Biological control in Africa: can it provide a sustainable solution for control of fruit diseases?

Abstract

Biological control has evolved from an obscure science to a well-established field of study. Biological control systems for citrus and subtropical fruit crops have been studied at the University of Pretoria for more than 20 years. Various Bacillus spp. originally isolated from leaf and fruit surfaces effectively controlled Cercospora spot of avocado, anthracnose of mango and avocado, Dothiorella/Colletotrichum fruit rot complex and stem end rot of avocado and mango, soft brown rot on mango and postharvest decay and secondary infections on litchi and citrus. Control was achieved through semi-commercial preharvest sprays or postharvest pack-house dip and spray applications. Integrated treatments involving antagonists combined with quarter-strength or recommended dosage of fungicides, disinfectants or natural plant extracts also effectively suppressed postharvest diseases of avocado, citrus and mango. Antagonist attachment, colonisation and survival were studied using several techniques including viable counts, electron microscopy and monoclonal antibodies. Modes of action of these different antagonist-pathogen-host combinations indicated several levels of interactions that were time and density-dependent and included secondary metabolites, nutrient competition volatiles and competitive colonisation. Innovative alternatives to apply the antagonists in the field were evaluated and included the use of foraging bees to disseminate the antagonists to flowers and woolly-based plastic caps to provide a slow release effect for the antagonist under field conditions. Commercialising the antagonists proved to be difficult due to limitations in local registration guidelines. The South African fruit industries and the University of Pretoria currently hold the patent with regard to the formulation and application of these biocontrol products. One of these products, Avogreen (B. subtilis), a control agent against Cercospora spot on avocado, has been commercialised. This review discusses the problems, opportunities and challenges in developing and commercialising biocontrol agents in the African context. Only once these complex antagonist–pathogen–host interactions, microbial dynamic systems and environmental impact on product performance are fully understood, can biocontrol be truly regarded as a viable alternative to pesticides and will it provide some solution to Africa's critical crop protection needs.