Coffee leaf rust: a complex disease

Coffee leaf rust in Bolivia. Photo by Neil Palmer (CIAT).

Coffee rust (Hemileia vastarix) is fungus that is one of the most devastating coffee diseases in the world. Native to Africa, it is now present in every coffee-growing nation. Infected leaves drop off, weakening the plant. Copper-based fungicides can be effective against coffee rust. However, it must be on the leaf surface to prevent infection, and copper can build up in soils, eventually reaching toxic levels.

The spores of coffee rust are spread by wind or rain, and will only germinate when exposed to one to two days of continuous wetness. Usually humidity is not enough; rain provides the necessary conditions.  Latin America has suffered through more frequent and prolonged rainy periods in recent years as have parts of Asia, thought to be due to climate change. As a result, coffee rust has been having increasing detrimental effects on coffee production.

Aside from fungicide application, there are two lines of defense against coffee rust: planting resistant hybrids, and modifying cultivation methods.

When coffee rust first arrived in the New World, around 1970 in Brazil, nearly all coffee being grown commercially was genetically nearly identical and very susceptible to rust. Thus, the development of rust-resistant hybrids has been an important element in battling this disease. Rust fights back, of course; there are over 40 strains of coffee rust, some of which now attack previously-resistant hybrids.

Colombia has made a large investment in “renovating” the nation’s coffee farms by swapping out older coffee varieties with rust-resistant types. This has been accomplished on a third of Colombia’s coffee lands so far. Some of the resistant varieties include Castillo, Tabi, and Colombia. One thing they all have in common is parentage that includes Hibrido de Timor, which in itself is a natural hybrid between arabica coffee (Coffea arabica) and robusta coffee (C. canephora).

Reliance on selective breeding and fungicides have more or less dominated the fight against rust, because there is debate about which cultivation method, sun or shade, helps mitigate this disease. This is in large part because the various factors that influence the severity of coffee rust outbreaks can be influenced, sometimes dramatically, by microclimate. This makes farm-wide management decisions tricky. Some examples of the quandary:

• In full sun, plants dry out faster, hindering the germination and spread of rust, and sun may kill the fungus. However, pores (stoma) on the surface of leaves open up at high levels of light intensity, allowing more frequent and deep penetration of rust spores that are present.
• Coffee grown in sun (especially if generously fertilized, as is usually the case) produces heavy yields. This over-bearing stresses the plants and makes them more apt to succumb to rust and other diseases.
• Shade increases the life of leaves and their size, so rust spores have more leaf area to colonize and time to be dispersed.
• Coffee types that are less genetically resistant to rust show lower levels of infection at lower levels of light intensity.
• Shade trees intercept raindrops in gentle rains, so spores are not dispersed. In heavier rains, however, larger drops of rain accumulate and splash on the coffee, releasing many spores.

One very interesting bit of research suggests that growing coffee under shade can facilitate complex ecological connections that help fight coffee rust in Latin America. Green coffee scale (Coccus viridus) is a small insect that has many hosts, including coffee (on which it is usually more of a nuisance than a pest). Like many scale insects, it has a mutalistic relationship with some species of ants. The ants feed on “honeydew,” a sweet substance exuded by the scale insects. In exchange, the ants defend the scale insects from predators.  Meanwhile, green coffee scale can be infected with a fungus (Lecanicillium [Cephalosporium] lecanii) …which also attacks coffee leaf rust. Researchers have found that where green coffee scales are protected by a particular common species of ant (Azteca instabilis) which nests primarily in the shade trees, the scale insects reach localized population levels that enable the Lecanicillium fungus to attack the coffee rust.

Although not conclusive, this study suggests that shade coffee systems may provide some level of biological control over coffee leaf rust, and in fact this ant-scale-fungi relationship may be one reason rust was not as devastating in the New World as it was in the Old World. (The ants, scales, and fungi are only part of this very complicated system, which also includes a decapitating fly, wasps, and a lady beetle; see Vandermmer et al. 2010 for more fascinating details.)

Avelino, J., Willocquet, L., & Savary, S. (2004). Effects of crop management patterns on coffee rust epidemics Plant Pathology, 53 (5), 541-547 DOI: 10.1111/j.1365-3059.2004.01067.x

Vandermeer, J., Perfecto, I., & Liere, H. (2009). Evidence for hyperparasitism of coffee rust (Hemileia vastatrix) by the entomogenous fungus, Lecanicillium lecanii, through a complex ecological web Plant Pathology, 58 (4), 636-641 DOI: 10.1111/j.1365-3059.2009.02067.x

Vandermeer, J., Perfecto, I., & Philpott, S. (2010). Ecological Complexity and Pest Control in Organic Coffee Production: Uncovering an Autonomous Ecosystem Service BioScience, 60 (7), 527-537 DOI: 10.1525/bio.2010.60.7.8

Posted in Research on coffee growing