Diseases and pests are detrimental not only reducing yield and coffee quality, but also harmful for the economic and livelihood of coffee farmers who depend on it.
The damage will typically start with an attack on flower buds, leaves, stems, branches and roots, thereby reducing the plant’s ability for photosynthesis and nutrient uptake. This results in the defoliation (losing leaves) of the plant, leading to bean death.
Coffee originated in Ethiopian from a single tree dated back to 1713 leading to genetic uniformity in coffee plants that we have today, which make more of the coffee plants susceptible to diseases. Most of the diseases are caused by pathogenic fungi. The most important diseases are Coffee Leaf Rust (CLR), Coffee Berry Disease (CBD) and Wilt Disease (CWD). The CLR and CBD are commonly affecting C. arabica (Arabica), and CWD is often affecting C. canophera (Robusta).
Pests can also damage coffee beans by female insects laying eggs inside the berries and complete its life cycle in the berry. The most important pest is the Coffee Berry Borer (CBB).
Coffee Leaf Rust
Image by Smartse
CLR is dreaded by coffee farmers. It was first discovered in Sri Lanka (then Ceylon) in the late 19th century, before causing the collapse of the country’s coffee industry. It significantly reduced coffee production in Ceylon from 42 million kilograms of exported coffee to less than 3 Kgs/ year. The damage was so severe, that it forced the country to abandon coffee production altogether, and shift to tea plantations.
CLR is a fungus caused by a pathogen (a bacterium, virus, or other microorganism that can cause disease), called Hemileia vastatrix. The pathogen prefers a temperature range of 20–28 °C and needs a wet leaf to grow, which makes coffee grown in the lower altitude more vulnerable to disease.
The infection starts when the fungus spore (i.e. plant reproduction system), also known as urediniospores, lands on the plant surface (leaf, stem, fruits, etc.) and is exposed to moisture for 24 - 48 hours. This condition is the perfect environment for fungi to grow. The fungus then germinates and infects the rest of the tree.
Once a coffee tree is infected, its leaf shows symptoms include small, yellowish, oily spots on the upper leaf surface. These areas expand into larger round spots that turn bright orange to red, and finally brown with a yellow border. On the underside of the leaf, it has orange-yellow powder like consistency and has a rusty appearance, which then turns black.
CRL may not kill the plant immediately, but will weaken it dramatically. The leaves of the infected tree will eventually fall, decreasing its ability to accumulate energy through photosynthesis for fruit production. As a result, the fruits don’t mature, and eventually fall off.
Coffee Berry Disease
Image by Scot Nelson
CBD was first discovered in Mt. Elgon, Kenya in 1922 and was responsible for up to 75% loss of the crop. From Kenya the disease spread to Angola in 1930, Zaire in 1937, Cameroon between 1955 and 1957, Uganda in 1959, Tanzania in 1964, Ethiopia in 1971 and Malawi in 1985.
CBD is caused by a pathogen Colletotrichum kahawae that can infect all organs of the host: flower buds, leaves, fruits and the maturing bark. It grows perfectly in high humidity, temperature range 15 - 27 deg C, with high altitudes being more susceptible. This disease spreads through insects, coffee pickers or infected seedlings, but never by wind due to its sticky constellation.
CBD doesn’t kill coffee trees but can cause crop losses of more than 80%. The early state of infection takes place during flower bud formation causing some losses in flowers. The infected tree shows noticeable symptoms during the expanding stage of berry development of sunken, black lesions (bruises) on the green pulp.
The pathogen never attacks mature coffee beans; it remains in the pulp. The infected pulp cannot be removed completely, causing so-called ‘‘stinkers’’ in the crop and reducing the quality, and eventually develops into mummified berries with no economical value.
Coffee Wilt Disease
CWD or tracheomycosis is a vascular disease caused by the fungal pathogen, Gibberella xylarioides (Fusarium xylarioides), and causes all infected trees to perish. The disease has been a serious problem to the production of Robusta coffee in DR Congo and Uganda since the 1990s. Although it affected Robusta the most, CWD was first observed in 1927 in the C. excelsa plantation.
The pathogen exists on coffee trees in two developing stages: Gibberella (sexual spore) and Fusarium (asexual spore). Infection mostly happens at the imperfect stage (Fosarium) penetrating through wounds into the base of the stem. The fungus blocks the water supply in the vascular system and causes a typical brown discolouration.
Detailed morphological studies and pathogenicity tests conducted in a Ph.D. thesis by Adugna in Ethiopia in 2004 found the CWD infecting C. arabica (Arabica) doesn’t infect C. canephora (Robusta). Thus, it was suggested that the coffee wilt population should be classified into two formae speciales: G. xylarioides f. sp. abyssiniae (F. xylarioides f. Sp. abyssiniae) from Arabica and G. xylarioides f. sp. canephorae (F. xylarioides f. sp. canephorae) from Robusta.
Coffee Berry Borer
Image by Georg Goergen
The Coffee Berry Borer (Hypothenemus hampei) is the most harmful pest to coffee cultivar. It's origin is Africa, but it has since spread to most coffee producing countries with the exception of Papua New Guinea and Nepal. The spread is mainly through seed contamination.
The CBB is a small beetle and spends most of its life in the well-developed berries, making it extremely difficult to control. It started with the impregnate female bore entering a berry and depositing her eggs in the seed. The eggs are hatched, fed, developed, and later mated inside the berry.
CBB causes two main pre-harvest damages: premature fall of young coffee berries and quality losses due to the feeding of larvae inside the berry. The impact it has on the cup quality depends on the damage it had. Slight damage to coffee seeds (one hole per bean) reduces the aroma, flavour and acidity. More severe damaged seeds can exhibit off-flavours, such as mouldy, chemical, fermented and tarry. Moreover, a severely damaged seed is fragile and breaks easily, thus burns faster during roasting and resulting in a bitter tasting cup.
Living most of its life mostly in the berry, CBB spreads and reproduces depending on fruit availability and rainfall. During the dry season it can survive in the berries that have fallen to the ground or left on the tree. Rain reduces the dry matter in the seed, which attracts the female berry.
However, rain is not the only major factor affecting the CBB population. Findings from Borbon-Martinez suggest that high humidity on the ground during the interseason can cause the fallen berry to rot and reduce the CBB population.CBB is extremely difficult to control both chemical and non-chemical. Biocontrol study indicates natural predators such as ants and birds. However, the study of ants as predators was conducted in the lab and might not necessarily be replicable in the field. Furthermore, other natural predators that are effective to reduce CBB are black thrips, Leptophloeus, Nematodes and Fungal entomopathogens.