Coffee leaf rust has been a problem for coffee farmers for over 150 years. Leaf rust co-evolved with coffee and appears naturally in wild coffee forests across Africa. But in those environments, rust populations are kept under control by natural enemies, especially mycoparasites (fungal parasites of other fungi).
Without natural enemies to control rust, farmers must rely on other techniques to control it—often, this includes expensive chemical fungicides whose use precludes organic certification. The other major alternative for fighting rust is replacing traditional varieties that are susceptible to the disease with rust-resistant varieties; however, farm renovation is costly and leads to lost income as farmers wait for new trees to mature, and many existing rust-resistant varieties appear to be losing their resistance.
Biological control is a potentially powerful tool for managing coffee leaf rust that would allow for organic certification and the continued use of traditional varieties. Biological control has been used extensively and successfully for the management of weeds and pests but, surprisingly, never for crop diseases. The concept is simple: In nature, major diseases and pests typically have co-evolved with their own natural enemies, which keep their population in check.
In the case of coffee leaf rust, Hemileia vastatrix, the most exploitable of these natural enemies are mycoparasites, which colonize and destroy the rust spores. Such mycoparasites of coffee rust already exist in Latin America but these are non-specialized opportunists and are unlikely to exert significant natural control without the use of costly technology to develop a formulated product (mycofungicide). Classic biological control exploits the ability of coevolved fungal natural enemies to produce massive quantities of innoculum on their hosts and to spread and propagate continuously within host populations; offering sustainable rather than short-term control.
In addition to reuniting coffee diseases with natural enemies, biocontrol can also reunine crops with their natural allies. These allies, called endophytes, are microorganisms that live symbiotically within the coffee plant and act as bodyguards helping the plant to fight off attacks. Biocontrol can be an effective, sustainable and ecologically benign method of fighting plant disease, as well as being highly cost effective.
Conducting collection missions to the centers of evolutionary origin for C. Arabica and C. canephora in Kenya, Ethiopia, and Cameroon, researchers have collected both mycoparasites of leaf rust and endophytes of coffee trees, and are testing them to see if they would be good candidates for farm-level control.
If a suitable biocontrol candidates can be identified to fight rust in the short term, they can be replicated and introduced relatively quickly (especially when compared with the amount of time it takes to develop a new rust-resistant variety of coffee through breeding). Biological control could be a cost-effective rust management technique for farmers that would allow them to maintain organic certification.
As the recent coffee leaf rust epidemic in Central America proved, rust is one of the most significant threats to coffee farmers. But farmers circumstances vary, and they need more than one option for how to manage threats like leaf rust. Biological control represents an innovative potential to help farmers address this threat.