Diagnosis: Coffee Wilt Disease

Coffee wilt disease (CWD), caused by the fungus Fusarium xylarioides, infects all wild and cultivated Coffea species—namely Arabica, Canephora (Robusta), and Liberica. As such, it poses a serious threat to biodiversity, cultivation, and trade alike.

CWD was first detected in the 1920s in Central Africa on Excelsa coffee, subsequently spread to Robusta, and is considered the principal cause of Excelsa’s collapse as a cultivated crop.

The second wave began in the 1970s in the Democratic Republic of the Congo (DRC) and reached Uganda in the 1990s; damage was particularly severe in the late 1990s/early 2000s, prompting neighbouring countries to tighten quarantine measures. Confirmed occurrences persist to date in the DRC, Uganda, Tanzania, and Ethiopia; recent indications also point to ongoing problems in Côte d’Ivoire. Reports from parts of West Africa remain unconfirmed in the absence of independent surveys. Probable introduction pathways include illicit coffee smuggling across remote border crossings and the movement of whole plants with attached roots and soil. Laboratory evidence further shows that certain Kenyan C. arabica varieties are susceptible in vitro. For southern Africa, only isolated case reports exist (South Africa, Eswatini).

Coffee wilt disease (CWD) exhibits clear, recurring field signs. As early as Fraselle (1950), the symptomatology was comprehensively described on Coffea canephora in the Democratic Republic of the Congo—many of these features are still regarded as classical today.

In addition to Fusarium xylarioides, other Fusarium species can induce similar damage on coffee but are distinguishable. Fusarium solani (syn. Haematonectria haematococca) likewise causes wilt and a dry root rot, but it is characterized by a purplish-brown discoloration of the wood, predominantly on the main roots and at the root collar. The black to dark subcortical discoloration typical of CWD is absent, as are—usually—stromata with perithecia in bark fissures. Microscopically, H. haematococca is readily recognized by abundant microconidia borne on long, branched conidiophores. Fusarium stilboides (syn. Gibberella stilboides) causes coffee bark disease: although subcortical discoloration and a decline associated with reduced vigor may occur, the blackish discoloration of the basal vascular tissues typical of F. xylarioides is lacking. The species is closely related to Fusarium lateritium (syn. Gibberella baccata), which also occurs on coffee and other perennial crops and can be pathogenic. Historical confusion with F. xylarioides has been documented, but it can now be resolved reliably based on the distinctly curved macroconidia of F. xylarioides (cf. Rutherford et al., 2010).

Fusarium xylarioides infects all wild and cultivated Coffea species and can therefore threaten coffee production broadly. Beyond coffee, there is—primarily from laboratory studies—evidence for a broader but heterogeneous host range: cotton seedlings (cv. IAC 20) proved susceptible; tomato fruits could be infected (fulfilling Koch’s postulates), whereas tobacco could not. This indicates that not all solanaceous hosts are equally susceptible. Of particular practical relevance is the detection of the pathogen in the banana cultivar Kayinja (“Pisang Awak”) in Uganda, which is frequently interplanted with coffee—representing a potential inoculum reservoir within the intercrop.

For production systems, this implies that the pathogen can persist outside the coffee plant, likely in certain intercrops and partly within weed communities. Accordingly, consistent surveillance of banana/plantain stands and weeds, strict hygiene (cleaning tools, crates, and footwear), prompt removal of plant and fruit residues, avoidance of moving soil or plant material between plots, and the use of certified planting stock from tightly supervised nurseries are essential.

The pathogen typically enters through wounds on roots and the lower stem (e.g., from cutting or hoeing injuries) and colonizes the xylem. This leads to vascular occlusion; plants wilt and die within weeks to months, depending on age.

Field studies in Uganda (Kangire et al., 2002) did not detect F. xylarioides in the companion plants examined or their roots; at the same time, the pathogen was found in banana plants—an important observation given the prevalence of coffee–banana intercropping. Spores (conidia and ascospores) are dispersed by wind and rain as well as by human activities such as harvesting and pruning—including when dead shrubs are dragged through the plantation as firewood. Because infection occurs via wounds, injurious practices (e.g., hoe-weeding) increase risk. Insect-mediated transmission is possible, although the pathogen has not been consistently isolated from pests or pollinators. Seed transmission remains inconclusive: it has been proposed in specific instances but not confirmed elsewhere.

CWD simultaneously affects plantations, biodiversity, and livelihoods: the pathogen also infects wild Coffea species and has even compromised genebanks (e.g., in Uganda), thereby eroding the genetic base for breeding. On the socio-economic side, the disease forces many farms to relocate or diversify (often with banana), drives rural–urban migration, and leads to substantial income losses.

Chemical interventions are only an option where they are legally approved. Preventive action hinges above all on awareness-raising and quarantine vigilance: CWD is endemic in parts of Africa, has spread across West and Central Africa, and is present in Ethiopia; to date, there are no confirmed reports from Asia or the Americas. Further dissemination can be curbed only by informed industry stakeholders and responsive quarantine services. Eradication is feasible only when the disease is detected early and few shrubs are affected: in such cases, plants must be uprooted and thoroughly burned in situ—an approach that is often difficult to implement for smallholders.

For practical diagnosis, two screening assays have proven to correlate closely with field incidence: (i) a seedling test in which wounded plants are inoculated with a spore suspension, and (ii) a germination test conducted directly on bark. Building on these tools, breeding programs in Uganda, Tanzania, and Ethiopia were initiated; several materials with at least partial resistance have already been released. From Tanzania, six Robusta lines have been reported that combine resistance to CWD and coffee leaf rust with good yields and cup quality; in multi-location trials (Kagera), they were rated resistant after 18 months, with prospects for commercial deployment. Indications of elevated caffeine and chlorogenic acid contents in resistant materials suggest possible biochemical contributions, although the exact resistance mechanisms remain unresolved.

Management of coffee wilt relies primarily on cultural measures and the deployment of resistant cultivars. Critical actions include early diagnosis and immediate uprooting and burning of affected shrubs; the plot should subsequently be left fallow for 6 months (12 months in some recommendations) to reduce soilborne inoculum. Historical measures such as 2.5% copper sulfate sprays or broad copper applications are now considered of limited value due to cost and environmental impacts; localized trunk paints with copper oxychloride have shown success in some locations but remain exceptions. Additional good practice includes avoiding wounding (e.g., during hoe-weeding), applying mulch to maintain soil health, and maintaining buffer distances between plots. Because biological control has so far shown promise only in vitro, integrated approaches are all the more important: continuous monitoring, training of extension agents and farmers (e.g., extension services and workshops, as recently in Côte d’Ivoire), and breeding programs with verified resistance. Where necessary, farms adapt their systems through relocation, diversification, or temporary fallowing to contain further spread and stabilize incomes.