The coffee genus (Coffea) comprises 124 species and is indigenous to the old world tropics (Razafinarivo et al., 2012). Coffee has a significant economic, social and spiritual impact on many communities with diverse cultural and/or psychological backgrounds (Chauhan et al., 2015). Coffee is the world’s second most traded commodity next to petroleum and serves as a direct source of income for growers in different parts of the world (FAO, 2015). In the second half of the nineteenth-century coffee was transformed into an industrial product as a consequence of the accelerated expansion of coffee production in Brazil, which in turn, nurtured the growth of a mass consumer market in the United States (Morris et al., 2018).
It is now grown in over 80 countries on more than 10.6 million hectares of land in the sub-tropical regions, especially in Africa, Asia and Latin America (FAOSTAT, 2020). On a social basis, it plays a relevant role notably in the subsistence of nearly 20 million coffee-farming families in under developed countries of Asia, Africa, and Latin America (Zhou et al., 2016). Coffee in Ethiopia accounts for 40% of total export and 10% of total government revenue (ICO, 2020).
The country has wider ecologies containing indigenous germplasm which offers the greatest opportunity to produce superior C. arabica that allowed Ethiopia to be competitive in the world market (Zenebe and Dawit, 2020). About 700,447 ha of land have been cropped with coffee (CSA, 2017). The Oromia Regional State is the homeland of C. arabica evidenced by the existence of wild coffee trees in the region with 464,426 ha (66%) of total area coverage and 317,316 tons of production per annum (CSA, 2017). Kaffa, Illubabor, Jimma, Wollega, Sidamo, Gedeo, Yirgachefe, and Hararghe are the topmost coffee-growing and producing areas (Hinsene et al., 2015).
Despite these, world coffee production has grown steadily since the 1960s, although it will be difficult to maintain this trend due to the continued rise in production costs, problems related to climate change, and the higher incidence of pests and diseases (Krishnaan, 2017). Arabica coffee in Ethiopia is threatened by several fungal diseases, including coffee leaf rust, coffee berry disease (causal agent Colletotrichum kahawae Waller & Bridge), coffee wilt disease (causal agent Gibberella xylarioides Heim & Saccas), and Armillaria root rot (causal agent Armillaria mellea Vahl ex and Fries) (Beyene et al., 2020). Coffee berry disease is an anthracnose of green and ripe berries caused by Colletotrichum kahawae (Waller and Bridge, 1993). In Ethiopia, the major factors contributing to low yield are the lack of improved cultivars for each ecological zone of the country, diseases, mainly coffee berry disease (CBD), and traditional management practices( Beyetta,2001). Colletotrichum species that cause anthracnose symptoms have been reported in several African countries (Cristobel et al., 2017).
Coffee berry disease, was first reported in Kenya in 1922 (McDonald 1926) and is caused by the fungal pathogen Colletotrichum kahawae, which is a specialized hemibiotrophic pathogen of C. arabica L. (Vieira et al., 2019). Unlike other Arabica coffee diseases, CBD is still restricted to the African continent despite favorable climatic conditions in certain high-altitude Arabica coffee-growing areas of Latin America and Asia (Van der Vossen et al., 2015). Colletotrichum kahawae infects green berries at the rapid expansion stage (4–16 weeks after flowering) and may also attack mature berries, 28 weeks after flowering (Gichimu et al., 2014). Epidemics of this disease can quickly destroy 50–80% of the developing berries on susceptible Arabica coffee cultivars during prolonged wet and cool weather conditions (Hindorf and Omondi, 2011).
In Ethiopia, CBD alone caused about a 24–30% loss of the national average yield on local landraces (Derso et al., 2000). Bayetta (2001) reported that the national average yield loss due to the disease in Ethiopia is estimated to be 20–25%, and also he reported that the loss may reach 100% during favorable seasons in some areas where altitude and rainfall are high.
Preventive control by fungicide sprays accounts for 30–40% of total production costs and leads to environmental pollution (Gichuru et al., 2008). Crop loss and cost of CBD control in Africa are estimated to be 300–500 million USD (Van der Vosen and Walyaro, 2008). Coffee Berry Disease, caused by Colletotrichum kahawae Waller and Bridge (Waller et al., 1993) is, to a large extent, responsible for reduced productivity and increased cost of production, thereby reducing the competitiveness of the region's coffee.
Symptoms on green berries appear as small dark sunken lesions typical of anthracnose which may spread to cover the whole berry. Under wet and cold conditions, the fungus readily sporulates forming a mass of pink conidia and penetrating the interior of the berries destroying the beans. The resulting dry, black mummified berries have no commercial value and may be shed off the tree or remain intact. The disease severity is higher on coffee trees exposed to sunlight than on those located under the shade (Mouen et al., 2008).
To overcome these biotic constraints management strategies have been used worldwide. Among these resistant varieties, cultural and chemical methods are widely used. Ethiopia mainly uses resistant varieties, mechanical and cultural disease management strategies to save the coffee crop and contribute to increased foreign exchange earnings from the coffee industry. Host resistance to CBD has been the subject of intense studies in the East African region (Van der Vossen and Walyaro, 1981).
It is widely accepted that resistant varieties provide an opportunity for farmers to produce coffee in a manner that is sustainable, competitive, and environmentally benign. The most reliable screening test for CBD resistance of mature trees to detect true resistance with little chance of escape will be artificial inoculation of berries on the trees at their most susceptible stage, i.e. 12–15 weeks after flowering when the berries are fully expanded but still soft (Mulinge, 1970). The discovery of hypocotyl infection on a four to six-week-old seedling using artificial inoculation with Colletotrichum kahawae spores contributes significantly to Arabica coffee breeding by shortening the time required to identify resistant progenies from crosses involving resistant and susceptible donors (Gichuru et al., 2008). A detached green berry inoculation test was used for resistance screening of C. Arabica accessions against CBD under artificial conditions. This offers the advantage of direct assessment of C. Arabica and Colletotrichum kahawae interaction at the fruit level and allowed an easy way of resistant genotypic characterization under field conditions (Pinard et al., 2012).
Coffee Berry Disease affected the production and productivity of Arabica coffee in all assessed areas in the Arsi zone, particularly the local cultivars which are preferred by the growers for their bean weight and typical quality (Bersisa at al., 2018). In most coffee-growing areas of Ethiopia, disease-resistant varieties have been developed and utilized by coffee growers. However, still, there is a lack of improved and resistant variety development in a few coffee growing areas including Arsi Zone. From Arsi zone, the “Gololcha “district is 14th among the top 25 coffee-producing districts in Ethiopia and 7th among the top 18 coffee-producing districts in Oromia (James et al., 2015). However, the “Gololcha” district still doesn't have improved and resistant varieties developed for the respective area. Therefore, the objectives of this study were to investigate the resistance of Arsi coffee collections to Coffee Berry Disease through artificial inoculation of green berries on mother trees, and younger coffee seedlings under controlled conditions.