an evidence for systemically induced disease resistance.