Coffee wilt disease

Coffee wilt disease (Tracheomycosis) is a common wilt that results in complete death of coffee trees it infects. This vascular disease is induced by the fungal pathogen known by it's teleomorph Gibberella xylarioides (Fusarium xylarioides). In 1927 coffee wilt disease (CWD) was first observed in Central African Republic (CAR) where it developed slowly and went on to cause two epidemics between the 1930s to 1960s. CWD was first seen in Coffea excelsa.

Host

Hosts of CWD include Coffea arabica (Arabica coffee), Coffea canephora (Robusta coffee), Coffea liberica (Liberian coffee), and Coffea excelsa (Excelsa coffee) Currently, the disease is limited to Eastern and Central Africa, however, studies have shown that most Coffea species are likely susceptible to the disease, which can potentially lead to more worldwide problems in the coffee industry.

Robusta coffee tree infected by coffee wilt disease (CWD), credit The Mediae Company

Symptoms and signs

Due to the nature of CWD, coffee plants often exhibit symptoms of disruption to vascular systems. Internal symptoms are disturbances to conduction of water in the plant. External symptoms include loss of moisture on leaves, discoloration, leaf loss, dieback of infected region, swelling of trunks, cracks in mature trees and lastly plant death. Signs of CWD include small blackish-brown perithecia caused by the sexual stage of the fungus from cracks in the bark which cause an observable bluish-black stain on the wood.

Disease cycle

The fungal pathogen responsible for CWD can exist on coffee trees as Gibberella xylarioides, the sexual or perfect stage, or as Fusarium xylarioides, the asexual or imperfect stage. CWD is spread by wind-born ascospores during its sexual stage or splash-borne conidia, where they land and can persist as a viable source of inoculum in the soil. Infection occurs via penetration of wounds at the base of stems mainly by the imperfect stage, Fusarium xylarioides. Once infected, fungus blocks the xylem system which induces host responses that inevitably result in plant death.

Importance

Coffee is a major cash crop, with over 2.5 million people directly depending on its production and trade as a livelihood. For some countries, coffee accounts for 50% of primary foreign exchange that is valued around 300-400 million dollars annually. Reduced coffee production causes decline of revenue for some African countries , which can also increase food insecurity and overall regression at grassroot level. In 1945 CWD destroyed most of Central Africa Republic Excelsa plantations, ultimately resulting in the complete collapse of the crop. This disease causes a threat for coffee growers around the globe as the disease reduces the quantity and quality of the coffee. Not only does this disease put an economic strain on countries that use coffee as a cash crop, but it also can lead to the rise of coffee prices for consumers everywhere.

Management

Once a coffee plant is infected by CWD, death is inevitable, making prevention the most beneficial for coffee growers. Diagnosis of CWD can be done by observing a blue-black staining after bark scraping. Additionally, regular root inspections are an effective measure to catch the disease in the early stages. Once disease is found, destruction of the coffee plant by cutting it at the ground level and burning it stops the spread of the infection to other plant. Restriction of movement for coffee plant, coffee husks mulch and planting materials are also useful. Relocation of coffee production or replanting of resistant C. canephora germplasm, assist with combating future spread. Preventive measures for CWD infection are to avoid wounding of trees for example when removing control weeds, fertilizing soil or by grazing of any animals. Additionally, maintaining plants’ health by using inorganic fertilizer, manure or mulch to conserve moisture are some ways to decrease the risk of CWD.