Current Status of Yam Diseases and Advances of Their Control Strategies

: Yam ( Dioscorea spp.) is an important tuber crop consumed globally. However, stable yam production faces challenges from a variety of diseases caused by fungi, nematodes, viruses, and bacteria. Prominent diseases such as anthracnose, leaf spot, yam wilt, dry rot


Introduction
Yam (Dioscorea spp.) is a crucial staple in people's diets in Africa and Asia's tropical and subtropical regions [1].These dioecious, monocotyledonous plants, which can be either annual or perennial, are primarily cultivated through the vegetative propagation of their tubers.Within the Dioscorea genus, which comprises approximately 600 species [2], around 60 of them serve as food and medicinal plants [3].Dioscorea rotundata (white yam) and D. alata (water tam) hold pivotal roles as essential food sources.Other significant cultivated yam species include D. opposita-japonica, D. bulbifera, D. esculenta, D. nummularia, D. cayenensis, D. pentaphylla, D. trifida, D. zingiberensis, and D. nipponica.D. polystachya (Chinese yam), native to temperate regions, demonstrates exceptional resilience to cold temperatures compared to other yam species [4].
Yam's global production has surged from 8.3 million tons to 88.2 million tons, covering an expanded production area from 1.15 million hectares (mha) to 10.3 mha [5].Overall, 60 to 100 million people mainly depend upon it for food, making it the fourth most important root crop by production after potatoes, sweet potatoes, and cassava.However, yams exhibit superior storage quality as compared to other crops, thus holding a significant potential for food security and to generate substantial income for farmers, processors, and sellers [6][7][8].In African nations like Nigeria and Ghana, yam is prevalent in mitigating hunger and establishing dietary stability across these regions [9].Small-scale farmers in the Ivory Coast, Ghana, Benin, Togo, and Nigeria predominantly cultivate yam, solidifying Africa as a central hub for yam production [10].In China, known as "Shanyao", yam significantly contributes to the global production of yams, accounting for 17% [11].Major yam-producing regions in China, including Henan, Hebei, Shandong, and Jiangsu provinces, collectively yield nearly 6 million tons annually [12].Notably nontoxic, Chinese yam can be consumed raw, distinguishing it from other edible species [13].
Agronomy 2024, 14, 1575 2 of 20 Yam has been recognized for its nutritional richness and medicinal properties for a long time.It is a significant energy source with abundant starch, protein, and fiber [14,15].Due to their high starch content, yams find application in animal feed, instant noodles, and various beverages, while the peel is primarily used in animal feed.Sourced from yams, diosgenin, a phytosteroid, contributes to a market share valued at USD 500 million [11].In China, the increasing consumption of yam in traditional medicines is attributed to its rich reserve of micronutrients, minerals, bioactive compounds, metabolites (resistant starches and steroidal sapogenins), antioxidants, and polysaccharides which play pivotal roles in combating various cardiovascular diseases, gastrointestinal disorders, diabetes, diarrhea, and asthma [4,16].High polysaccharide contents contribute to managing obesity and improving insulin resistance [17].
Despite its importance, yam production still falls behind other tuber and cereal crops [18].Increasing local and export demands contrast with limited output due to factors such as high cultural costs, labor intensiveness, diminished soil fertility, shifting climatic conditions, and the prevalence of numerous pathogens and insects [9].Global food production will need to increase by about 60% by 2050 to meet the food needs of 10 billion people.As the global population expands and arable land becomes scarcer, ensuring food security has emerged as a critical challenge.Plant diseases are a global concern, significantly impacting food crop production and nations' social and political stability.The yield losses of different crops due to these diseases and crop pathogens are substantial [19].Various plant diseases due to different phytopathogens, including fungi, nematodes, bacteria, and viruses, pose severe threats to global agricultural crops and yields of yam [20,21].Fungal pathogens such as Colletotrichum spp., Sclerotium spp., Fusarium spp., Alternaria spp., and Rhizoctonia spp., along with nematodes like Scutellonema bradys and Meloidogyne spp., contribute to these diseases.Additionally, viral infections from yam mosaic virus (YMV) and yam mild mosaic virus (YMMV), Dioscorea bacilliform virus (DBV), and bacterial strains like Xanthomonas campestris and Corynebacterium spp.further exacerbate the biotic disease burden on yams.
Traditional agricultural practices have reduced the genetic diversity of major crops, making them more susceptible to substantial losses caused by plant diseases.These practices are not sustainable in the long-term for managing diseases and conserving resources [22].Various methods to combat plant diseases in agriculture have been developed, including host resistance, agrochemicals, and biological agents [23].While these methods have helped prevent and control epidemics in the short-term, their deployment without a comprehensive understanding of epidemiological and evolutionary impacts offers limited sustainability.The primary management of plant disease losses still relies heavily on introducing significant resistance (R) genes into crops with desirable agronomic traits [24].During the growing season, when crops are most vulnerable to epidemics and lack sufficient genetic defense, agrochemicals often represent the only effective control option.The development and application of modern agrochemicals have undoubtedly been crucial in enhancing global food supply and security [25].However, achieving long-term sustainable disease management requires a shift from focusing solely on yield and productivity to integrating these goals with ecological, economic, and environmental considerations.Current disease control strategies need to be adaptable, yet they often remain static.For these strategies to be widely adopted, they must be both practical and economically viable, integrating sustainable farm profitability, disease epidemiology, and pathogen evolution into formulating optimal strategies [26,27].
This review extensively investigates the significant diseases caused by phytopathogens, investigating fungi, nematodes, viruses, bacteria (Figure 1), their epidemiology, and diverse control strategies employed to manage these diseases.To optimize the plants' defense capabilities, sustainable management of yam diseases can be achieved by adjusting agricultural practices.However, successful outcomes require careful consideration and proactive measures to address the continually evolving abilities of pathogens.agricultural practices.However, successful outcomes require careful consideration and proactive measures to address the continually evolving abilities of pathogens.

Anthracnose of Yam
Yam anthracnose is one of the most devastating diseases across global yam-growing regions, causing severe yield losses ranging from 50 to 90% [28,29].Rapid pathogen establishment triggers aggressive infectivity, leading to substantial yield loss [30].The Colletotrichum genus, a significant contributor to diseases in major crops worldwide, ranks as the eighth most decisive group of fungi globally [31,32].This exhibits diverse growth characteristics like varied growth rates, distinct conidia, and appressoria morphology, infiltrating yams across global growing regions.Colletotrichum gloeosporioides, also known as

. Anthracnose of Yam
Yam anthracnose is one of the most devastating diseases across global yam-growing regions, causing severe yield losses ranging from 50 to 90% [28,29].Rapid pathogen establishment triggers aggressive infectivity, leading to substantial yield loss [30].The Colletotrichum genus, a significant contributor to diseases in major crops worldwide, ranks as the eighth most decisive group of fungi globally [31,32].This exhibits diverse growth characteristics like varied growth rates, distinct conidia, and appressoria morphology, infiltrating yams across global growing regions.Colletotrichum gloeosporioides, also known as Colletotrichum alatae, emerges as the primary agent behind yam anthracnose [32], causing up to 90% yield losses [33].While it affects all parts of the yam plant, it predominantly targets leaves, stems, and vines.This disease is characterized by various spots, often black or brown, and defoliation occurs in uncontrolled conditions, leaving behind naked scorched vines (Figure 2A) [34].The early symptoms include small round or oval dark black or brown spots that appear on leaves, petioles, and stems, mostly round or oval, possessing slight depression in the center of the spot.As the disease progresses, the center of the spot is often grey-white or grey-brown, with a yellow halo around it.The spots merge to form patches, and the expansion of the spots frequently leads to necrosis of the edges of the leaf blades.The infection usually starts at the base of the stem, with black dots appearing on the stem surface, expanding into long stripes or pike-shaped spots, often blackish brown.The spots gradually expand into patches, leading to the blackening and drying out of the stems [29,35,36].Multiple methods can spread anthracnose, but it is mainly dispersed through rain splashes of the contaminated soil with spores [37].The infection initiates with the penetration of the pathogen through natural openings like the stomata and cuticle [32].The optimum temperature for activating the pathogen Colletotrichum gloeosporioides to cause disease is 25 • C to 30 • C [38].However, spore germination and viability are relatively sensitive to relative humidity [39].Given the severity of yam anthracnose, continued research and farmer education are crucial to mitigating its impact.
Colletotrichum alatae, emerges as the primary agent behind yam anthracnose [32], causing up to 90% yield losses [33].While it affects all parts of the yam plant, it predominantly targets leaves, stems, and vines.This disease is characterized by various spots, often black or brown, and defoliation occurs in uncontrolled conditions, leaving behind naked scorched vines (Figure 2A) [34].The early symptoms include small round or oval dark black or brown spots that appear on leaves, petioles, and stems, mostly round or oval, possessing slight depression in the center of the spot.As the disease progresses, the center of the spot is often grey-white or grey-brown, with a yellow halo around it.The spots merge to form patches, and the expansion of the spots frequently leads to necrosis of the edges of the leaf blades.The infection usually starts at the base of the stem, with black dots appearing on the stem surface, expanding into long stripes or pike-shaped spots, often blackish brown.The spots gradually expand into patches, leading to the blackening and drying out of the stems [29,35,36].Multiple methods can spread anthracnose, but it is mainly dispersed through rain splashes of the contaminated soil with spores [37].The infection initiates with the penetration of the pathogen through natural openings like the stomata and cuticle [32].The optimum temperature for activating the pathogen Colletotrichum gloeosporioides to cause disease is 25 °C to 30 °C [38].However, spore germination and viability are relatively sensitive to relative humidity [39].Given the severity of yam anthracnose, continued research and farmer education are crucial to mitigating its impact.

Concentric Leaf Spot of Yam
Concentric leaf spot of yam is considered yam's second most destructive foliar disease [40].This disease is caused by Sclerotium rolfsii, a significant soil-borne pathogen with a wide host range of approximately 500 plants, contributing substantially to global yield losses [41].Under adverse conditions, yam crops can experience yield losses of up to 50%.Visual symptoms include brownish spots, often surrounded by the chlorotic halo, that progressively expand concentrically.Circular leaf spots of varying sizes form concentric rings, and at maturity, fungal sclerotia can be observed at the center of these leaf spots.As necrosis progresses, these lesions may merge with the center, causing the affected areas to drop out and have a distinctive ring pattern compared to other spot diseases.Concentric leaf spots can be observed in nurseries and fields, leading to spot disease and complete

Concentric Leaf Spot of Yam
Concentric leaf spot of yam is considered yam's second most destructive foliar disease [40].This disease is caused by Sclerotium rolfsii, a significant soil-borne pathogen with a wide host range of approximately 500 plants, contributing substantially to global yield losses [41].Under adverse conditions, yam crops can experience yield losses of up to 50%.Visual symptoms include brownish spots, often surrounded by the chlorotic halo, that progressively expand concentrically.Circular leaf spots of varying sizes form concentric rings, and at maturity, fungal sclerotia can be observed at the center of these leaf spots.As necrosis progresses, these lesions may merge with the center, causing the affected areas to drop out and have a distinctive ring pattern compared to other spot diseases.Concentric leaf spots can be observed in nurseries and fields, leading to spot disease and complete blight of germinating yams [40,42].Sclerotium rolfsii colonies exhibit a brown-to-black coloration and are globose and compact.Under microscopic examination, the mycelium is typically light and straight, lacking conidia [43].Sclerotium rolfsii produces abundant hyphae and sclerotia as asexual resting structures, facilitating the pathogen's survival without a host [41].The optimum temperature for initiating Sclerotium rolfsii pathogenesis is 25 • C to 30 • C, with intermediate soil moisture at around 70% of field capacity [44].

Leaf Blight of Yam
Leaf blight of yam, caused by Alternaria tenuissima, was first reported by [45].Alternaria tenuissima is a vital pathogen that economically impacts several important crops, including tomato, broad bean, potato, watermelon, and muskmelon [46].This pathogen predominantly affects the aerial parts of crops, leading to blight, fruit rot, and stem canker.Outbreaks of foliar diseases caused by Alternaria tenuissima are often widespread and particularly devastating.Initial symptoms appear on the petioles and then spread to the leaf bases, forming irregular spots.The diseased leaves exhibit inward curling from the edges and develop chlorosis.In later stages, a stale mildew forms around these irregular spots.The colony of Alternaria tenuissima grows slowly, forming round, thick, and dark gray-brown patches [45].High humidity and relatively elevated temperatures rapidly initiate the infection [46].

Yam Wilt
Yam wilt is one of the most damaging emerging diseases, attributed to Fusarium spp., a fungal genus comprising approximately 3000 phylogenetic species distributed worldwide [47].This particular ascomycete genus is recognized for causing the most economically damaging plant infections globally, affecting nearly all commercially significant crops and resulting in substantial annual economic losses totaling billions of dollars within the global agricultural sector [48].First reported in 1988, yam wilt was initially associated with Fusarium oxysporum as the causal agent, and recent studies continue to identify Fusarium oxysporum as the dominant pathogen responsible for this disease [49].In China alone, Fusarium-induced disease losses range from 30 to 70%, affecting both above and below-ground parts of yam.The infection begins early, causing continuous color changes in the infected tissues, leading to wilting, rot, and plant death (Figure 2B).Yam wilt also presents a major post-harvest threat to yams [50].As a soil-borne disease, high temperatures and rainy weather can rapidly accelerate the onset of pathogen infection.

Leaf Spot of Yam
Leaf spot is the most prevalent foliar disease in yam fields, and a variety of pathogens cause it.These include Cylindrosporium dioscoreae, Cercospora dioscoreae, Pseudocercospora contraria, Nigrospora oryzae, Curvularia eragrostidis, and Alternaria alternata [51][52][53].Initially, symptoms appear on the leaf surface as white to grey mycelial masses.As the infection progresses, these masses disappear.Leaf symptoms also include chlorosis lesions with dark brown spots that enlarge over time (Figure 2C).Occasionally, irregular spots merge, forming extensive necrotic areas on the leaves.The fungus thrives on crop debris, with conidia primarily produced on the surface of infected leaves and dispersed by rain.The disease is more prevalent during wet and warm weather conditions.Fungal colonies exhibit white margins and fluffy white mycelium, later transitioning to grey-brown during sporulation.Microscopic observations show light to olive brown conidiophores, occurring single or in clusters, septate, and straight with short beaks.

Collar Rot of Yam
Collar rot caused by Rhizoctonia spp., primarily affects the roots of yam tubers, leading to subsequent decay.Dark-colored spots on the stem indicate collar rot, with brown marks near the soil level as significant symptoms of this disease.Although the roots are the main sites of infection, any leaf symptoms usually appear exclusively on one side.Rhizoctonia spp.pose a more significant threat during the summer months, thriving under conditions of low moisture and temperatures ranging from 12 • C to 32 • C, promoting the disease's development.The susceptibility of plants increases with the presence of wounds near the soil surface [36].

Tuber Rot of Yam
Yam tuber rot is a critical factor influencing the post-harvest lifespan of tubers, making them unpalatable and affecting their market value.Post-harvest losses range between 25% and 50% of the yield due to various fungal and bacterial pathogens, with viruses and nematodes contributing to additional yield losses.These pathogens typically produce extracellular enzymes that degrade cell wall polymers, ultimately affecting parenchymatous tissues [54,55].The severity of tuber rot varies based on the pathogen attack, host specificity, and yam varieties [56].Reported rot-causing fungal pathogens include Fusarium, Aspergillus, Macrophomina, Rhizopus, Penicillium, Sclerotium rolfsii, Botrydiplodia species, and bacterium Erwinia carotovora [57,58], with Fusarium tuber rot being particularly destructive.
Fusarium spp.belong to Ascomycota and contribute to important diseases in crops and fruits worldwide [59,60].Tuber rot caused by Fusarium is primarily prevalent in China, India, Japan, and Nigeria, with the pathogen favoring infection under high humidity and temperatures ranging from 23 • C to 29 • C [61].Disease symptoms often remain undetectable until the tubers are cut transversely, revealing brown-colored dry rot in the infected tissues (Figure 2D).The detailed review highlights the significant impact of yam tuber rot on post-harvest losses, particularly Fusarium spp., to improve the market value and lifespan of yam tubers.

Prevention, and Control Strategies 2.2.1. Cultural Control
Cultural practices in agriculture are geared towards growing disease-free plants and are recognized for their cost-effectiveness and environmental sustainability.These practices help prevent the build-up of disease-causing pathogens in soil and plants and generally provide more long-term efficacy than reactive measures like pesticides.Effective methods to prevent disease include early planting, timely removal of alternative hosts, using healthy seeds, and maintaining field sanitation [32].Specific strategies for controlling diseases caused by Sclerotium rolfsii include removing and destroying infected plants that act as inoculum sources, using resistant crop varieties, and rotating with non-host crops [41].Additionally, collecting and burning vines post-harvest can significantly help control fungal infections.Rhizoctonia, a soil-borne pathogen, can be effectively managed using disease-free tubers.Keeping diseased plants and residues out of growing areas is crucial in mitigating disease impacts [36].Minimizing physical damage to tubers during post-harvest handling is also vital, and any wounded tubers should be placed in conditions conducive to rapid healing.Proper storage conditions, particularly temperature, significantly reduce post-harvest losses of yam tubers.A study showed an average temperature of 29.7 • C and relative humidity of 78.6% in a special wooden box compared to a platform with a temperature of 30.7 • C, was completely absent of the rotting of tubers [62].Utilizing both traditional and modern yam barns can further enhance the qualitative and quantitative control of diseases [55].Maintaining low shelf temperatures is essential for post-harvest handling systems to preserve marketability by controlling post storage disorders such as rooting and decay.The comprehensive findings indicate that an optimized post-harvest program, incorporating heated-air curing, appropriate storage conditions, and low shelf temperature, can effectively extend the storage potential of Chinese yam to beyond seven months [63].

Chemical Control
The primary management of fungal diseases such as yam anthracnose typically involves the use of copper-based fungicides like mancozeb or maneb, chlorothalonil, and benlate.These fungicides require biweekly or monthly applications.However, the excessive use of these chemicals raises significant environmental concerns globally and also contributes to the development of fungicide-resistant strains of pathogens [64].Sclerotium rolfsii can be treated with potassium salt, salicylic acid, carbendazim, and seed treatments with fungicides like metalaxyl to prevent infection.Fungicides such as Topsin-M (thiophanate methyl) and prothioconazole have also proven effective against leaf spot disease.Dipping yam tubers in fungicides like benomyl, captan, and thiabendazole can help control yield losses due to Fusarium spp.Synthetic chemicals such as thiobendazole, mancozeb, borax, and others have been shown to be effective against tuber rot in yam [55,65].

Biological Control
Biological control, utilizing microbial antagonists, offers an eco-friendly alternative to chemical methods [66].Soil actinomycetes, known for producing various secondary metabolites, can suppress numerous pathogens.Among these, Streptomyces species are particularly effective against soil-borne fungal pathogens.A novel strain, Streptomyces MJM5763, has shown efficacy in controlling yam anthracnose.Application of MJM5763 and the crude culture filtrate extract (CCFE) validated significant efficacy in reducing anthracnose severity by 85-88% and incidence by 79-81% in vitro, 90 days post-inoculation.Comparably, field trials exhibited a reduction in anthracnose severity and incidence by 86% and 75%, respectively [67].The use of biological agents like Trichoderma spp., Pseudomonas spp., Penicillium spp., plant extracts (such as garlic, ginger, neem, turmeric), and techniques like soil solarization are significant and environmentally friendly strategies to control diseases like Sclerotium rolfsii [44].Additionally, Bacillus species isolated from the yam farm soil and microflora of yam tubers and Trichoderma harzianum have effectively reduced tuber rot and yam rot.Studies showed a 50.61% inhibition rate compared to control when treated with Trichoderma harzianum [36,68,69].Plant extracts containing flavonoids, glycosides, and alkaloids also aid in controlling tuber rot.Extracts from bitter melon (Momordica charantia L.) are used to combat leaf spots in yams due to their antifungal activity against various pathogens including Alternaria alternata and Curvularia eragrostidis [70].The effectiveness of Bacillus subtilis, obtained from the microflora of cow dung, as a biocontrol agent against post-harvest pathogens of yam has been substantiated in previous studies.The in vivo study demonstrated that Bacillus subtilis strains inhibited the growth of Fusarium oxysporum and Botryodiplodia theobromae in wound cavities of yam tubers by up to 83% [71].The peel extract of water yam (Dioscorea alata) exhibits potential for controlling rot in post-harvest yam tubers [72].

Breeding Control
Due to environmental and health concerns, a concerted effort has been made to develop and utilize resistant varieties and lines to combat yam anthracnose [73].While developing these varieties through classical breeding has been slow, some progress has been achieved in creating resistant hybrids using traditional breeding methods.The interspecific hybridization between Dioscorea alata IN2x females and Dioscorea nummularia Lam.Vanuatu polyploid males demonstrated effectiveness, showing significant variability in pollination efficiency, seed set, and seedling survival rates (49.0%,20.8%, and 35.3%, respectively).The hybrids verified intermediate traits between the two parent species, mainly exhibiting high tuber yield and resistance to anthracnose [74].In another study, researchers explored the potential of genomics-assisted breeding and genetic engineering strategies to control yam anthracnose [32].Genome-wide association studies (GWAS) are currently being conducted to find quantitative trait loci (QTLs) associated with yam anthracnose in Dioscorea rotundata and Dioscorea alata.These efforts aim to enhance markerassisted breeding in yam [75].Additionally, leveraging non-pathogenic races of pathogens to enhance resistance has shown promising results.For instance, the use of non-pathogenic races of Fusarium oxysporum to control Fusarium wilt has been successfully implemented in several crops, including basil, cucumber, banana, and watermelon [76].Nematodes play a crucial role by diminishing tuber yield and quality, expediting bacterial and fungal outbreaks [77].The plant-parasitic nematode Scutellonema bradys is the primary agent responsible for dry rot in yams, posing a significant challenge to yam cultivation worldwide.This nematode is prevalent in tropical regions and seriously threatens yam production.This disease reduces the quantity of yams and adversely impacts the market value of affected tubers, with diseased tubers weighing approximately 20-30% less than healthy ones.Scutellonema bradys is an endoparasite primarily affecting the roots and tubers, confined to the tuber's outer one to two centimeters.Its ability to move intercellularly and destroy cell walls results in the formation of cavities and necrosis.Due to its capacity for rapid dissemination within yam tubers, Scutellonema bradys represents a high risk factor and a significant challenge in yam-growing areas.Diseased seed tubers, rather than soil, serve as a considerable nematode inoculum source.Scutellonema bradys also exhibits a broad host range, affecting most yam species cultivated for consumption.Storage conditions significantly influence the population dynamics of Scutellonema bradys, with relative humidity levels between 40 and 80% and temperatures ranging from 22 • C to 32 • C playing crucial roles in population growth [36,78].The data emphasize the critical threat posed by Scutellonema bradys to yam yield and quality.Tackling its rapid dissemination and impact on tuber weight is essential for sustaining yam production in tropical regions.

Crazy Root Syndrome of Yam
Root-knot nematodes (RKN) are identified as the causal agents of crazy root syndrome in yams.RKNs have a widespread impact on yam cultivation.This disease alters the appearance of yams and adversely affects their tuber quality, making them less appealing and reducing market value.In China, RKNs are responsible for substantial yield losses, ranging from 24% to 80%, and have led to the decline of numerous cultivars [2,78].Various Meloidogyne species, particularly Meloidogyne incognita, Meloidogyne javanica, Meloidogyne arenaria, and Meloidogyne hapla, are known to cause yam diseases.Among these, Meloidogyne incognita and Meloidogyne javanica are considered the most significant.Infestations by Meloidogyne spp.led to the galling of roots and tubers, resulting in stunted and chlorotic plants.In severe cases, these infections can even lead to the death of young seedlings.RKNs primarily affect underground tubers and roots, forming bulbous, irregular tumors that often appear white or yellowish-white on the surface of fibrous roots, eventually turning brown to yellow-brown (Figure 3).Such damage can severely inhibit tuber formation and lead to rotting [2,36,79].Under favorable conditions, the population of RKNs in the soil can increase rapidly, with the optimal temperature range for their development being around 25

Cultural Control
Hot water treatment effectively controls the nematode Scutellonema bradys [81].By heating water to 50 • C to 55 • C and dipping the tubers for 45 min, this technique effectively controls Scutellonema bradys infections without damaging the tubers.However, this application is limited to small-scale operations.On a larger scale, challenges such as maintaining consistent temperatures, high labor costs, fuel expenses, and managing substantial quantities of yam make it impractical.For cultural control measures, phytosanitation is employed to manage nematode populations [82].It is also essential to segregate infected tubers from healthy ones before storage and planting to prevent the establishment of nematode infections.

Cultural Control
Hot water treatment effectively controls the nematode Scutellonema bradys [81].By heating water to 50 °C to 55 °C and dipping the tubers for 45 min, this technique effectively controls Scutellonema bradys infections without damaging the tubers.However, this application is limited to small-scale operations.On a larger scale, challenges such as maintaining consistent temperatures, high labor costs, fuel expenses, and managing substantial quantities of yam make it impractical.For cultural control measures, phytosanitation is employed to manage nematode populations [82].It is also essential to segregate infected tubers from healthy ones before storage and planting to prevent the establishment of nematode infections.
Additionally, disease-free planting materials can be produced through tissue culture techniques.Traditional methods like incorporating cow dung into yam mounds and applying wood ash prior to planting have also been shown to reduce nematode populations.Agronomic practices, such as crop rotation and allowing fields to lie fallow, are highly effective in controlling nematodes [36,78,83].Growing sweet potatoes as a trap crop alongside yam to manage root-knot nematodes (RKN) and using nematode-free tubers for propagation are recognized as successful strategies.However, a significant challenge remains with Meloidogyne spp., which has a wide host range [36,79].Additionally, disease-free planting materials can be produced through tissue culture techniques.Traditional methods like incorporating cow dung into yam mounds and applying wood ash prior to planting have also been shown to reduce nematode populations.Agronomic practices, such as crop rotation and allowing fields to lie fallow, are highly effective in controlling nematodes [36,78,83].Growing sweet potatoes as a trap crop alongside yam to manage root-knot nematodes (RKN) and using nematode-free tubers for propagation are recognized as successful strategies.However, a significant challenge remains with Meloidogyne spp., which has a wide host range [36,79].

Chemical Control
The use of nematicides has also proven effective against nematodes.Treating planting materials with nematicides to obtain nematode-free seed yam is cost-effective.Successfully combating nematodes, the bare root-dip method involves treating infected yams with dasanit, nemafos, and nemagon at concentrations of 1250 ppm and 645 ppm for durations of 15 min and 30-60 min, respectively.Ebufos and carbosulfan are recommended nematicides for combating Scutellonema bradys [84].In the market, nematicides effective against root-knot nematodes include fluopyram, imidacloprid, thiodocard, and abamectin [79].Furthermore, carbofuran and granular oxamyl, applied at rates of 3 kg active ingredient per hectare and 3-6 kg active ingredient per hectare, respectively, have been reported as effective against Meloidogyne javanica on Dioscorea rotundata, with treatments repeated at 3-4 week intervals [36].Fulan, a synthetic nematicide applied at two-bed types (ridging and mounding), suppressed the nematode densities of Meloidogyne incognita and Pratylenchus coffeae around 93% compared to control [85].

Breeding Control
The progression of resistant yam varieties has been hindered by the lack of specific and effective screening methods and the difficulty in producing sufficient planting material for consistent year-round phenotyping.Traditionally, yam cultivation employs whole tubers or tuber segments as planting materials [93].Resistant varieties are proving valuable in the battle against Scutellonema bradys.Notably, one variety from Dioscorea esculenta and Dioscorea dumetorum has demonstrated effectiveness against this nematode [36].Utilizing resistant cultivars is recognized as a safe, environmentally friendly, sustainable, and costeffective strategy for managing root-knot nematodes.The resistance of yam accessions typically hinges on reproductive factors and the galling index.Recent research on resistant white yam cultivars showed that accession TDr1515OP16/0030 produced 19.4 g of mini tubers, almost 74% higher than its predecessor [94].The adequate inoculum levels required for screening yam accessions for their response to nematodes, specifically Meloidogyne incognita, Scutellonema bradys, and Pratylenchus brachyurus, can be significantly reduced by 60% through the use of vertical sacks containing ten vines.Compared to the traditional method of using a single set, this approach offers additional benefits, including conserving space, labor, and tubers [95].A novel method utilizing rooted yam vine cuttings and yam plantlets generated through semi-autotrophic hydroponics (SAHs) propagation has been proven effective for phenotyping yam genotypes for nematode resistance which will be helpful in the advancement of resistant varieties against nematodes.This study's results showed a potential resistance and tolerance against Scutellonema bradys in 58% of the genotypes, and against Meloidogyne javanica, Meloidogyne arenaria, Meloidogyne incognita, and Meloidogyne enterolobii in 88%, 65%, 65%, and 58% of the genotypes, respectively [96].

Yam Mosaic Virus (YMV)
Viral diseases significantly threaten yam cultivation worldwide, with the yam mosaic virus (YMV) being particularly destructive.YMV belongs to the potyvirus group and is one of the major concerns in yam agriculture, although data on the variability of these viruses remains insufficient.YMV typically causes symptoms including mottled dark green and chlorotic patches on leaves.Severe infections lead to leaf chlorosis, pronounced green veining, leaf curling, stunting, and overall plant distortion.Aphids transmit YMV in a non-persistent manner and can also spread through the sap.Additionally, mechanical inoculation by aphids facilitates horizontal transmission.The virus can also be spread through the vegetative propagation of infected plant material.Notable yam species vulnerable to YMV include Dioscorea alata, Dioscorea cayenensis, Dioscorea esculenta, and Dioscorea rotundata.The tuber yield reduction is higher at the early stages of infection as compared to the maturity level [7,34,36,99].The above data highlight the severe impact of the yam mosaic virus on yam yield, and addressing its transmission, especially through aphids and vegetative propagation, is crucial for sustainable yam farming.

Yam Mild Mosaic Virus (YMMV)
Yam mild mosaic virus (YMMV), second to YMV in significance, is another member of the Potyvirus genus affecting yams worldwide.Its symptoms, which include mild mottling and mosaic patterns, are generally seen in Dioscorea alata and Dioscorea trifida, but interestingly, Dioscorea rotundata shows no symptoms.Initially identified as Yam virus 1 or Dioscorea alata virus (DAV), YMMV particularly affects Dioscorea alata species, primarily in African regions [100].Distinguished from other potyviruses based on International Committee on Taxonomy of Viruses (ICTV) species classification criteria, YMMV exhibits less than 76-77% nucleotide and less than 80% amino acid identity in the coat protein (CP) region, with a 57.1% amino acid sequence similarity between YMV and YMMV in the CP region [101].YMMV mainly spreads through the vegetative propagation of infected tubers and vines and is primarily transmitted by the aphid vector Aphis craccivora.

Dioscorea Bacilliform Virus (DBV)
Badnaviruses, belonging to the plant pararetrovirus group within the family Caulimoviridae, are significant pathogens causing diseases in almost all major crops worldwide.The economic impact of these viruses is considerable, with losses ranging from 10 to 90% in some cases.Badnaviruses affect a diverse array of crops, including yam, citrus, banana, sugarcane, black pepper, grape, cocoa, sweet potato, and taro.Dioscorea bacilliform virus (DBV) is notably prevalent in yams, representing the primary viral threat.DBV mainly spreads through vegetative propagation and is also transmitted by various mealybug species, especially Planococcus citri.While DBV-infected plants often show no apparent symptoms, some may exhibit chlorosis, mosaic patterns, crickling, and necrosis, generally ranging from mild to moderate in severity.The association of Badnaviruses with yams was initially reported in relation to Brown spot disease in D. alata [102][103][104].

Prevention, and Control Strategies
Plant viruses are intracellular obligate parasites that can only survive within a living host [105], making viral infections in plants difficult to cure compared to bacterial or fungal infections.In yams, it is hypothesized that the primary mode of virus transmission is using infected cuttings or tubers as planting materials [104].Numerous studies have demonstrated that the secondary or horizontal spread of viruses in vegetatively propa-gated crops is facilitated by insect vectors and/or mechanical transmission.Nevertheless, the transmission mechanisms of other viruses infecting yams remain unidentified, impeding the development and implementation of effective control strategies against these pathogens [106,107].Effective virus diagnosis is fundamental for successful disease management, particularly in sanitation programs critical for clean seed production.Inaccurate diagnoses can result in false negatives or positives, with potential outcomes ranging from minor misrepresentations of virus distribution to major consequences, such as spreading viruses to new areas through the exchange of infected germplasm [97].The development of virus-free plants through in vitro culture techniques has been established as a viable method for eliminating viruses.These techniques include micrografting, thermotherapy, chemotherapy, shoot-tip cryotherapy, and meristem culture [105,108].A thorough understanding of the genetic diversity of the yam mosaic virus (YMV) is essential for developing diagnostic tools to enhance and improve yam cultivation.YMV can persist from one season to the next and from generation to generation, so disease-free planting material is the best strategy to prevent its spread.Techniques such as tissue culture and mini tuber production in a disease-free environment are also effective control strategies for YMV.Cryotherapy of shoot tips has been shown to aid in producing YMV-free Dioscorea opposita plants [104,109].Cryotherapy of axillary buds resulted in approximately 76.33% plantlet regeneration and completely eradicated yam mosaic virus [110].Advanced detection techniques, such as TAS-ELISA, ICRT-PCR, CT-RT-LAMP, and RT-RPA, have been developed to identify YMV [30].Quantitative PCR (qPCR) is infrequently employed for the routine diagnosis of yam viruses despite being reported as more sensitive than conventional RT-PCR in detecting YMV [110].However, it is crucial to establish sensitive and robust detection methods that target small viral genes and those characterized by low abundance to effectively identify viral sequences [111].Sensitive diagnostic tools are essential for detecting the full range of yam mild mosaic virus (YMMV) diversity globally, helping to prevent the exchange of YMMV isolates and avoiding potential recombination that could lead to new virulent strains.Using disease-free planting material is crucial to prevent the spread and mitigate the impact of YMMV.Hot water treatment at 32 • C and 36 • C has been reported to eliminate YMMV efficiently at around 90% [112], and water-dissolved ozone application has also successfully sanitized potyviruses of around 63% [104,113].The high genetic variability of badnavirus complicates the development of diagnostic tools for their detection [114].The discovery of endogenous DBV in the yam genome adds further complexity to detecting yam badnaviruses [115].The subsiding costs of next-generation sequencing (NGS) hint that it may soon become the preferred method for screening promising yam breeding lines or landraces for viral and eDBV content before 'seed yam' multiplication, as this has already proven to be adequate for characterizing unknown viruses [116].Until then, the approach of ELISA with existing antisera as well as PCR used by [114] could be effective against DBV.Nonetheless, control strategies similar to those used for YMV and YMMV may also be effective against DBV.Xanthomonas campestris [117] and Bacillus pumilus [118] are the reported causative agents of bacterial leaf spot disease in yams.Xanthomonas exhibits a broad host range, affecting over 60 genera in monocotyledonous and 160 genera in dicotyledonous families, encompassing 9 monocotyledonous and 49 dicotyledonous families, respectively [119].Bacterial infection of plant foliage, fruit, and stems necessitates the presence of openings, such as lesions, to facilitate entry.Upon reaching the target site within the plant cell, Xanthomonas induces tissue softening and blockade of xylem vessels [120].In the initial stages, characteristic angular choleretic lesions emerge from the leaf margins, progressing to wilting and necrosis as the disease advances [121].The optimal conditions for the rapid growth of Xanthomonas involve cool and wet environments, and the pathogen's distribution is notably hindered if these conditions are not met [117].Bacillus pumilus infection initially establishes as brownish, water-soaked lesions on the leaves.As the infection advances, these lesions turn black and develop yellow halos.In severe cases, the diseased leaves wilt and eventually abscise [118].

Bacterial Soft Rot
Bacterial soft rot caused by Pantoea agglomerans was first reported in 2022 by [122] on Chinese yam (Dioscorea opposita).Pantoea agglomerans is a Gram-negative aerobic bacillus that occurs in plants as a mutualist.This bacteria is a causal agent of multiple cultivated crops such as rice, cotton, maize, sorghum, and an ornamental plant named Chinese Taro [123].Water-soaked lesions are the primary and typical symptoms of bacterial soft rot, which can be frequently seen on the infected tubers.Bacterial colonies are rod-shaped with peritrichous flagella.

Prevention, and Control Strategies
Infections caused by Xanthomonas are highly destructive and difficult to manage due to various virulence factors.Once the bacterium is introduced, its flagellum can quickly infiltrate the plant's vascular system [124].Utilizing different forms of copper, zinc, and magnesium can mitigate the severity of the disease [125].A mixture of streptocycline and copper oxychloride, at concentrations of 500 ppm and 2000 ppm, effectively controls the bacterial leaf spot caused by Xanthomonas campestris.Alternatively, Moringa oleifera leaf extracts offer a natural solution due to the adverse effects of chemical treatments.Moringa oleifera, a widely utilized tropical plant, is particularly noted for its antibacterial properties and bioactive compounds that combat Xanthomonas [126].Leaf extracts of Eucalyptus officinalis, Azadirachta indica, Cymbopogon citratus, and Ocimum gratissimum showed good potential for controlling bacterial leaf spot caused by Xanthomonas campestris.Additionally, copper-based nanoparticles have shown potential in controlling bacterial leaf spots in yams, further highlighting the role of innovative materials in plant disease management.Oxolinic acid showed better control efficacy than the other nine commercial pesticides [118].

Research Gaps and Limitations
Due to the limited research devoted to it, yam, categorized as an "orphan" crop, requires the development of enhanced technologies.This is particularly important in the context of climate change and the increasing demand for yams as a food source [127].The research gap in the field of yam diseases highlights a lack of comprehensive understanding and detailed exploration of various aspects of these diseases.This includes an insufficient investigation of specific pathogens affecting yam crops, a partial grasp of disease transmission mechanisms, an incomplete understanding of the genetic factors that confer disease resistance in yam varieties, and a limited exploration of effective disease management strategies specifically tailored for yam cultivation [128].Moreover, constraints in existing research on yam diseases stem from several factors.These include limited access to adequate funding and resources necessary for in-depth studies, challenges associated with conducting field research due to the complex nature of yam cultivation and disease dynamics, limitations in the research methodologies and technologies available for studying yam diseases, and difficulties in obtaining accurate and timely data on disease incidence and prevalence in yam-growing regions.Addressing these research gaps and limitations is essential for advancing the field of yam diseases.Doing so will contribute to the development of effective disease management strategies and ensure the sustainable production of yam crops, thereby supporting food security and enhancing livelihoods in yam-growing areas.

Future Prospective
The future of studying yam diseases should target several critical areas to bridge existing research gaps and foster sustainable yam production.These areas include the following: Comprehensive Disease Surveillance: Establishing robust disease surveillance systems is crucial for monitoring yam disease incidence, prevalence, and distribution across different regions.This effort can leverage advanced technologies such as remote sensing, molecular diagnostics, and participatory approaches that engage farmers and local communities.
Pathogen Characterization and Diversity: Detailed molecular studies are needed to characterize yam pathogens.This includes exploring their genetic diversity, population dynamics, and interactions with host plants.Such research can enhance our understanding of disease epidemiology and evolution, which is vital for developing targeted disease management strategies.
Host-Pathogen Interactions: Investigating the genetic foundations of disease resistance in yam varieties is essential.Studies could include genomic, transcriptomic, and functional genomics approaches to pinpoint genes linked to resistance and susceptibility.This research will deepen our understanding of the complex interactions between yams and their pathogens.
Capacity Building and Extension Services: It is imperative to strengthen research capacity and enhance extension services to effectively transfer knowledge and technologies for managing yam diseases to farmers and agricultural stakeholders.Initiatives might include training programs, farmer field schools, and the creation of extension materials adapted to specific local needs.
By focusing on these strategic areas, researchers can help develop sustainable and resilient yam production systems that minimize the impact of diseases and support the livelihoods of millions who rely on yam cultivation for food and income.

Conclusions
Yams are crucial to global food and nutritional security, especially considering future needs.However, the yield and quality of yam crops are severely limited by a number of biotic and abiotic threats, including insect pests, fungi, bacteria, viruses, and nematodes.Researchers employ host resistance, agrochemicals, and biological agents to combat these threats, with integrated disease management offering a comprehensive approach (Figure 4).Currently, yam disease management heavily relies on plant protection products, which are not sustainable in the long-term due to environmental concerns.Additionally, emerging challenges such as climate change, the introduction of invasive species, and the emergence of new species or strains of pests and pathogens are exacerbating existing management issues.While there have been advancements in yam disease management, there is a pressing need to shift towards more sustainable practices that can be maintained and improved upon by future generations.Given yams' critical role in ensuring global food security, it is imperative to rethink and innovate disease management strategies to enhance the sustainability of yam cultivation.This approach will not only help in overcoming current limitations but also in securing a resilient food future.
security, it is imperative to rethink and innovate disease management strategies to enhance the sustainability of yam cultivation.This approach will not only help in overcoming current limitations but also in securing a resilient food future.

Data Availability Statement:
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Figure 2 .
Figure 2. Fungal disease symptoms.(A) Yam anthracnose, small round or oval dark black or brown spots.(B) Yam wilt, vascular discoloration resulting in wilting, rot, and plant death.(C) Leaf spot, chlorosis lesions with dark brown spots.(D) Tuber rot, brown-colored dry rot in the infected tissues.

Figure 2 .
Figure 2. Fungal disease symptoms.(A) Yam anthracnose, small round or oval dark black or brown spots.(B) Yam wilt, vascular discoloration resulting in wilting, rot, and plant death.(C) Leaf spot, chlorosis lesions with dark brown spots.(D) Tuber rot, brown-colored dry rot in the infected tissues.

Figure 3 .
Figure 3. Crazy rot syndrome of yam.(A) whole yam tuber with numerous dark and scabby lesions, (B,C) cross-sectional views revealing internal rot and discoloration beneath the surface lesions.

Figure 3 .
Figure 3. Crazy rot syndrome of yam.(A) whole yam tuber with numerous dark and scabby lesions, (B,C) cross-sectional views revealing internal rot and discoloration beneath the surface lesions.

Figure 4 .
Figure 4. Integrated yam disease control strategies.Strategies to combat yam diseases through a combination of cultural, breeding, biological, and chemical methods.(Created with BioRender.com).Author Contributions: Conceptualization, D.D. and D.S.; writing-original draft preparation, H.T., C.X., L.W., H.G. and G.W.; writing-review and editing, D.S.; visualization, H.T.; funding acquisition, D.D. All authors have read and agreed to the published version of the manuscript.

Funding:
This research was funded by grants from China Agriculture Research System (CARS-21).Data Availability Statement:The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Figure 4 .
Figure 4. Integrated yam disease control strategies.Strategies to combat yam diseases through a combination of cultural, breeding, biological, and chemical methods.(Created with BioRender.com).

Funding:
This research was funded by grants from China Agriculture Research System (CARS-21).