Map 2.1. Distribution of coffee species in the wild. (From Maurin et al., “Towards a Phylogeny for Coffea,” 1571)
Figure 2.1. Coffee branch and berries. (In Thurber, Coffee, frontispiece)
Genetically, the arabica coffee plant differs from other Coffea species in one significant way. It is self-fertile (autogamous), meaning that only one parent plant is necessary to flower and produce seeds. All other species of Coffea are allogamous, requiring two parents to flower and produce seeds. Arabica’s distinctive genetic makeup made it suitable for transplant over great distances, since only a single plant (or seed) was necessary to establish a new population.3 It also meant that populations of arabica tended to be homogeneous, which, as we shall see, could be environmentally risky but commercially desirable. Arabica’s self-fertility made it easy for farmers to produce a consistent product from one harvest to the next. These advantages came at a price, however. Arabica coffee is also less genetically variable than other coffee species, leaving it susceptible to diseases and pests.4
One of these is the coffee leaf rust, caused by the coffee rust fungus known scientifically as Hemileia vastatrix. It is an obligate parasite of coffee; that is, it can only complete its life cycle on plants of the genus Coffea. The fungus begins its life cycle as a tiny spore. The spore will only germinate in specific conditions: it must be deposited on the underside of a coffee leaf, the air temperature must be 15°C–28°C (optimally 21°C–25°C), and water droplets must be present on the underside of the leaf. A coffee writer in the 1920s aptly described fungi like H. vastatrix as the “vampires of the vegetable world” since they feed on the tissue of other organisms. Once H. vastatrix germinates, it penetrates the leaf and sends shoots into the leaf tissue. The fungus creates a branching mycelium that feeds on the surrounding leaf tissue, forming circular orange pustules. Ultimately, these shoots produce spore buds that pierce back out through the underside of the leaf. Each pustule can contain as many as one hundred thousand spores, each of which can begin the infection cycle anew (see fig. 2.2). The spores can be dispersed by winds and rain, or by the many insects, animals, and people that pass through the ecosystem. During a severe rust outbreak, rust pustules can cover the coffee leaves, causing them to fall prematurely. Defoliation deprives the coffee plants of vital nutrients. Repeated infections debilitate the plant, preventing the branches and fruit from developing fully.5
The precise geographic distribution of H. vastatrix in the wild remains unclear. The coffee expert Albertus Eskes argues that “it is most likely that H. vastatrix has coevolved simultaneously on many coffee species from all over Africa.”6 In principle, the geographic range of H. vastatrix in the wild could have been as large as the range of the Coffea genus. But some fragmentary historical evidence, discussed in later chapters, suggests that H. vastatrix’s wild range spanned the Great Lakes region and Ethiopia in East Africa, as well as the eastern half of the Congo River basin. The genetics of the coffee plant and the fungus also offer clues about the fungus’s historical distribution. The fungus has evolved into strains (physiological “races”) that specialize in attacking particular species of coffee. Most Coffea species, in turn, have developed some degree of resistance to the fungus. The most highly rust-resistant species, such as C. canephora, grew in warm and humid areas favorable to the fungus. The least rust-resistant species, including C. arabica, grew in cooler and drier areas that were less hospitable to the fungus.7 The presence of resistant genes in each coffee species therefore offers clues to the presence of the rust in its habitat.
Figure 2.2. Coffee rust infection process, simplified. A. Rust spores are disseminated by wind, humans, rain splash, insects, and animals. B. Spores are deposited on healthy coffee leaf. C. Spores germinate and infect leaf tissue, producing characteristic orange-colored lesions on the leaf. D. Infection causes premature leaf fall. E. Cross-section of infected leaf, showing how fungus colonizes leaf tissue and then sporulates, beginning the infection process anew. (Illustration by Angel Luis Viloria Petit)
Although H. vastatrix was widely distributed across Africa, serious outbreaks were unknown. “The available information,” writes Eskes, “suggests that most African coffee species have developed a balanced relationship with H. vastatrix, showing generally little disease under natural conditions.”8 Both the historical and genetic records suggest that the rust fungus was widespread in the wild home of arabica coffee, yet it is unlikely that there was ever a major outbreak there. Wild arabica, like other coffee species, developed ways of coexisting with the rust. All arabica varieties have some degree of resistance, but genetic resistance alone does not explain the absence of major outbreaks in Ethiopia. Under the right conditions, as coffee farmers would later discover to their dismay, arabica could be highly susceptible to the rust. But the rust was also kept in check by the environment of Ethiopia’s highland forests. The average temperatures of the highlands are, in places, cooler than ideal for the rust spores to germinate. The dense forest intercepted rainfall, which made it more difficult for the spores to germinate and spread. The forest also blocked the wind, which limited the circulation of rust spores. The comparatively low density of coffee plants in the forest also limited the opportunities for spores to find a host to reproduce upon. And, in Ethiopia, H. vastatrix was itself parasitized by hyperparasitic fungi found in the forest, of the genera Darluca and Verticillium.9
“Arabian” Coffee in the Islamic World, 1450–1700
The first people to regularly consume coffee likely left the forest cover intact, at first. The people of southwestern Ethiopia may have first consumed the leaves of the coffee—as a tisane—rather than the fruit. But at some point, they also began consuming the fruit. According to one often-repeated legend, a goatherd named Kaldi discovered coffee’s stimulant properties when his goats started dancing after eating coffee fruit. Although this charming story is likely not true in its specifics, it does suggest one way that people may have discovered the bean’s stimulant properties. There is some debate as to whether Ethiopians consumed coffee beans as a food—mixed with butter, honey, and spices—or as a drink.10 At first, the people of Kaffa likely foraged for coffee, harvesting the fruit from wild trees. At some point they realized, speculated the botanist Pierre Sylvain, that plants exposed to the sun yielded more coffee. So they began to manage the forest canopy, reducing the shade to increase the yields of wild plants. Some people transplanted wild coffee seeds and seedlings from the forest to gardens near their houses, where they cultivated coffee alongside other crops. As coffee became more popular in Ethiopia, people started moving coffee plants beyond their native range.11 These changes presented the rust with new opportunities to spread, though if it did, the levels of infection likely remained low. “Diseases and pests do not seem to be a problem in the coffee forest,” wrote Sylvain in 1956, “where man has not changed the biological equilibrium.”12 The rust was likely not a major problem on the semiforest or garden coffees either. The coffee plants were protected by a measure of genetic resistance, cropping practices, and temperatures that were favorable to the plant but inimical to the rust.
Coffee’s life as a global commodity began early in the fifteenth century CE, carried along trade routes that linked southern Ethiopia to the Red Sea through the port of Zeila. At first, the growth of coffee consumption in precolonial Africa did little to alter the relationship between the plant and the pathogen in Ethiopia because most coffee was harvested from wild plants.13 Sufi Muslims were instrumental