Biological control is the use of one living organism to control another living organism that is considered a pest species. In addition to breeding new and better genetic material and the use of good crop husbandry, the development of an effective biological control strategy could provide another tool to manage coffee rust which would allow for organic certification and the continued use of heirloom varieties. If a suitable agent(s) can be identified in the short term, then this approach would be available in significantly less time than that needed to develop a new variety. CBC of fungi exploits the ability of coevolved fungal natural enemies in order to produce massive quantities of inoculum on the host plant and allow them to spread and propagate continuously within the host population. It offers a sustainable control method but has and has, surprisingly, never been used for crop pathogens (diseases). The concept is simple and follows the enemy-release hypothesis whereby an exotic or alien species increases its fitness, and hence its invasiveness, because it arrives without its guild of co-evolved natural enemies.
Examples
i) Bacteria
Bacteria such as Bacillus and Pseudomonas are known to produce compounds that negatively affect fungal pathogens of plants. Such bacteria evolved in the soil and utilize antifungal compounds to compete with soil dwelling fungi. A number of studies have shown how coffee rust development in greenhouse settings or in the lab can be retarded by Bacillus and Pseudomonas. For example, a study by Haddad et al, 2009 showed for the first time that certain strains of< i>Bacillus and Pseudomonas reduced coffee rust on organic farms in Brazil. In follow up work the same team (Haddad et al 2014) found 17 different bacterial isolates collected from leaves, leaf debris, and soil reduced both the infection frequency and the number of H. vastatrix urediniospores produced per leaf by more than 70%.
ii) Other fungi
White halo fungus, Lecanicillium lecanii, has been suggested as a potential biological control agent of coffee rust by Prof. John Vandermeer and collaborators at the University of Michigan (Vandermeer et al 2009). White halo fungus has been shown to be hyperparasitic on Hemileia vastatrix in laboratory conditions and it has also been observed attacking the fungus in the field. White halo fungus often infects green coffee scale which feed on coffee. These insects are frequently tended by ants which collect the sugar that they excrete. The ants often create clusters of scale insects on the plants which are infected with white halo disease. It is postulated that white halo fungus may attack and kill the coffee leaf rust fungus or may simply reduce its abundance due to crowding effects or produce chemicals to attack it. Currently, the fungus does not appear be a viable biological control agent because it has not evolved to parasitize the fungi, rather it evolved to infect insects. Promising attempts have been made to culture the fungus and apply it as a topical spray to control the rust fungus.