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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 packhouse 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
antag-onist–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.

South African Journal of Botany
2004, 70(1): 128–139