Hemileia vastatrix. It first entered the written record in 1869, when it was found on a remote coffee farm in Ceylon (now Sri Lanka)—then the world’s third-largest coffee producer. A little more than a decade later, the rust had driven Ceylon’s coffee growers to abandon coffee. Between 1870 and 1990, the rust slowly made its way around the world’s coffeelands, first striking Asia and the Pacific, then Africa, and finally reaching Latin America’s vast coffeelands in 1970. By 1990, it had reached virtually every major coffee-growing region in the world except Hawaii. In some places, as in Ceylon, it was a catastrophe. The rust helped drive the collapse of coffee farming in Java, an island whose name remains synonymous with coffee. As the rust made its way across the globe, however, farmers and scientists gradually learned how to adapt their farms and farming practices to the disease. Farmers were supported by a complex network of national and international organizations. By the 1990s, it seemed that the rust was just another disease. Coffee communities had adapted to the rust, much the same way that communities around the world had adapted to the influenza virus. Like the flu, the rust could be a nuisance. But properly managed, it was nothing more than that—at least in theory.
Disease, Landscape, and Society
To understand the coffee rust’s tangled history, it is helpful to understand how crop epidemics work. The coffee leaf rust is much more than the fungus alone. The fungus is present in many coffee ecosystems; in some, the coffee plants have mild infections that never develop into full-blown, disruptive epidemics. So clearly the epidemic is much more than the pathogen. We need to look beyond the pathogen alone and ask, What makes the disease a disease? To answer that, we need to consider how the coffee rust fungus interacts with the rest of the coffee ecosystem. It is helpful to consider an epidemic as a system with three major elements: the pathogen (the fungus H. vastatrix), a susceptible host (in this case the coffee plant), and the appropriate environmental conditions (rainfall, temperature, sunshine, cropping patterns, etc.). These three elements—virulent pathogen, susceptible host, and environmental conditions—can be represented as a triangle (fig 1.1).2
Epidemics are only possible if all three elements are in place. Most obviously, if the pathogen is not present, there can be no outbreak. But while the fungus is necessary for an outbreak, it is not in itself sufficient to cause one. The fungus and the susceptible coffee plant may be present in an ecosystem, but environmental conditions—say, the temperature or the farm structure—may prevent the fungus from reproducing rapidly, so there is no outbreak. In still other cases, the fungus may be present and the environmental conditions may favor the disease, but the coffee cultivar is resistant to the rust, so there is no outbreak. Furthermore, none of the three elements is absolute; different strains of the fungus can be more or less virulent, and different coffee cultivars can be more or less resistant. The environmental conditions also favor the epidemic to a greater or lesser degree. We can use the disease triangle to understand how the host, pathogen, and environment interacted in each place to produce an outbreak.
Each of these three elements is not only biological, it is also historical. Each element changes over time, the product of interactions among human and natural forces. People have, by planting thousands or even millions of susceptible plants together, unintentionally created environments that favor rust outbreaks. They have unintentionally carried spores of the rust farther and faster than the rust would have traveled on its own, infecting coffee zones that had previously been free of the disease. As farmers and scientists learned more about how the disease worked, they manipulated the host, pathogen, and environment to limit the rust. They tried to contain the pathogen through quarantines and to kill it with chemicals. They have strengthened the host by breeding rust-resistant coffee varieties. They have altered the coffee ecosystem, in places, by reducing or eliminating shade trees (fig 1.2), hoping that exposing the coffee farm to full sun would inhibit the rust.
Figure 1.1. The disease triangle, showing how the pathogen, host, and environment interact to produce a disease outbreak.
The rust attacks the leaves of the coffee plant, but it harms the whole plant. A healthy coffee tree obtains most of its nutrition through its leaves, by photosynthesis. Nutrients allow the tree to produce new branches and buds, which in due course flower and develop into the fruit. In shaded forests, the coffee plants produce few flowers, and the leaves can provide more than enough nutrients to allow the fruit to develop properly. On the farm, farmers often manipulate the plant and the landscape to encourage the plant to produce more fruit. They reduce or eliminate shade, which encourages the plant to produce more flowers and, in turn, more fruit. They can also increase crop yields by pruning and manuring the trees. But they have to be careful not to ask too much of the tree, particularly the leaves. If the nutrition required by the fruit is greater than the tree can provide, the fruit may fail to develop properly; in some cases, the branches can become starved and die. So even in disease-free ecosystems, farmers have to ensure that they do not ask the tree for more nutrients than it can deliver. Sometimes they cut it close. Heavy fruit loads can cause coffee harvests to fluctuate widely from one season to the next. A heavy crop one season can draw so many nutrients that it inhibits the growth of branches and flowers the following season, leading to a lower fruit load—a pattern commonly described as biennial bearing.3
Figure 1.2. Coffee monoculture, nineteenth century. Dense stands of coffee like this were highly productive but also highly susceptible to diseases and pests. (In Thurber, Coffee, facing p. 7)
The rust disrupts this delicate balance. When a microscopic rust spore germinates, it sends shoots into the leaf and develops into a mycelium that colonizes the leaf and feeds off the leaf tissue. It creates a circular rust-colored lesion on the leaf, which gives the disease its name. If a leaf has just a few lesions, it can continue functioning more or less normally (see fig. 1.3). But if the conditions favor the fungus, the leaf can develop many lesions, making it difficult for it to deliver nutrients to the plant. Badly infected leaves can drop off altogether. During severe outbreaks, trees can be defoliated, depriving the tree of vital nutrients. Branches fail to develop normally and die back. The fruit, likewise, may not develop properly, or at all (see fig. 1.4). Severe rust outbreaks cause significant losses during the current season (primary losses).
The most serious effects of the rust, however, are typically felt in later seasons (secondary losses). After an outbreak, the trees may seem to recover; the next season they produce a new flush of leaves, and to a casual observer all seems well. But damaged or dead branches can no longer produce fruit. A recent study of coffee diseases and pests in Central America found that primary losses could be as high as 26 percent, while secondary losses reached 38 percent.4 The rust exacerbated the patterns of biennial bearing; both the troughs and the peaks were lower. Once the fungus was present in the ecosystem, it was effectively impossible to eliminate. The coffee farm itself became a reservoir of infection. Farmers had to find ways of coexisting with the disease.
Picture a tropical mountainside, then divide it into three belts by altitude. The fungus is highly sensitive to temperature, and in the tropics, temperature is in turn heavily influenced by altitude. In the highest belt, temperatures are relatively cool. The rust fungus may be present but does not cause any significant damage to the plant. Coexistence is easy; farmers in this belt do not usually have to take any special measures to control the rust. Conversely, in the lowest and warmest belt coexistence is virtually impossible. The fungus flourishes and causes such extensive damage that farmers cannot produce the crop profitably. Yields are too low, or the costs of managing the disease are too high. Those farmers typically abandon coffee cultivation. Much of our story will focus on the middle belt, the space where coexistence is possible but requires effort. Here, the rust can cause significant damage, but coffee may still be profitable if farmers can manage it. The three belts lie on a continuum.