Yam Nematode Research Articles

Plant-biomass-based hybrid seed wraps mitigate yield and post-harvest losses among smallholder farmers in sub-Saharan Africa

Sustainable practices that reduce food loss are essential for enhancing global food security. We report a ‘wrap and plant’ seed treatment platform to protect crops from soil-borne pathogens. Developed from the abundantly available wastes of banana harvest and recycled old, corrugated cardboard boxes via chemical-free pulping, these paper-like biodegradable seed wraps exhibit tunable integrity and bioavailability of loaded moieties. These wraps were used for nematode control on yam (Dioscorea cayenensis-rotundata) seed pieces in Benin, a major producer of this staple crop in the sub-Saharan African ‘yam belt’. Our seed wraps loaded with ultra-low-volume abamectin (1/100 ≤ commercial formulation) consistently controlled yam nematode (Scutellonema bradys) populations while considerably increasing the yield at various locations over 2015–2018. Substantial reduction in post-harvest tuber weight loss and cracking was observed after 3 and 5 months of storage, contributing to increased value, nutrition and stakeholders’ preference for the wrap and plant treatment.

Fluctuation in Yam Nematodes Depending on the Phenological Stages of the Main Yam Species (Dioscorea alata L.) Cultivated in Côte d’Ivoire

The telluric factors favorable to nematode pathogenic diversity make yam nematode control ineffective. This work aims at studying the fluctuation in yam nematodes depending on yam phenological stages. Trials were implemented in four yam production areas in Côte d'Ivoire. After inventory of symptoms on yam tubers, the nematodes associated with the symptoms were extracted and identified. The correlation coefficients between the severity of symptoms on tubers and the size of the associated nematode populations were determined. The size of nematode populations associated with the symptoms were determined in 100 g of soil and 100 g of yam peel were determined depending on the phenological stages of yam plants. Galls, cracks, dry and wet rot were observed on harvested yam tubers. Globodera, Meloidogyne spp., Pratylenchus coffeae and Xiphinema were the nematodes associated with the symptoms. Pratylenchus coffeae was strongly involved in the development of cracks (r = 0.75) and dry rot (r = 0.86) then Meloidogyne spp. in that of galls (r = 0.78). Pratylenchus coffeae and Meloidogyne spp. fluctuation in cultivation soils and yam tubers is influenced by yam phenological stages. Their numbers increase in soils and tubers before tuberization initiation. Producers could draw on the results of this study to establish a schedule of nematicide treatments that could start as soon as yam seeds are planted.

Morphological and molecular characterization of Pratylenchus species from Yam (Dioscorea spp.) in West Africa.

The root-lesion nematodes (RLN), Pratylenchus spp., are among the major plant-parasitic nematodes affecting yam (Dioscorea spp.) production in West Africa. The distribution and diversity of RLN species associated with yam was investigated through a soil and tuber survey of the main producing areas in Nigeria and Ghana. Pratylenchus spp. were detected in the yam rhizosphere in 59% of 81 soil samples from Ghana and 39% of 114 soil samples from Nigeria. Pratylenchus spp. were detected in 24 of 400 tubers examined, in combination with root-knot nematodes (Meloidogyne spp.) and their associated damage of galls and crazy roots (79%), and with yam nematode (Scutellonema bradys) and their associated damage of dry rot (17%), although no specific additional symptoms were observed for Pratylenchus spp. Species of Pratylenchus were identified by their morphological features and by sequences of the D2-D3 region of the 28 S rDNA gene and the mitochondrial cytochrome oxidase I gene (COI). Pratylenchus brachyurus was the most frequent RLN species in both the rhizosphere and tubers of yam. Pratylenchus hexincisus was recovered from one tuber collected in Nigeria. While further investigations are required to establish the host status of yam for this nematode, this appears to be the first record of P. hexincisus on yam. The present taxonomical status of P. scribneri and P. hexincisus is discussed.

Morphometric characterisation of Scutellonema bradys populations from the Guinea Savanna and Transitional Rain Forest ecological zones of Ghana

Yam (Dioscorea spp.) forms a large portion of Ghanaian diets and is cultivated in most regions. However, yam is highly susceptible to parasitism by the yam nematode, Scutellonema bradys. Ten major yam communities from 10 districts (Yendi, Karaga, East Gonja, Kumbungu, Savelugu/Nanton, Kintampo North, Techiman, Wenchi, Atebubu/Amanten and Ejura/Sekyeredumase) were selected for soil and yam tuber sampling. Body lengths ranged from 624.6 to 858.5 µm, and 609.5 to 854.1 µm for female and male S. bradys populations, respectively. Tail lengths ranged from 5.3 to 9.6 µm, and 5.2 to 8.6 µm for female and male S. bradys populations, respectively. Principal component analysis did not reveal distinct groups among the female and male populations. This was further confirmed through agglomerative hierarchical clustering with subgroupings showing less than 20% dissimilarity for both female and male populations, and nematodes from different locations clustered together. Significant morphological variation therefore exists both within and among female and male S. bradys populations as reflected in the formation of subgroups.

Morphological and molecular characterisation of Scutellonema species from yam (Dioscorea spp.) and a key to the species of the genus.

SummaryThe yam nematode, Scutellonema bradys, is a major threat to yam (Dioscorea spp.) production across yam-growing regions. In West Africa, this species cohabits with many morphologically similar congeners and, consequently, its accurate diagnosis is essential for control and for monitoring its movement. In the present study, 46 Scutellonema populations collected from yam rhizosphere and yam tubers in different agro-ecological zones in Ghana and Nigeria were characterised by their morphological features and by sequencing of the D2-D3 region of the 28S rDNA gene and the mitochondrial COI genes. Molecular phylogeny, molecular species delimitation and morphology revealed S. bradys, S. cavenessi, S. clathricaudatum and three undescribed species from yam rhizosphere. Only S. bradys was identified from yam tuber tissue, however. For barcoding and identifying Scutellonema spp., the most suitable marker used was the COI gene. Additionally, 99 new Scutellonema sequences were generated using populations obtained also from banana, carrot, maize and tomato, including the first for S. paralabiatum and S. clathricaudatum, enabling the development of a dichotomous key for identification of Scutellonema spp. The implications of these results are discussed.

English

Using two commercially available arbuscular mycorrhizal fungal (AMF) products, one based on Funneliformis mosseae and the other on Glomus dussii, an experiment was conducted to assess their effect on yam growth and ability to suppress nematode damage in pots. Four yam cultivars (cvs) were used: two Dioscorea alata cvs (TDa98-01183 and TDa98-165), and two Dioscorea rotundata cvs (TDr97-00551 and TDr 745). Micropropagated yam plantlets were inoculated either with F. mosseae or with G. dussii at the stage of transplanting into 2L pots and - one month later - with 500 vermiform Scutellonema bradys. The plantlets were grown for further six months in the greenhouse at IITA-Ibadan. The results showed that the presence of AMF tended to lead to improved growth of yam, especially D. alata cvs, as compared with the non-arbuscular mycorrhizal control plants. When challenged with the yam nematode S. bradys, plantlets of the two D. alata cultivars pre-inoculated with F. mosseae and cv TDr97-00551 pre-inoculated with G. dussii yielded significantly higher tuber weights compared to non-AMF control plantlets. S. bradys densities on yam plantlets pre-inoculated with AMF were generally suppressed, although no differences were observed in visible damage scores, which remained low or absent across treatments.     Key words: Bio control,  Scutellonema bradys, suppression, yam growth.

Tropical cover crops for the management of the yam nematode, Scutellonema bradys

ABSTRACTYam tuber dry rot caused by Scutellonema bradys results in the reduction of tuber quality. Studies were conducted on the effect of intercropping cover crops with yams on nematode damage to tubers. Eleven cover crops were intercropped with yams in pot screen house and field experiments. Plants were inoculated with S. bradys in water suspension in pots and by incorporating infected yam peels into soil around plant roots in field experiments with un-inoculated plants serving as control. The experiments were factorial with six replications per treatment in pot and four in field experiments. In pot experiments, populations of S. bradys recovered from tubers and nematode damage were lower in yam tubers intercropped with Tagetes erecta, Aeschynomene histrix, Stylosanthes guianensis, Mucuna pruriens and Pueraria phaseoloides compared to where no cover crops were intercropped with yams. Conversely, S. bradys populations from pots with Crotalaria ochroleuca, Crotalaria juncea, Centrosema pubescens, Cajanus cajan and Lablab purpureus were similar to Vigna unguiculata, a known host of S. bradys. The use of A. histrix, T. erecta and S. guianensis, M. pruriens and P. phaseoloides offers promise for inclusion in yam-based cropping systems as a measure for S. bradys management in yam production.

Molecular and morphological characterisation of Scutellonema bradys from yam in Costa Rica and development of specific primers for its detection

The yam nematode, Scutellonema bradys, which can cause dry rot disease of yam (Dioscorea spp.), was recorded for the first time from Costa Rica in four species of yam occurring in the Atlantic and north regions. Morphometric measurements from two populations from each region using ten female and 11 male characters corresponded with previous descriptions of this species. Canonical discriminant analysis of the female morphometric data separated the populations by region, whereas no separation by region was evident using the male data. Analysis of DNA sequences from the ITS region indicated that populations from Costa Rica were monophyletic with S. bradys from West Africa and clearly distinct from other Scutellonema species. No genetic separation by geographic region or Dioscorea species host was observed between Costa Rica populations. Species-specific primers were developed from the ITS region and supported the identity of 17 populations from 15 locations in Costa Rica as S. bradys: 14 populations from D. alata (greater or water yam) and one each from D. trifida (white yampee), D. cayenensis (yellow yam) and D. rotundata (white yam). Yam production in Costa Rica began in the Atlantic region, where the yam nematode was likely introduced from the Caribbean, progressively spreading to other locations through the use of infected vegetative planting material.

Infectivity and reproduction of Scutellonema bradys on weeds and cultivated plant species

The nematode Scutellonema bradys is the main phytosanitary problem of yam (Dioscorea rotundata) in the Brazilian State of Bahia. Little is known about the host range of this nematode in Brazil, especially on weeds and plant species cultivated together with yam. This study aimed at evaluating different plant species to determine their host status to S. bradys and to study the relative infectivity of different stages of the nematode on selected host plants. Among the 48 evaluated plant species, 12 were infected by S. bradys. Yam was considered to be the only good host of the nematode, while Luffa algyptiaca, Momordica charantia, Heliotropium indicum, Vigna unguiculata, Cucurbita pepo, Abelmoschus esculentus, Sicana odorifera, Solanum lycopersicum and Ipomoea batatas were classified as bad hosts because of the relatively low reproduction factor as compared to yam. Crotalaria juncea and Cajanus cajan behaved as trap plants because large numbers of S. bradys penetrated their roots but populations decreased over time. Our results showed that juveniles are more infective than females and these are more infective than males on roots of five host plants. We also demonstrated that S. bradys prefers certain host plants as inferred by their lower reproduction on roots of these hosts. Penetration of juveniles tended to be spread over 16 days after inoculation, whereas for adults it was more concentrated at 8 days after inoculation. These results may be important to plan crop rotation schemes to control the yam nematode.

The yam nematode (Scutellonema bradys), a potential threat to potato (Solanum tuberosum) production in West Africa

Potato (Solanum tuberosum) production in Africa is rapidly expanding and becoming increasingly important. As its geographical production range broadens, so does its potential to host new pests and diseases. Following the discovery that potato can be affected by Scutellonema bradys, further studies were undertaken to assess its potential pathogenicity on potato under screenhouse and field conditions, and on marketed tubers. Potato plants inoculated with S. bradys produced tubers with substantial cracking and evident tuber rot, compared with tubers from uninoculated plants. Symptoms of nematode infection on tubers included a scaly appearance, surface cracking as well as deeper tissue cracks, distortions, and darkened surface patches. In most cases these patches were related to sub-surface rot. Nematodes were recovered from the soil, roots and tubers of inoculated plants. Eight weeks after inoculation, the reproduction factor of the nematode was greatest (2·0) at the lowest inoculation rate assessed (1000 nematodes per 2·5-L pot) and least (0·4) at the highest inoculation rate (5000 nematodes per pot). In the screenhouse, potato tuber weights were low and mostly unaffected by nematode inoculation rate, except at 5000 nematodes per pot. In the field, non-inoculated plants yielded over nine times more tubers than plants inoculated with 2000 S. bradys. Low densities of S. bradys were also recovered from 10 of 15 (67%) samples collected from market stalls, indicating field infection. This study confirms that potato can host and be damaged by S. bradys, raising its prospect as a likely significant biotic constraint to the crop.

Scutellonema bradys, the yam nematode, newly reported affecting Irish potato (Solanum tuberosum) in Nigeria

Scutellonema bradys, the yam nematode, was recovered from potato tubers (Solanum tuberosum) originating from the potato growing area of Jos, Nigeria, representing the first record of this nematode damaging potato. Infested tubers showed cracking, similar to the symptoms S. bradys causes on yam (Dioscorea spp.). Although some Scutellonema spp. have been recovered from potato, such as S. brachyurum (Schuerger & McClure, 1983), S. paralabiatum (Coyne et al., 2003) and S. unum (Njuguna & Bridge, 1998), tuber damage has not been reported. Scutellonema bradys is recorded mostly from West Africa and responsible for significant yam tuber losses, especially during storage, but also from the Americas and Asia (Bridge et al., 2005). To confirm the effects of S. bradys, potato tubers sourced locally in Ibadan, Nigeria were transplanted from sawdust sprouting beds into sterile soil-filled pots. Two weeks later half of the pots were inoculated with 2000 S. bradys nematodes (recovered from yam). Plants were harvested at onset of haulm senescence. Plants inoculated with S. bradys produced tubers with substantial cracking of the cortex and evident tuber rot, unlike plants that had not been inoculated. Symptoms included scaly appearance, surface cracking, deep tissue cracks and distortions, and sub-surface rot, which tend to differ from external symptoms caused by Ditylenchus destructor and D. dipsaci (and the knobbly and pimply appearance resulting from Meloidogyne spp. infestations). Scutellonema bradys was recovered only from soil, roots (8 per g root) and tubers (14 per g peel) in inoculated pots. Morphological identification confirmed mature female and male nematodes as S. bradys. This report establishes that Irish potato is prone to damaging infestation by S. bradys. With increasing movement and exportation of yam tubers, there is increased likelihood of a spread of S. bradys. For example, S. bradys has been intercepted on yam imported to South Africa (Esther van den Berg, ARC, Pretoria, Republic of South Africa, personal communication). This note therefore reinforces the importance for both phytosanitary staff and farmers to ensure correct diagnosis of damage to tubers.

Management of crotalaria and pigeon pea for control of yam nematode diseases

Management of plant-parasitic nematodes with the use of nematicides has not been recommended for small farmers that grow yam in the Northeastern region of Brazil, due to its high cost and residue toxicity. The use of plants with antagonistic effect to nematodes and green manure which improves soil chemical, physical and biological characteristics can be a viable and low cost alternative to control parasitic nematodes. This work aimed to evaluate the effect of crotalaria (Crotalaria juncea) and pigeon pea (Cajanus cajan) plants on the control of yam nematodes. Three experiments were carried out. The first was conducted under in vitro conditions to evaluate the nematostatic and nematicide effect of extracts from fresh and dry matter of the above ground parts of crotalaria, pigeon pea, and the combination of both. The second experiment was carried out under greenhouse conditions to evaluate the effect of soil amendment with crotalaria, pigeon pea, and the combination of both in the infectivity of Scutellonema bradys, using tomato plants as the host plant. The third experiment was conducted under field conditions to evaluate the effect of crotalaria, pigeon pea, and the combination of both, cultivated between yam planting rows and incorporated to soil surface, on yam nematodes. The aqueous extract obtained form fresh matter of crotalaria had a nematicide effect of 100% for S. bradys. Extracts from dry matter of both crotalaria and pigeon pea did not have any nematicide effect, but had a nematostatic effect. Incorporation of crotalaria to soil inhibited infectivity of S. bradys in tomato seedlings. These results showed that planting crotalaria alone or in combination with pigeon pea, between the yam planting rows, is an efficient method for controlling S. bradys and Rotylenchulus reniformis associated with yams. Crotalaria can be used for controlling these plant-parasitic nematodes in soil.

Predatory ability of Arthrobotrys musiformis and Monacrosporium thaumasium on Scutellonema bradys

Scutellonema bradys (Steiner & LeHew) Andrassy is the most important yam nematode in the State of Bahia, Brazil, being responsible for the decay of yam tubers, known as dry rot disease. Nematode-trapping fungi are potential biocontrol agents against plant parasitic nematodes. The in vitro predatory ability of Arthrobotrys musiformis Drechsler and Monacrosporium thaumasium Drechsler on S. bradys was evaluated. The fungi were grown in PDA medium, than transferred to the center of Petri dishes with 2% agar plus water. After 14 days of the fungal cultures incubation, 150 nematodes were added to the dishes. For a period of 5 days, at 24-hour intervals, the number of captured nematodes was counted. Both fungi formed trapping structures of single ring and three-dimensional adhesive network types, 24 hours after the addition of the nematodes to the fungal cultures. The percentage of nematodes captured by each fungus increased linearly with time, reaching 94.6% and 97.3% of captured nematodes by A. musiformis and M. thaumasium, respectively, at the fifth day of evaluation. Both fungi presented good predatory ability upon S. bradys. This is the first report of nematophagous fungi capturing S. bradys. Further studies should evaluate the potential of these fungi as biocontrol agents of S. bradys in yam plantations.

Effect of fertiliser application on yam nematode (Scutellonema bradys) multiplication and consequent damage to yam (Dioscorea spp.) under field and storage conditions in Benin

Studies were carried out at the International Institute of Tropical Agriculture (IITA-Cotonou Station, Benin) and at Save (central Benin), on the influence of chemical fertiliser application on yam nematode (Scutellonema bradys) densities and damage in the field and in storage to three yam (Dioscorea spp.) cultivars namely two D. rotundata cvs, TDr 131 and Ala, and one D. cayenensis cv, Kokoro. Treatments with nematodes (1000 per plant) and NPK, DAP (Diamonic Phosphate) and KCl at rates of 300, 155, and 60 kg/ha, respectively, were compared to un-inoculated and no-fertiliser control in field plots in terms of the damage to the crop and the multiplication of the nematode. At harvest, yield and the number of tubers per plant were not significantly affected by fertiliser application or nematode inoculation. However, the fertilisers suppressed nematode multiplication in tubers, especially DAP. In storage, tuber weight loss was recorded in all treatments. Weight loss was more pronounced in tubers from plots receiving fertiliser application than from those receiving none and when infected with S. bradys than when healthy, and mostly in tubers from infested plots which had received fertiliser. Nematode multiplication rates were greater in tubers during the first 3 months of storage, declining in the fourth and fifth months.

The Mechanism of Action of the Yam Nematode, Scutellonema Bradys

Chromatographic analysis of the incubation solution of S. bradys showed that five amino acids--aspartic acid, phenylalanine, hydroxyinol acetic acid, leucine and isoleucine were discharged by this nematode. Pectinase and amylase activities were detected in homogenates of S. bradys. The absence of the steroid group of compounds in the nematode-infected yam tubers revealed by spectrometric analysis might be disease-related as evidenced by its appearance in the healthy yam tubers.