Coffee is one of the most important cash crops in the world, generating significant foreign exchange and supporting the livelihoods of millions of people globally. Over the last 30 years, demand for coffee has grown steadily, leading to an expansion in production and exports.
There are 124 species in the Coffea genus known to science (Davis, et al., 2011), with two that are cultivated widely and on a global scale —Coffea arabica (commercially known as arabica) and Coffea canephora (commercially known as robusta). Throughout this essay and the catalog generally, we use this term “robusta” to refer to the entire C. canephora species and all its subtypes.
Until recently, arabica held reign over most of the coffee market due to preferences for its cup quality, but various factors, including the increasing demand for coffee, have led to expansions in the production of robusta, as the species requires less stringent growing conditions and possesses a certain level of resistance to pests and diseases that often plague farm productivity. Robusta production expanded rapidly after the emergence of soluble coffee in the 1950s.
Presently, approximately 60% of the coffee produced and marketed in the world comes from arabica plants and 40% comes from robusta plants (ICO, 2021).
The top global producers of robusta are currently Vietnam, Brazil, Indonesia, Uganda, and India, which together produce over 90% of the world’s robusta (Slipchenko, 2021). Of these producers, Vietnam and Uganda are the foremost exporters of robusta (Brazil, for example, retains a substantial portion of its production for internal consumption). However, an increasing number of countries that currently restrict or have previously restricted coffee production to arabica are beginning to explore robusta; these include Mexico, Nicaragua, Guatemala, and Colombia, among others. Additionally, there is growing interest in exploring the potential of increasing the cup quality of robusta.
Coffea canephora Pierre ex A. Froehner is a species of coffee that originated in central and western sub-Saharan Africa. In the wild, it is found mainly in the understory of humid, evergreen forests (but sometimes in seasonally dry humid forests or gallery forests) with elevations ranging from 50 to 1500 m above sea level (Davis, et al., 2006).
The interest in producing Robusta at a global level resides in the fact that it can be grown in a wider range of climates and altitudes compared to arabica, which requires precise conditions in order to thrive, like heavy shade and high altitudes. In contrast to arabica, robusta plants typically have a greater crop yield, contain higher levels of caffeine, lower levels of sugar, higher levels of soluble solids, and are less susceptible to damaging pests and diseases (Goldemberg et al., 2015). Further, robusta can be grown in hotter, more humid temperature ranges, found in lower altitudes between 200 – 800 meters (Slipchenko, 2021), and often requires less maintenance via herbicide and pesticide (Daviron & Ponte, 2005). Despite these attributes, robusta is still sensitive to environmental disturbances. Research suggests that robusta’s ability to thrive in hotter climates may be overstated and that temperatures over 20.5 degrees centigrade can have a significant negative impact on yields Kath et al., 2020). Additionally, many robusta varieties are still susceptible to key diseases and pests, such as coffee leaf rust, stem borer, coffee berry disease, coffee berry borer, and nematodes, among others (Vega et al., 2006)
Due to the aforementioned benefits, though, robusta is often easier to farm, allows for greater productivity, and is more cost-effective to produce than Arabica. Ongoing climate predictions of rising temperatures and altered precipitation patterns by 2050 indicate that arabica cultivation may no longer be sustainable in the coming years, which may, in turn, increase the production of robusta by a significant margin (Bunn, et al., 2015, Kath, et al., 2023, Dinh, et al., 2022, Kath et. al, 2022, de Aquino, et al., 2022). Even so, robusta faces its own limitations and climate vulnerability (Tournebize, et al., 2022).
However, the beans that come from robusta production generate differences in terms of taste and cup quality (Leroy, et al., 2006). For instance, coffee brewed from robusta beans is often lower in acidity, higher in bitterness, and more “full-bodied” due to its pyrazine content (Miyanari, 2008), an aromatic known for its earthiness. But when handled and processed properly, Robusta can serve as a product for specialty markets (Uganda Coffee Development Authority, 2019).
Many different common terms are used to describe robusta in the areas where it is grown. These include “robusta,” “conilon,” “nganda,” “koillou/quillou,” and others. These terms are generally regional, colloquial, and do not necessarily correspond to specific genetically distinct varieties/clones that have been developed and released by breeders over the years. Because robusta cross-pollinates — a single robusta tree cannot successfully pollinate its own flowers, as Arabica trees can do; scientists call this “allogamous” (Nowak, et al., 2011) — subtypes grown in the same field typically interbreed (Thomas, 1935). A consequence of this mating system is that the majority of cultivated robusta is still made up of unselected populations obtained from open-pollinated seeds (Labouisse et al, 2020). For more background on Robusta breeding, see Montagnon, Thierry, and Eskes, 1998a & b.
Put simply, robusta plantations are not genetically uniform; consequently, many robusta farmers have little awareness of which variety or subtypes they are growing. This is one reason why colloquially, C. canephora is often referred to as simply “robusta,” as described and commercialized by Linden in 1900 (Dagoon, 2005).
Because robusta is a cross-pollinating species (i.e., it requires pollen from two different types of plants in order to produce new cherries), it is necessary for farmers to grow more than one type of robusta in their fields in order to have successful pollination and fruit production. Some breeding programs have developed and released “polyclonal” or “multiline” varieties to address this challenge where the “variety” is an intentional mix of genetically distinct clones (Campuzano, et al., 2022, Montagnon, et al., 2003, Berthaud & Charrier, 1998).
However, not all Robusta types can successfully grow together in a field — the cross-compatibility of types is genetically controlled. Some varieties are unable to fertilize one another (Lashermes et al., 1996, Prakash, 2018). So far, research on optimal combinations of subtypes in production has been scarce, but one key consideration is simultaneous flowering.
In different production regions, how such mixes are released and distributed for farmers is handled differently. It is common in West Africa, for example, for breeders to create polyclonal seed varieties (i.e., multiple different types of robusta are distributed together in the same seed packets to farmers). In Brazil, it is more common for breeders to create multiple unique clones that are then tested for compatibility; the highest-performing complimentary clones are then propagated and released to farmers (Depolo, et al., 2022, Surya, 2018).
The scope of genetic diversity in robusta coffee is much larger than that of arabica. There are many unknown variations (including traits related to cup quality) in the robusta gene pool. By and large, these hidden variations are yet to be explored by breeders.
Robusta originates from humid lowland forests in tropical areas of Africa, an area with a wide natural geographic distribution from Guinea to Uganda and Angola, growing in numerous forms and ecotypes. It has been surveyed and prospected by ORSTOM and FAO missions (Dussert et al., 1999). The exact natural origins of the cultivated types are difﬁcult to know for certain given the widespread introduction and naturalization of different subtypes around the region and the geneflow between wild and nearby plantations (Davis, 2006, Kiwuka et al., 2021).
Cultivation of the species began around 1870 in Congo, using material coming from Zaïre’s Lomami River region, now known as the Democratic Republic of Congo (Berthaud & Charrier, 1988). A subtype of robusta called “kouillou” (later renamed “conilon” via linguistic distortion when it was introduced to Brazil) was observed in the wild by the French in 1880 between Gabon and the mouth of the Congo River, mainly along the Kouilou-Nari River region. The species was named C. canephora by the botanist Louis Pierre in 1895. Pierre, who worked in France at the Muséum National d’Histoire Naturelle, received a sample of the plant collected in Gabon by the Reverend Théophile Klaine. The name was first published along with a description of the species by Froehner in 1897. In 1898, Edouard Luja, in preparation for the 1900 Paris Exposition, was sent to collect 10 species with economic potential in the Congo. During this mission, Luja collected several thousand seeds in the surroundings of Lusambo of a ‘new’ coffee species (Benoit, 1968). These seeds were probably collected on an early robusta plantation in the region. Belgian Congo became one of the principal breeding centers, from which breeding lines were distributed throughout the tropics.
At the turn of the century, the species began to spread to other parts of the world. Robusta seeds from Congo were sent to Brussels, and from there it was sent under the name “robusta” to Java, Indonesia, where it was quickly accepted by farmers due to its productivity and apparent resistance to coffee leaf rust (Cramer, 1957), as a major outbreak occurred in Southeast Asia in the late 1800s. These materials were later enriched with those from Gabon and Uganda. Around the same time, other Robusta material selected from wild populations was brought to areas of Ivory Coast, Guinea, and Uganda (Charrier and Eskes, 1997).
From here, robusta continued to move around the world, entering India by way of Java (with later introductions from west Africa). Material selected in Java was reintroduced to central Africa from 1910 onward, and to the Belgian Congo in 1916 at the Institut National pour Étude Agronomique du Congo (INEAC), which served as the home to the majority of selection from 1930 to 1960. Within Africa, robusta production grew in Madagascar, Uganda, Ghana, and the Ivory Coast, often intermingling endemic variants with those introduced from commercial production in other parts of the continent.
As noted previously, much of the movement of robusta and the increase in the popularity of its production during this period may be attributed to the spread of coffee leaf rust (CLR), a fungal disease that ravages coffee plants. One of the greatest benefits of robusta production is that the species possesses a natural resistance to some of the major pests and diseases that impact coffee production; they can thrive under harsh conditions (Campuzano, et al., 2022).
Robusta was later introduced to Latin America, and in particular Brazil, with some additional commercial introductions in Central America via Guatemala between 1930 – 1935. Further, CATIE in Costa Rica introduced robusta plants called “French lines” between 1981 – 1983.
In present day, countries that lie within Asia and Oceania are collectively the largest producers of robusta, generating 60% of the world’s output at 41.5 million 60 kg bags annually. This region is followed by South America, which produces 28% of the world’s share of robusta, generating 19.8 million bags of coffee in the 2020 – 2021 year.
C. canephora is is a diploid (2n=2x=22) species divided into two broad genetic groups, Guinean and Congolese. The Guinean group originated in central-west Africa, while the Congolese group originated in central Africa. Among these two groups, the Guinean is the most widespread. In addition, within each group, there are different populations, or subgroups. Within the Guinean group, there are at least two subgroups, named “kouilou” or “conilon,” and “robusta.” However, more recent studies using advanced genetics techniques, have further refined the robusta species into eight subcategories. Studies of the genetic relationships within C. canephora have shown that, in general, these populations are well differentiated and genetically isolated (Berthaud, 1986, Montagnon, 1992, Cubry, et al., 2008, Musoli, et al., 2009, Dussert et al., 1999, Gomez et al. 2009, Mérot‑L’Anthoëne et al., 2019). Montagnon (1992) proposed a substructure within the Congolese group with two subdivisions, SG1 and SG2. Dussert (1999) added two extra groups (including B and C, as referenced below) to the Congolese group. However, these subgroups are not necessarily visually distinct from one another (Chadburn & Davis, 2017, Charr et al., 2020).
Using RFLP and SSR markers, Gomez et al. (2005) pooled C. canephora genetic diversity into five genetic groups (A, B, C, D, and E). Geographically, genetic group A comprised wild populations from Congo and Cameroon, group B from eastern-central Africa, group C from western-central Africa, Cameroon and northeastern Congo, group E from Congo and southern Cameroon, while group D consisted of wild populations from Côte d’Ivoire and Guinea, separated geographically by the Dahomey Gap from the other diversity groups. Musoli et al. (2009) further determined that some Ugandan wild populations clustered into another distinct group (group O). Finally, Mérot‑L’Anthoëne et al. (2019), using a genome-wide Coffee 8.5K SNP array, described C. canephora genetic diversity with eight distinct genetic groups, including the Ugandan one (group O), thus identifying two new genetic groups, (comprising samples from southern Democratic Republic of the Congo) and G (comprising samples from Angola), whereas the differentiation between groups E and R was weaker.
Wild populations are the primary genetic relative of robusta coffee, and cultivated coffee has changed little from its wild progenitors. It is also a secondary genetic relative of arabica, conferring potential disease and pest resistance (Chadburn & Davis, 2017).
As a part of the genetic conservation of the species, gene banks of robusta were established in several producing countries in Africa and Asia. There are currently 40 known collections of this species held in ex-situ collections (Tram, et al., 2022, Botanic Gardens International, PlantSearch). The species was set into collection in Côte d’Ivoire, with 700 wild genotypes by ORSTROM in collaboration with the Center de Coopération Internationale en Recherch Agronomique Pour Development.
In addition, the species was collected in Guinea, Cameroon, the Congo, and Central African Republic and later introduced into field gene banks. The species is found in protected areas such as Mangala Forest Reserve in Tanzania, Bia National Park in Ghana, Isalowe Forest Reserve in the Democratic Republic of Congo, and Reserve du Dja in Cameroon.
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