INTRODUCTION

Coffee is a major crop worldwide, with an estimated three billion cups consumed daily. It is also the second most traded commodity worldwide, after oil (Davis et al., 2012). In 2009∼2010, coffee exports amounted to US$15.4 billion, with at least 93.4 million bags shipped worldwide. Over 100 million people depend on coffee production for their livelihoods (Iscaro, 2014). Although coffee is consumed more frequently in developed countries, it is mainly produced in developing nations along the equator (Rwigema, 2021).

Uganda is Africa’s second largest coffee exporter, after Ethiopia, currently ranked eighth globally. The coffee industry significantly contributes to the country’s export earnings, accounting for one-third. This sector provides a livelihood to 1.8 million households, and nearly all of them (99%) are smallholder farmers (<2 acres) (World Coffee Research, Uganda, 2024). Coffee in Uganda is produced by 125 districs (88 produce Robusta, 15 Arabica, and 9 Robusta and Arabica), with an annual production of approximately 282,000 tons (Uganda Coffee Development Authority (UCDA), 2024).

Uganda grows two coffee types: Robusta (Coffea canephora Pierre ex Froehn) (80%) and Arabica (Coffea Arabica) (20%). Robusta coffee requires an average air temperature of 24∼30 ºC (75∼86 ºF), a rainfall of 2,000 mm per year, and is grown at 1,200 meters above sea level (m.a.s.l.) (Masters et al., 2009). On the other hand, Arabica’s optimal temperature is 15∼24 ºC (59∼75 ºF), a rainfall of 2,000 mm per year, and grows well above 1,500 m.a.s.l. (Musoli et al., 2008;United Nations Development Program, 2005).

The Uganda National Development Plan III (NDP III) identified coffee as a priority crop for improving economic stability and creating opportunities for rural development (Government of Uganda, 2020). Uganda hopes to quadruple its coffee production in the coming years; however, this project is threatened by pests and disease incidence (World Coffee Research, Uganda, 2024). Pests and diseases impact coffee yield and affect coffee’s health, nutrition, development, and physical and chemical qualities (Ogundeji et al., 2019). Major coffee pests include white coffee stem borer, coffee berry borer, black coffee twig borer, root mealy bugs, lace bugs, skeletonizers, and coffee leaf miners. Primary coffee diseases include wilt disease, brown eye spots, leaf rust, and berry disease (Table 1) (Kagezi et al., 2018;Rutherford and Phiri, 2006).

Despite the importance of coffee production in Uganda, farmers are still faced with pest and disease management challenges. Therefore, documenting and making pest and disease management strategies known to farmers is crucial. This review discusses two diseases, two major pests prevalent in Uganda’s coffee systems, and the mechanisms by which pathogens and pests invade and establish in coffee plants. A review of secondary data was conducted using Google Scholar, Research Gate, and PubMed. It also evaluates the challenges and prospects for improving pest and disease management, which is essential for agricultural stakeholders to adopt effective strategies to manage pests and diseases in Uganda.

MAJOR COFFEE PESTS IN UGANDA

1. Insects

A. Coffee Berry Borer Hypothenemus hampei Ferrari (Coleoptera: Curculionidae)

The coffee berry borer (CBB) is the most devastating coffee pest worldwide, causing 50∼100% yield loss amounting to US$ 500M in damages annually (Johnson et al., 2020). The first report of CBB was in France in 1867. Reports in Africa began in Gabon in 1901 and Zaire in 1903. Many theories have been proposed regarding the geographical origin of CBB. It is suspected to have originated from West and Central Africa, Angola in Southwest Africa, and North East Africa. It is more evident that CBB’s original host was C. canephora and that it originated somewhere in Africa (reviewed in Vega et al., 2012;Damon, 2000). Uganda recorded CBB as a pest in 1908, and the author suspects CBB to be indigenous, feeding on wild species of coffee (Hargreaves, 1926). Previously, CBB was severe at low altitudes; however, more recent studies indicate high CBB incidence at 1,864 m.a.s.l. (Whittaker et al., 2024;Kagezi et al., 2018;Damon, 2000). CBB attacks both mature and immature coffee berries of Arabica and Robusta in all parts of Uganda (Liebig, 2017;Wang et al., 2015). High relative humidity (> 90%) and increased temperatures, especially after rainfall, stimulate CBB emergence. CBB has an optimal temperature of 15∼32°C (Jaramillo et al., 2009).

CBB life cycle starts between 90∼120 days after coffee plant flowering. Green and ripe coffee berries are vulnerable to CBB attacks, although CBB prefers berries with more than 20% dry weight. The female CBB needs two to eight hours to enter and create galleries inside the coffee bean. The total CBB progeny can reach up to 300 individuals in a single coffee seed (Vega et al., 2019;Jaramillo et al., 2009). Larvae and adults feed on the endosperm. CBB is associated with mutualistic gut microbiota that helps detox caffeine (Moreno-Ramirez et al., 2024;Robusta Coffee Handbook, 2019;Arabica Coffee Handbook, 2019;Infante, 2018). CBB infestation causes three kinds of losses: reduced final product yield and quality, mature berries becoming vulnerable to infections and other pest attacks, and loss of immature berries due to premature fall, arrested development, and decay (Damon, 2000).

Although CBB infestation typically occurs when the beetles attack the berries still attached to the bushes, CBB reproduction can continue in fallen and processed berries as long as the humidity level is at least 13.5%. In Uganda, coffee is produced year-round; thus, CBB populations can exceed eight generations yearly (Damon, 2000). There are varying theories on what attracts CBB to coffee berries. Some suggestions include the attraction of CBB to caffeine, and the importance of ethanol and methanol-based solvents as components, as caffeine is not volatile (Gutiérrez-Martinez and Ondarza, 1996). However, this has been debated in recent studies suggesting that CBB is only attracted to an ethanol-methanol mixture but not caffeine (Dufour and Frérot, 2008). Furthermore, the authors noted that CBB can survive without food for 80 days and becomes inactive at 15 °C. During the dry season, CBB becomes inactive in old berries while waiting for the first rains, which renders the berries uninhabitable due to waterlogging. At this point, the female CBB emerges and starts searching for new berries (Baker and Barrera, 1993; Baker et al., 1992; de Kraker, 1988). The dispersal of CBB can occur through long and short-distance flights. Passive dispersal is assisted by animals, vehicles, humans, and wind. Triggers for CBB dispersal include; the first rains after an inter-harvest period, depletion of food sources within the berry, overcrowding, and the search for a mate or a suitable berry for oviposition (de Oliveira-Filho, 1927). The coffee berry borer is polyphagous and has been found on Rubiaceae tree species, but further research is needed to determine whether the CBB can feed and reproduce on alternative hosts (Vega et al., 2012).

CBB Management

B. Black Coffee Twig Borer Xylosandrus compactus Eichhoff (Coleoptera: Curculionidae)

The Black coffee twig borer (BCTB) (Xylosandrus compactus) also known as ambrosia beetles referring to its exclusive feeding on fungi (ambrosia fungi) in galleries created into wood by adult female beetles. BCTB is native to Asia and is adopted to warm environments (Greco and Wright, 2015). The beetle was first reported in Uganda in 1993 and has since infested all Robusta coffee-growing regions in the country; about 68.8% of coffee plantations are infested with black coffee twig borer (BCTB). Kagezi et al. (2013) reported 100% and 50% of BCTB prevalence in central and southwestern Uganda, respectively. The beetle infests 40% of trees per farm and kills 8.6% of twigs, resulting in losses of US $40 million in foreign earnings. It particularly threatens the livelihood of small-scale farmers (Robusta Coffee Handbook, 2019;Wu, 2016;Kagezi et al., 2013). Additionally, BCTB is a polyphagous pest infesting 224 plant species, 40 of which exist in Uganda (Kagezi et al., 2013).

Female BCTB beetles are the ones that cause damage, as the males are unable to fly. The females of the new generation leave their parental galleries after 29 days and establish new galleries using the same entry hole that was used by the maternal borer. Typically, the emergence of the beetles occurs in the afternoon, between noon and 5:00 PM (reviewed in Greco and Wright, 2015). Oviposition occurs between the fourth and seventh day after females bore into the twig. It takes three to five days for the eggs to hatch. Unfertilized eggs become male progeny, and fertilized eggs become female progeny. The complete lifecycle of BCTB takes an average of 28.5 days at 23 to 27 °C and 50 to 60% relative humidity. Mating is sibling mating, pupation, and mating of brood adults take place inside the infested twig. The lifespan of female borers is 58 days, and 6 days for males (reviewed in Greco and Wright, 2015).

Coffee plants suffer two kinds of damage from BCTB; physical damage and the introduction of ambrosia fungus which may be phytopathogenic. The Physical damage involves tunneling to the phloem if the borer rejects the twig or BCTB cuts out vascular tissue from the twig as it creates a brood chamber where the eggs are laid either way the plant suffers wilting, and necrosis eventually, defoliation is observed. Female BCTB requires 3.7∼5.3 h to bore an entrance tunnel into twigs. Within hours of the attack, leaves on the infested twig turn light green and start wilting. The bark of the twig and the wilted leaves turn black after a few days. The invasion of the xylem leads to obstruction of the flow of water and nutrients (Bukomeko et al., 2018;Greco and Wright, 2015;Burbano et al., 2012).

BCTB Management

2. Diseases

A. Coffee Wilt Disease/Tracheomycosis caused by Fusarium xylarioides

Coffee Wilt Disease (CWD) is a significant threat to Robusta coffee production, and its occurrence can be sporadic, leading to epidemics. Fusarium xylarioides is a soil-inhabiting fungus that enters through roots and lower stems that are wounded due to management practices. After infection, the disease can spread to adjacent trees through root-to-root contact or short-distance dispersal of fungal material, such as through rain splash. On-farm practices, like hoe weeding and slashing, also spread the disease if the tree is injured (Flood, 2021;Robusta Coffee Handbook, 2019;Musoli et al., 2008). The optimal conditions for F. xylarioides include 25 ± 1° C, pH 5.5, aeration, and water activity (Alemu, 2012).

When the pathogen invades a coffee plant, it colonizes the vascular system, disrupting water conduction. This manifests as leaf wilting and desiccation, followed by de-foliation and dieback of the affected branches. Coffee berries on the affected tree ripen prematurely and dry up but remain attached to the primary branches. An infected and dried-up coffee plant remains firmly rooted in the ground, symptoms appear at any growth stage, and the symptom development rate is variable. Juvenile plants may die in weeks, while mature trees may last 3∼25 months (Peck and Boa, 2024;Musoli et al., 2008;Rutherford and Phiri, 2006). Older and shaded coffee trees or those planted on loamy soils in humid and wet areas are highly susceptible to CWD attack (Alemu, 2012).

CWD Management

B. Coffee Leaf Rust caused by Hemileia vastatrix

Coffee leaf rust (CLR) caused by the fungus Hemileia vastatrix (Berk. and Broome) is the second most devastating coffee disease in Africa after coffee berry disease. CLR became significant in Uganda in the 1940s, affecting Arabica and Robusta species. However, it is severe on Arabica plants with an estimated loss of 49.5% when no control measures are undertaken. CLR occurs in all coffee- growing districts in Uganda, especially in the lowlands (Luzinda et al., 2015;Matovu et al., 2013). H. vastatrix is an obligate host-specific pathogen, and its optimal conditions include a temperature of 21∼25°C and 100% humidity. Temperatures below 15°C and above 35°C slow its development (Gichuru et al., 2021). Leaf wetness is only required during spore germination and penetration of the host stomata (Yirga, 2020). An infection’s severity is determined by rainfall intensity and distribution, residual inoculum, and tree leafiness (Gichura et al., 2021). The fungus tolerates longer seasons without rainfall and is dispersed by rain, wind, animals, and some insects (Yirga, 2020).

The infected spots grow and produce orange spores, while the older parts of the lesion become necrotic and turn brown. Although CLR can occur in any part of the leaves, lesions tend to cluster on the leaf margins (Berihun and Alemu, 2022). CLR causes the plant to lose its leaves prematurely, resulting in a loss of photosynthetic surfaces. This, in turn, forces the plant to use the stored carbohydrates in its roots to sustain the developing berries, leading to the loss of fine feeder roots. The expanding berries fail to fill up due to lacking required nutrients, and young berries fall off prematurely. Repeated attacks of CLR-causing fungi can slowly lead the coffee bush to decline, resulting in reduced yield (Talhinhas et al., 2017).

CLR Management

OVERVIEW OF COFFEE PEST AND DISEASE MANAGEMENT IN UGANDA

Disease-causing organisms and pests cannot be eliminated from the environment and are predicted to increase in the face of climate change. Increased temperatures will expand pest distribution and increase the incidence of diseases, as noted with CBB (Jaramillo et al., 2009). Additionally, the stress caused by climate change can weaken plants, making them more susceptible to infections caused by opportunistic pathogens (Agegnehu et al., 2015). Moreover, pest control strategies often target individual pests without adequately considering the broader implications for the entire coffee pest complex and agroecosystem (Nyambo et al., 1996). Therefore, unconventional management approaches must be taken to reduce pest and disease impacts on coffee productivity.

1. Cultural control

Cultural control is the deliberate alteration of the production system to reduce pest and disease populations (Zalom, 2010). Ugandan farmers have adopted cultural practices to manage pests and diseases since they are cheap, readily available, and provide multiple benefits. Coffee trees benefit from pruning, which improves microclimate, air circulation, nutrient and water efficiency, and spray efficacy (Gichuru et al., 2021;Judith et al., 2018;Damon, 2000). Phytosanitary measures can reduce the spread of coffee wilt diseases(Alemu, 2012). Other cultural practices in Ugandan coffee systems include stamp strip pruning, harvesting efficiency, crop sanitation, and proper postharvest handling (Luzinda et al., 2015).

The effectiveness of cultural practices depends on several factors, including the farmer’s willingness, skill, dedication, and community participation. Thus, cultural methods may be inadequate on their own; they are also labor- intensive and unfeasible. For example, timely harvesting is paramount to managing CBB, ensuring < 5 over-ripe berries per tree; this cultural control is partially effective if more berries are left on the tree (Aristizábal et al., 2017). Additionally, working on a coffee plantation can be challenging due to steep slopes, large trees, and many fallen berries, making manual labor less effective (Moreno‑ Ramirez et al., 2024).

2. Agroforestry

Agroforestry intentionally integrates trees and shrubs into crops and animal farming systems to create environmental, economic, and social benefits. Shade trees planted near coffee reduce the coffee temperature by 4°C, creating low temperatures mainly required for Arabica coffee (Bongase, 2017). Shade trees have many benefits, including providing a refuge for natural enemies, thus promoting biological control. Additionally, the canopy of shade trees reduces the energy of raindrops and diverts them from their trajectory, preventing the dislodging and dispersal of disease-causing conidia that can infect coffee. Furthermore, shade trees alter the microclimate; Barradas and Fanjul (1986) recorded a temperature decrease of 2.6°C in shaded coffee gardens. Lastly, shade trees improve soil properties within coffee plantations. Overall, shade trees are essential in maintaining a healthy and balanced environment. However, choosing shade trees that will not promote pests and disease incidence in coffee is critical. The microclimate created by shade trees should not favor coffee pests or stages of pathogen development. Noticeably, some shade trees act as cohosts of pests and, thus, should be avoided. For instance, BCTB and CBB have roughly 200 and 40 plant species hosts, respectively, mainly from the Rubiaceae and Fabaceae families. These alternative host plants can sustain pest populations, especially between coffee harvesting seasons (Vega et al., 2019;Kagezi et al., 2013;Damon, 2000). Gichuru et al. (2021) cite the increased severity of Coffee berry disease in shaded coffee since the latter retains more infected leaves than open coffee. Similarly, López-Bravo et al. (2012) reported an increased incidence of CLR under shaded coffee due to a lower maximum temperature and higher leaf wetness, leading to uredospore germination and infection.

Conversely, managing shade trees and coffee influences pathogen incidence. For example, pruning shade trees and coffee trees increases aeration and sunlight infiltration, thus drying leaves and fruits and negatively affecting pathogen dispersal and spore germination (Ayalew et al., 2022). Therefore, the shade tree choice is context-dependent and should balance ecological control, minimizing adverse effects, and incorporate farmers’ preferences (Gichuru et al., 2021).

3. Chemical control

Chemical control involves using specially formulated pesticides to kill or control plant pests and diseases. Uganda has low pesticide application rates (0.01 kg/ha), and most of the used pesticides are classified as moderately hazardous (WHO class II). The most used insecticide and fungicide are cypermethrin and mancozeb (Staudacher et al., 2020;Kagezi et al., 2018).

The timely application of pesticides can provide adequate control, but it is usually beyond the financial means of resource-constrained farmers. Besides the cost, pesticide adulteration is common; 10∼15% of agrochemicals in Uganda are fake. Therefore, certain pests cannot easily be managed by pesticides on the market (Luzinda et al., 2015;ASARECA, 2010). Due to inadequate technical advice from extension workers, farmers often use the wrong pesticides. Nyeko et al. (2002) found that farmers in Kabale advised themselves to use Dithane (fungicide) to control aphids.

Additionally, Uganda lacks clear guidelines on pesticide application and has limited resources to deal with the environmental and health consequences of pesticides. Although the Food and Agricultural Organization (FAO) has set Good Agricultural Practice guidelines, they are minimally enforced. Farmers do not follow recommended mixing concentrations and preharvest intervals as instructed on labels. Furthermore, farmers do not follow personal protective guidelines, although they are aware of the health risks associated with pesticides (Ssemugabo et al., 2022;Staudacher et al., 2020;Kaye et al., 2015). Pesticide sprays are mainly calendar-based, as this method is convenient. However, it can lead to wastage and occasional failure to effectively target the intended pest (Nyambo et al., 1996). Finally, to optimize the farm’s microeconomy, farmers in Uganda often intercrop coffee with food crops, fodder plants, and trees. Thus, considering the effect of pesticides on the overall coffee farming system is essential (Nyambo et al., 1996).

4. Biological control

Biological control manages pest populations using living organisms and can reduce pest-caused damage. Robusta is native to Uganda, while Arabica originated in Ethiopia and Malawi; therefore, Africa is the ideal location for natural enemies of coffee pests which include birds, arthropods, and entomopathogens (Kagezi et al., 2015).

Notable natural enemies in the Ugandan coffee systems include the formicid ant (Plagiolepis sp.), the entomopathogens Metarhizium anisopliae and Beauveria bassiana for the control of coffee twig borer, the entomopathogens Cephalonomia stephanoderis, Prorops nasuta, Phymastichus coffea, and Beuveria bassiana, and the parasitoid braconid Heterospilus coffeicola (Schmiedeknecht) (Robusta Coffee Handbook, 2019;Egonyu et al., 2015). Interestingly, there is a tritrophic interaction between coffee, coffee pests, and their natural enemies, all influenced by the environment.

5. Development of resistant varieties

Resistance refers to host strategies reducing infection, thus limiting the pathogen’s fitness (Roy and Kirchner, 2000). Using resistant varieties is the most appropriate, efficient, and economical method for managing pathogens. The National Agricultural Research Organization (NARO)/ National Coffee Research Institute (NaCORI) has developed five Arabica varieties awaiting assessment from the Ministry of Agriculture, Animal Industry and Fisheries, and ten wilt-resistant varieties. More varieties are still under field evaluation(NARO, n.d). Despite their superiority, resistant varieties are not widely adopted by smallholder farmers in Uganda, probably because they are expensive. One resistant seedling costs US $ 0.39 compared to US $ 0.13 for an ordinary seedling. Once established, coffee can be productive for 40 years, which makes farmers reluctant to renovate their plantations with new cultivars.

6. Integrated pest management

According to the United Nations FAO, Integrated Pest Management (IPM) considers all available pest control techniques and other measures to discourage the development of pest populations while minimizing risks to human health and the environment. IPM combines cultural, biological, and chemical measures to manage diseases, insects, weeds, and other pests, considering all relevant control tactics and methods locally available and evaluating their potential cost-effectiveness (FAO, 2024).

Studies indicate that the successful establishment of an IPM program combines control strategies based on regional phenology and cost/benefit analysis of specific tactics (Kogan, 1998). The UCDA and NARO/NaCRRI are developing IPM packages for key Arabica and Robusta pests. The activities involved include profiling pests in each coffee variety, determining economic threshold injury levels for key insect pests, and testing selected IPM options. Some specific activities underway include the shade effect on soil fertility and pH; the shade effect on insect pests, yield, and quality; shade effect on coffee diseases; weed management; change of cycle; soil improvement; soil erosion control; upscaling stem smoothening and stem wrapping technologies for stem borer management; and biological and economic studies of CBB and three of its natural enemies (UCDA Annual Report 2011/12).

FUTURE DIRECTION FOR COFFEE PESTS AND DISEASES MANAGEMENT IN UGANDA

Controlling coffee pathogens is an ongoing necessity, particularly in the face of climate change and globalization, causing increased pests and diseases within the coffee system. New pathogen races are reported, and pathogens appear in areas where they did not exist. This issue is compounded by unpredictable weather patterns, distorting predicted disease progressions (Alwora and Gichuru, 2014). Developed countries, particularly in Europe, North America, and Japan, prefer organically produced coffee, which comes with a premier price twice that of traditional coffee (Bailey et al., 2010).

Therefore, more sustainable pest control strategies must be developed, considering farmers’ and consumers’ demands. Researchers, extension workers, and farmers should work together for tailor-made solutions. Liebig et al. (2016) observed that farmers do not follow nonconventional strategies advised by experts for pest management, and resistant coffee varieties are not adopted as much. The authors also noted the lack of strategy for specific regions and farming systems in Uganda, leading to the low adoption of management practices.

Notably, research progress on the pathogenesis and life cycles of major coffee diseases and pests; discovering caffeine detoxification in CBB (Infante, 2018), ambrosia fungus elimination from BCTB (Kagezi et al., 2015), the complexities of coffee wilt diseases (Floods, 2021), and CLR pathogens (Merle et al., 2020). These findings open new research options for managing coffee pests and diseases. Future collaborations between researchers from different countries will drive the research agenda to great heights.

Precision Agriculture (PA) is defined as “management strategy that gathers, processes and analyzes temporal, spatial and individual plant and animal data and combines it with other information to support management decisions according to estimated variability for improved resource use efficiency, productivity, quality, profitability and sustainability of agricultural production.” (ISPA, 2024). Utilizing PA tools, such as GPS, Global Navigation Satellite System, and Artificial Intelligence (AI) for pest surveillance and management is the future Uganda must embrace. These PA tools can monitor insect pests using a radar to track pest migration, video equipment to monitor flying insects, chemiluminescent tags to track insect movements at night, echo-sounding and habitat mapping to detect larval movements, and smartphones to monitor pests and disease development (Toscano-Miranda et al., 2022).

Such tools will allow us to manage coffee production in an environmentally friendly way. Using site-specific knowledge, PA can tailor real-time pest and disease management, ultimately reducing waste and safeguarding the environment through timely control. PA technologies ensure that pathogen control is given where and when needed. Applying pesticides when required reduces pesticide resistance development (Bongiovanni and Lowenberg- Deboer, 2004).

CONCLUSION AND RECOMMENDATIONS

Cultural control practices, such as pruning, stumping, desuckering, fertilizer application, clean harvesting, uprooting and burning infected trees, and coffee tree spacing, are commonly used with promising results; however, they remain inadequate. Using fungicides like copper oxychloride and insecticides exhibits moderate control; unfortunately, most farmers are resource-constrained. Using carefully selected shade trees is also commendable in coffee pest and disease management. Since all standard management practices have merits and demerits, site-specific IPM offers a valuable solution, considering the coffee system as a whole. Moreover, planting resistant varieties is an economically viable option that farmers must adopt. Lastly, Uganda must integrate digital technology like AI in pest and disease management. Using digital technology is a means to PA that encourages pest management where and when needed.

적 요