Biocontrol of vascular streak dieback (Ceratobasidium theobromae) on cacao (Theobroma cacao) through induced systemic resistance and direct antagonism

Abstract

ABSTRACTVascular streak dieback (VSD), caused by Ceratobasidium theobromae (P.H.B. Talbot & Keane), has a considerable impact on cacao (Theobroma cacao L.) production in Southeast Asia. Two biocontrol experiments were set up to explore alternatives to ineffective chemical control of VSD. The effects on VSD development of (i) Trichoderma harzianum, superficially applied to leaves of mature cacao plants, and (ii) an inoculation with fungal (Trichoderma asperellum) and bacterial (Bacillus sp., Pseudomonas sp. and Enterobacter spp.) elicitors of systemic resistance in young cacao plants, were tested. After three months, cacao leaves treated with T. harzianum exhibited a significantly (p < .05) lower disease score compared to the untreated control leaves, with no significant differences or interaction effects between three cacao clones tested. Six months after inoculation of cacao seedlings with elicitors, a significant (p < .05) effect on the number of VSD-affected leaves per plant was evidenced on seedlings treated with Bacillus sp. and Enterobacter spp. After transplanting to the field (8 months after inoculation), both number of VSD-affected leaves per plant and number of VSD-affected leaves per branch per plant were significantly (p < .05) lower as compared to control plants following inoculation by all bacterial elicitors tested, with no significant differences in the VSD controlling effect observed between the different bacterial elicitors tested. T. asperellum did not show potential as an elicitor of systemic resistance in our experiment. More research is needed on the lasting effect of biocontrol treatments, as well as on their economic and ecological sustainability.