Unveiling Ceratocystis wilt disease: a review of cocoa’s unforgiving foe

Cocoa, a tropical evergreen tree native species to the Amazon rainforest, is a highly favored plantation crop of smallholder farmers and a source of livelihood in tropical lowlands of South America, West Africa, and Southeast Asia. Ceratocystis wilt is one among many fungal diseases that cause economic losses in cocoa production. This narrative review aims to evaluate gaps in understanding Ceratocystis wilt of cocoa. The study utilized the PRISMA framework which serves as guide for a semi-structured review. Through the scientific databases, a total of 202 articles were identified, comprising 51 from the Web of Science and 151 from ScienceDirect. Data extraction was done by categorizing the obtained literature. Results revealed that C. cacaofunesta is an ascomycetes fungus and was first described as C. fimbriata and later re-defined as C. cacaofunesta verified using molecular technique and morphological differences. Its symptoms include leaf chlorosis, and entry of infection is through the wound in the stem caused by contaminated cutting tools and insect attack. As a necrotrophic fungus, it feeds on its host’s nutrients. It is also a homothallic species that can self-fertilise and strive at a temperature between 18 to 28 degrees Celsius. Effective control measures can be costly and negatively impact the environment. However, mitigation measures can be taken, such as exclusion and eradication as preventive measures. This narrative review provides a significant future research undertaking, such as identifying diagnostic protocol for C. cacaofunesta and helping mitigate the disease through providing information dissemination on the farmer’s level for early detection.


Introduction
Cocoa (Theobroma cacao, Linn) is a tropical evergreen tree belonging to the family Sterculiaceae which is a native species to the Amazon rainforest [1].The primary producers of cocoa beans are the tropical lowlands of South America, West Africa, and Southeast Asia, mainly consisting of smallholder farmers [2].As a perennial crop, it is one of the favored plantation crops of over fifty countries, consisting of over 12 million hectares for cocoa production and about 5.3 million tons of world production, with the majority of production concentrated in West Africa, particularly in the Ivory Coast and Ghana which accounts of over 60% of world supply [3][4][5].The consumer's increasing demand of about 5% for cocoa products reflects the global market value of the chocolate industry of approximately US$ 101 billion, and the cocoa industry provides livelihoods to over 6 million farmers and 40 million people worldwide [6,7].
In recent years, cocoa productivity has been visibly decreasing.It may be due to ecological and environmental factors, including ageing trees, degraded soils, and pests and diseases in cocoa production [2].One of the diseases capable of killing the cocoa plant is Ceratocystis wilt [8].This review examined the current knowledge of Ceratocystis wilt affecting cocoa production and evaluated the gaps in understanding Ceratocystis cacaofunesta as a causal agent of Ceratocystis wilt and provides recommendations for future research, including the development of effective management strategies to mitigate the global impact of the disease to the cocoa crop.

Define Research Question and Literature Search
To understand better the disease dynamics, this paper utilized the PRISMA framework to guide a semistructured review of the Ceratocystis wilt disease of cocoa [9].The research question was "What is the disease dynamics of Ceratocystis wilt of cocoa?".Inclusion and exclusion criteria are set to capture the most recent and relevant published articles in the scientific databases.The following search strings "(TS) cocoa OR cacao OR Theobroma cacao AND (TS) "Ceratocystis Wilt" OR Ceratocystis OR "Ceratocystis fimbriata" OR "Ceratocystis cacaofunesta" were utilized to extract diverse and specific outcomes for the topic under study.Selected studies were obtained only from the databases of Web of Science (Clarivate Analytics) and ScienceDirect, which contain an extensive peer-reviewed process and highly regarded published articles.

Data Gathering, Screening Process, and Data Extraction
The semi-systematic review obtained a total of 202 records, subdivided into 51 from the Web of Science and 151 from ScienceDirect databases.After obtaining the potential sources, a screening process was undertaken by screening from titles, abstracts, and full texts.The most relevant articles that were directly relevant to the review were selected and excluded others which are not related to the topic.The selected studies went through in-depth and detailed notetaking and organized them according to themes and key concepts.

Data synthesis
The data synthesis was articulated by the different categories of understanding the disease dynamics.The obtained data were categorized as to the significance and impact, causal agent, symptoms, disease cycle, epidemiology, and management strategies, which will be discussed further in this paper.

Cocoa Ceratocystis wilt: an overview
3.1.1.Significance and impact.The Ceratocystis genus has a global distribution.It was first established in 1890 as a causal pathogen of sweet potatoes' black rot (Ceratocystis fimbriata) and then encompasses several fungi associated with insects which cause and produce sap stains to wood.The taxonomic complication of the fungal pathogen provides confusion and controversy to the wilt species [10].As of 2023, there are 15 species of Ceratocystis that was included in the list of European and Mediterranean Plant Protection Organization since 1998 including Ceratocystis cacaofunesta, the causal agent of cocoa which make it a significant fungal pathogen globally [11].In most infected areas, Ceratocystis wilt resulted to significant economic losses, especially in cocoa producing countries [12], and the spread of the fungal disease does not bring economic and ecological advantages to some of the most economically challenged and environmentally fragile region of the world [8].

Ceratocystis cacaofunesta, the causal agent of cocoa's Ceratocystis wilt. The ascomycete
Ceratocystis cacaofunesta is the cause of cocoa's Ceratocystis wilt.The ascomycete fungus belongs to Sordariomycetes class and Microascales order and is the specialized host to Theobroma cacao, which infected grafted seedlings and adult cloned cocoa tree [12].The Sordariomycetes class is the second largest of ascomycete, encompassing terrestrial species, as well as some are phytopathogens, causing diseases in leaves, stems, and roots of the host plants [13].The causing agent was first described as C. fimbriata but was re-defined as C. cacaofunesta, which reclassification was verified using the molecular technique and morphological differences [14].
3.1.3.Symptoms.The Ceratocystis spp pathogens cause diseases to various tree species (Table 1).The disease is distributed along the countries from South America such as Brazil, Costa Rica, Venezuela, and Colombia; Africa such as South Africa, and Asia such as China, Indonesia, Japan, Pakistan, Oman, Malaysia and Vietnam, to name a few.Temperate and Mediterranean regions such as the United States, Albania, Greece, and Australia also have visibility of Ceratocystis spp.Moreover, the disease has been transferred from one tree species to another as alternative host and eventually evolved which was happened to C. cacaofunesta which was first named as C. fimbriata and later renamed with the aid of molecular technique and morphological differences.
C. cacaofunesta as compared to other Ceratocystis spp shares similarity in terms of symptoms.As a fungal pathogen, it causes leaf chlorosis and wilt within 2-4 weeks after infection [15].The main signs of disease in cocoa trees and other hosts such as Mangifera, Acacia spp, and Prunus is a wound caused using contaminated pruning tools or insect damage.Another recognizable sign is that the leaves on the affected plant suddenly wilt and remain attached to the branches for several weeks after the tree's death [8,16].Prior to wilting, no signs of hyphae and conidia in inoculated susceptible and resistant cocoa seedling, thus, reproductive structures of the fungal pathogen in planting materials like stem and trunk can only be noticed at the late stage of the infection process [8].[41], and contaminated cutting tools [8,42].The entry of infection for Ceratocystis spp is mainly limited to the xylem during the disease cycle, and some species break down the walls of the xylem vessels to spread nearby parenchyma cells, and often secrets structures of microsclerotia, chlamydospores, thick-walled mycelium and spore-bearing coremia in soil or dead plant tissues, which serve as their resting and overwintering stages [43].
The entry of infection of C. cacaofunesta to cocoa is passively through the stem wounds by insects or contaminated tools.At the same time, some literature observed necrosis as the effect of C. cacaofunesta shows soil-borne pathogens characteristics of the disease.Moreover, C. cacaofunesta pathogen after entering the target host germinates chlamydospores and shows different patterns of fungal colonization.Ceratocystis spp fungal colonization is limited in some parts of the infected areas for natural host while for susceptible host, fungal colonization is extensive at the radial direction in the stem's parenchyma tissues [44].Studies of the histopathology of C. cacaofunesta in T. cacao seedlings that are both resistant and susceptible showed that fragments of the fungus grow well in both genotypes and form perithecia [45].C. cacaofunesta is a type of necrotrophic fungus, which obtains its nutrients from the cells of dead hosts before or during its colonization [44].C. cacaofunesta is homothallic species together with C. virescens, C. pinicola, and C. fimbriata, where MAT-2 strains are self-fertile.However, some other Ceratocystis spp, such as C. eucalypti, is strictly heterothallic with single ascospore strains representing one of two opposite mating types [46].
3.1.5.Epidemiology.South American tropics have a higher number of species and appearances, suggesting that the region is probably the birthplace of Ceratocystis species.The genetic markers can help determine the diversity of pattern and distinguish them from the native to the introduced population [44].Cocoa is grown in various tropical and subtropical regions globally.The temperature for its lies between 15 to 32 degrees Celsius and level of precipitation between 1500 to 3000 millimeters however Ceratocystis cacaofunesta species has optimum growth when the temperature is between 18 to 28 degree Celsius [47].The temperature and precipitation conditions in these regions support the growth of the cocoa tree, but it also creates an environment suitable for C. cacaofunesta.The temperature is within the range of infection, and the spread of the disease is higher, which could lead to reduced yield and decreased profitability for cocoa farmers.
3.1.6.Management strategies.The aim of implementing effective control measures for Ceratocystis spp can be costly and have a negative impact on the environment.Therefore, prevention is the recommended strategy for cocoa farmers.However, when the plantation is at high risk of infection and already infected with the fungal pathogen, mitigation measures such as exclusion and eradication can be implemented, and control measures such as chemical and biological controls and the use of hostresistant genotypes all aim to disrupt the critical stages of the fungus' infection cycle [48].Management strategies for Ceratocystis cacaofunesta can be categorized as avoidance, exclusion, eradication, protection, host-plant resistance, chemical control and biological control strategies [44], which can be assessed based on the cost-effectiveness depending on the severity of the disease.

Conclusion
It is important to emphasize that Ceratocystis cacaofunesta is a serious threat to cocoa production.Moreover, the availability of resistant cocoa varieties to C. cacaofunesta is fragmented and limited [49][50][51] and requires updating and upgrading.The disease is difficult to control, and the ability to spread is alarming since the temperature and precipitation conditions for cocoa production create an environment for C. cacaofunesta for optimal growth [47].Finally, C. cacaofunesta imposes economic losses on infected cocoa areas, and the spread of the disease does not bring economic and ecological advantages to the world's most economically challenged and environmentally fragile cocoa regions.
There is still room for development to elaborate the biology and epidemiology of C. cacaofunesta to understand better the pathogen's life cycle, which can lead to effective control strategies.Moreover, it is essential to research the development of disease-resistant cocoa varieties that will help the efficiency of the breeding process.Previous studies on disease-resistant varieties need updating and upgrading [49][50][51] and integration of the current and modern breeding process will be helpful.Integration of multiple control methods can be further studied to identify the cost-effectivity and best combinations of control strategies.It is also imperative to find new avenues for control and address the challenges of C. cacaofunesta for the future of the cocoa industry.

Table 1 .
Some Ceratocystis spp.associating fungal wilt in trees 3.1.4.Disease cycle.C. cacaofunesta as a causal agent of Ceratocystis wilt of cocoa has a complex life cycle which involves both sexual and asexual reproduction.As part of the latin American clade of Ceratocystis, infections commonly linked to cocoa is caused by bark and ambrosia beetle