An Improved Phenotyping Method for Evaluation of Yam (Dioscorea spp.) Resistance to Nematodes Belonging to the Genera Meloidogyne and Scutellonema.

Phenotyping yam (Dioscorea spp.) germplasm for resistance to parasitic nematodes is hampered by the lack of an efficient screening method. In this study, we developed a new method using rooted yam vine cuttings and yam plantlets generated from semi-autotrophic hydroponics (SAHs) propagation for phenotyping yam genotypes for nematode resistance. The method was evaluated using 26 genotypes of D. rotundata for their reaction to Scutellonema bradys and four root-knot nematode species, Meloidogyne arenaria, M. enterolobii, M. incognita, and M. javanica. Yam plantlets established in nursery bags filled with steam-sterilized soil were used for screening against single nematode species. Plants were inoculated four weeks after planting and assessed for nematode damage eight weeks later. A severity rating scale was used to classify genotypes as resistant, tolerant, or susceptible determine based on the nematode feeding damage on tubers and the rate of nematode multiplication in the roots of inoculated plants. The results demonstrated putative resistance and tolerance against S. bradys in 58% of the genotypes and 88%, 65%, 65%, and 58% against M. arenaria, M. javanica, M. incognita, and M. enterolobii, respectively. The method is rapid, flexible, and seasonally independent, permitting year-round screening under controlled conditions. This method increases the throughput and speed of phenotyping and improves the selection process.

Planting Material of Enset (Ensete ventricosum), a Key Food Security Crop in Southwest Ethiopia, Is a Key Element in the Dissemination of Plant-Parasitic Nematode Infection.

Enset (Ensete ventricosum), is a perennial herbaceous plant belonging to the family Musaceae, along with banana and plantain. Despite wild populations occurring in eastern, central and southern Africa, it is only in Ethiopia that the crop has been domesticated, where it is culturally and agriculturally symbolic as a food security crop. Although an under-researched orphan crop, enset serves as a staple food for about 20% of the Ethiopian population, comprising more than 20 million people, demonstrating its value in the country. Similar to banana and plantain, enset is heavily affected by plant-parasitic nematodes, with recent studies indicating record levels of infection by the root lesion nematode Pratylenchus goodeyi. Enset is propagated vegetatively using suckers that are purposely initiated from the mother corm. However, while banana and plantain suckers have proven to be a key source of nematode infection and spread, knowledge on the infection levels and role of enset suckers in nematode dissemination is lacking. Given the high levels of plant-parasitic nematodes reported in previous surveys, it is therefore speculated that planting material may act as a key source of nematode dissemination. To address this lack of information, we assessed enset planting material in four key enset growing zones in Ethiopia. A total of 340 enset sucker samples were collected from farmers and markets and analyzed for the presence of nematodes. Nematodes were extracted using a modified Baermann method over a period of 48 h. The root lesion nematode P. goodeyi was present in 100% of the samples, at various levels of infection. These conclusive results show that planting material is indeed a key source of nematode infection in enset, hence measures taken to ensure clean suckers for planting will certainly mitigate nematode infection and spread. The effect of nematode infection on yield and quality on enset remains to be investigated and would be a way forward to complement the nematode/disease studies conducted so far and add valuable knowledge to the current poorly known impact of pests and diseases.

Global Cropland Connectivity: A Risk Factor for Invasion and Saturation by Emerging Pathogens and Pests.

The geographic pattern of cropland is an important risk factor for invasion and saturation by crop-specific pathogens and arthropods. Understanding cropland networks supports smart pest sampling and mitigation strategies. We evaluate global networks of cropland connectivity for key vegetatively propagated crops (banana and plantain, cassava, potato, sweet potato, and yam) important for food security in the tropics. For each crop, potential movement between geographic location pairs was evaluated using a gravity model, with associated uncertainty quantification. The highly linked hub and bridge locations in cropland connectivity risk maps are likely priorities for surveillance and management, and for tracing intraregion movement of pathogens and pests. Important locations are identified beyond those locations that simply have high crop density. Cropland connectivity risk maps provide a new risk component for integration with other factors-such as climatic suitability, genetic resistance, and global trade routes-to inform pest risk assessment and mitigation.

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.

Management of Cyst and Root Knot Nematodes: A Chemical Ecology Perspective.

Plant parasitic nematode infection of crops can be highly detrimental to agricultural production. Since the discovery that plant roots release chemicals that attract the infective stage of plant parasitic nematodes some 80 years ago, significant progress in identifying the signaling molecules has occurred only relatively recently. Here, we review the literature on chemical ecological studies of two major plant parasitic nematode groups: root knot nematodes in the genus Meloidogyne and cyst nematodes in the genus Globodera because of the negative impact their parasitism has on farming systems in Africa. We then highlight perspectives for future directions for their management.

Plant-Parasitic Nematodes and Food Security in Sub-Saharan Africa.

Sub-Saharan Africa (SSA) is a region beset with challenges, not least its ability to feed itself. Low agricultural productivity, exploding populations, and escalating urbanization have led to declining per capita food availability. In order to reverse this trend, crop production systems must intensify, which brings with it an elevated threat from pests and diseases, including plant-parasitic nematodes. A holistic systems approach to pest management recognizes disciplinary integration. However, a critical under-representation of nematology expertise is a pivotal shortcoming, especially given the magnitude of the threat nematodes pose under more intensified systems. With more volatile climates, efficient use of water by healthy root systems is especially crucial. Within SSA, smallholder farming systems dominate the agricultural landscape, where a limited understanding of nematode problems prevails. This review provides a synopsis of current nematode challenges facing SSA and presents the opportunities to overcome current shortcomings, including a means to increase nematology capacity.

Diversity of Root-knot Nematodes Associated with Tubers of Yam (Dioscorea spp.) Established Using Isozyme Analysis and Mitochondrial DNA-based Identification.

The root-knot nematodes (RKN), Meloidogyne spp., represent an important threat to yam (Dioscorea spp.) production in West Africa. With the aim to establish the diversity of RKN species affecting yam tubers, for control and resistance screening purposes, surveys were conducted in the main yam producing areas of Nigeria. Galled tubers (N = 48) were collected from farmers' stores and markets in nine states in Nigeria and in one district in Ghana. RKN isolated from yam tubers were identified using enzyme phenotyping (esterase and malate dehydrogenase) and mitochondrial DNA (mtDNA) NADH dehydrogenase subunit 5 (Nad5) barcoding. Examination of 48 populations revealed that yam tubers were infested by Meloidogyne incognita (69%), followed by M. javanica (13%), M. enterolobii (2%), and M. arenaria (2%). Most of the tubers sampled (86%) were infected by a single species, and multiple species of RKN were detected in 14% of the samples. Results of both identification methods revealed the same species, confirming their accuracy for the identification of these tropical RKN species. In addition to M. incognita, M. javanica, and M. enterolobii, we report for the first time M. arenaria infecting yam tubers in Nigeria. This finding extends the list of yam pests and calls for caution when developing practices for yam pest management.

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

The 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.

Agricultural nematology in East and Southern Africa: problems, management strategies and stakeholder linkages.

By 2050, Africa's population is projected to exceed 2 billion. Africa will have to increase food production more than 50% in the coming 50 years to meet the nutritional requirements of its growing population. Nowhere is the need to increase agricultural productivity more pertinent than in much of Sub-Saharan Africa, where it is currently static or declining. Optimal pest management will be essential, because intensification of any system creates heightened selection pressures for pests. Plant-parasitic nematodes and their damage potential are intertwined with intensified systems and can be an indicator of unsustainable practices. As soil pests, nematodes are commonly overlooked or misdiagnosed, particularly where appropriate expertise and knowledge transfer systems are meager or inadequately funded. Nematode damage to roots results in less efficient root systems that are less able to access nutrients and water, which can produce symptoms typical of water or nutrient deficiency, leading to misdiagnosis of the underlying cause. Damage in subsistence agriculture is exacerbated by growing crops on degraded soils and in areas of low water retention where strong root growth is vital. This review focuses on the current knowledge of economically important nematode pests affecting key crops, nematode control methods and the research and development needs for sustainable management, stakeholder involvement and capacity building in the context of crop security in East and Southern Africa, especially Kenya, Tanzania, Uganda and Zimbabwe.