All about nematodes
Review offers hope for breaking soilborne curse in Africa
Root-knot nematodes, a highly destructive group of soil dwelling worms that infect most, if not all cultivated crops, are probably one of the greatest threats to agriculture in SSA. Therefore, for the region’s food security goals to be achieved, these pests, alongside other nematode species, must be wholly integrated into efforts to improve agricultural productivity. Encouragingly, slow but steady progress made over the past 20 years indicates an optimistic outlook for an otherwise daunting endeavour.
These findings are presented in a review published recently by researchers from the International Institute of Tropical Agriculture (IITA), International Centre of Insect Physiology and Ecology (icipe), Kenya, Queen’s University Belfast, United Kingdom, University of Ibadan, Nigeria, Sugar Cane Research Institute, Tanzania, and Makerere University, Uganda.
The first of its kind in terms of scope, the study presents a thorough overview of nematode pests in SSA, locating this menace within the continent’s highly intricate agricultural landscape. As food requirements heighten in response to a rapidly rising population and escalating urbanisation, the need to intensify agricultural production in Africa is greater than ever. This scenario poses huge challenges in a region characterised by smallholder systems, diverse agroecologies, low agricultural productivity and increasing and adverse impacts of climate change. Moreover, intensified crop production in traditional sites as well as emerging peri-urban and urban systems leads to elevated pest and disease threats, and unattractive tradeoffs between food safety, health of people, animals and the environment, due to improper use of pesticides.
“Against this background, our review aims to provide knowledge to support comprehensive incorporation of nematode management as a critical component for sustainable intensification of agriculture in Africa,” notes Dr Danny Coyne, soil health and nematology scientist, IITA. “We highlight aspects such as the nematode diversity, impact, obstacles and the improving progress in this field to tackle them effectively.”
Complexity, damage and challenges
“This report highlights the complex, highly diverse community of nematodes in SSA, which makes it difficult to assess the importance or disease transmission potential of individual species,” says Dr Solveig Haukeland, nematologist, icipe. “The two most important groups are root-knot nematodes, known in short as RKNs, and lesion nematodes. In particular, RKNs, a pervasive group that is widely distributed across SSA, are hugely destructive and are considered among the world’s most damaging crop pathogens; a malignant soilborne curse, persistently undermining crop production.”
The study further provides an insight into the magnitude and complexity of the damage caused by nematode pests, including how infected plant roots prevent water and nutrient uptake. In addition, nematode infection leads to higher levels of root and tuber rot as a consequence of wounds caused by the pests feeding on plant roots. Nematodes have the ability to infect, feed on, and reproduce on an astonishing range of crops and plant species. These pests are also difficult to identify and because they have high reproductive and short generation times, they spread rapidly. Moreover, climate change will alter the geographical spread of nematode pests and lead to the emergence of new nematode problems.
Importantly, the significance of nematode pests in SSA is often obscured by their non-specific, cryptic disease symptoms, resulting in misdiagnosis or attribution to other causes or secondary infections. Further, across the continent, diagnostic capabilities for nematodes are limited, although the situation is gradually changing as expertise develops and more reliable scientific techniques become available. Nematology as a discipline is neglected, restricting the development of skilled capacity.
Farmers and agricultural staff typically have an inadequate understanding of nematodes or the expertise to manage them. Deficiencies in the availability of resistant varieties, use of quality agricultural inputs and improved farming techniques are other contributing factors to the spread of nematodes. In addition, the often unregulated nature of seed supply systems, as well as the informal exchange of infected planting material facilitates the spread of nematodes.
Nematode damage on crops
The researchers note that although no reliable estimates of economic losses due to nematodes in SSA are available, the impact of these pests is undoubtedly significant. This point is illustrated through case studies of the impact of nematodes on a range of crops important on the continent, as follows:
- Banana (and plantain) is one crop that can be used to demonstrate the impact of nematodes on crop production in SSA. Unless tissue culture plants have been used, it is presumed that every banana plant is infected with nematode pests. Feeding by nematodes creates root necrosis and death. Damaged root systems result in weakened plant anchorage, poor bunch weight, inability to support stems and plant susceptability to weather elements.
- Yam is affected by both lesion nematodes and RKNs, with the former traditionally more significant. The lesion nematode, Scutellonema bradys, causes dry rot disease. However, there is growing evidence of increasing incidence of RKNs on yam and higher levels of damage. Recently, the aggressive RKN species Meloidogyne enterolobii, was recovered from yam for the first time in Nigeria.
- Maize lesion nematodes and RKNs, and numerous other nematode species may may occur simultaneously, causing significant damage to production. Cereals, including maize, are often mistakenly viewed as poor hosts of RKNs, perhaps because of the lack of the typical galling symptoms that Meloidogyne spp. cause..
- Rice: The cyst nematode, Heterodera sacchari, has been recorded on upland rice in West Africa, causing serious losses and, potentially, represents a major constraint for regional rice production intensification.
- Cassava, probably best known for its ability to withstand most afflictions, is generally viewed as being unaffected by nematodes. However, the naturally knobbly roots conceal RKN galling damage, and roots infected early in the season appear to deteriorate and die. The absence of infected roots at harvest disassociates nematodes from the significant yield loss that they cause.
- Potato: Numerous nematode species and genera are reported from potato. In SSA, RKNs and lesion nematodes appear to be the most important. The recent discovery of the potato cyst nematode in Kenya illustrates the serious threat of nematodes to this crop.
Recommendations
To hasten progress in managing the nematode curse in SSA, it is necessary to create greater awareness of the pests and the damage they cause, across society.
"For example, adoption of innovations, such as traditional varieties that are genetically transformed to withstand nematode infection, would provide a formidable weapon in the battle against these pests. Indeed, formulating more conducive regulations for transgenic crops in SSA countries can become a real game changer in the fight against nematodes,” notes Dr Johnathan Dalzell, Lecturer, Queen’s University Belfast, School of Biological Sciences, Medical Biology Centre.
Regarding the capacity building and socio-economic context that frames soil health research in Africa and in particular on the nematology discipline, Dr Laura Cortada, soil health and nematology researcher, IITA concludes: “Although there has been considerable progress since 1985, there is great need to amplify and hasten efforts to deal with the threat of nematode pests. Based on the authors’ personal experiences in agriculture and capacity building in the region they make six key recommendations.”
- Creation of public awareness and understanding across the agricultural landscape, including farmers, extension services, policymaking bodies, donors and development partners, academia and the scientific community.
- Establishment of networks or platforms towards building a critical mass of nematology expertise, while generating relevant activities and publicity. Good practice examples include: Nematology platform developed through icipe and IITA Nematology Initiative for Eastern and Southern Africa Nematological Society of Southern Africa; Nigerian Society of Nematologists; and Specialised Nematology Training by Ghent University, Belgium.
- Knowledge advances on nematode management needs to be fully integrated within agriculture, academia and policy.
- Adequate coordination, motivation, and financial support is required to institute necessary agricultural policies.
- Develop suitable, adoptable and relevant nematode management options, and make them available across SSA.
- Stimulate wider and greater multidisciplinary interaction in open access journals, and beyond specialised scientific literature.
Notes for Editors
Publication details: Coyne D.L., Cortada L., Dalzell J.J., Claudius-Cole A. O., Haukeland S., Luambano N., Herbert Talwana H. (2018) Plant-Parasitic Nematodes and Food Security in Sub-Saharan Africa. Annual Review of Phytopathology, 56, https:// doi.org/10.1146/annurev-phyto-080417-045833
Corresponding author: Danny Coyne – d.coyne@cgiar.org
Research collaborators: International Institute of Tropical Agriculture; Queen’s University Belfast, School of Biological Sciences, Medical Biology Centre, United Kingdom; Department of Crop Protection and Environmental Biology, University of Ibadan, Ibadan, Nigeria; International Centre for Insect Physiology and Ecology (icipe), Nairobi, Kenya; Sugar Cane Research Institute, Kibaha, Coast, Tanzania; and Department of Agricultural Production, School of Agricultural Sciences, Makerere University, Kampala, Uganda.
The International Institute of Tropical Agriculture (www.iita.org) is a not-for-profit institution that generates agricultural innovations to meet Africa’s most pressing challenges of hunger, malnutrition, poverty, and natural resource degradation. Working with various partners across sub-Saharan Africa, we improve livelihoods, enhance food and nutrition security, increase employment, and preserve natural resource integrity. IITA is a member of CGIAR, a global agriculture research partnership for a food secure future.
The International Centre of Insect Physiology and Ecology (www.icipe.org), headquartered in Nairobi, Kenya, is the only research institution in Africa working primarily on insects and other arthropods. icipe’s mission is to ensure better food security, health and livelihoods in Africa, by producing world-class knowledge and then developing solutions that are environmentally friendly, accessible, affordable and easy-to-use by communities. These objectives are delivered through four thematic areas — Human Health, Animal Health, Plant Health and Environmental Health. icipe gratefully acknowledges the financial support of the following core donors: Swiss Agency for Development and Cooperation (SDC); Swedish International Development Cooperation Agency (Sida); UK Aid, from the government of the United Kingdom; the Ministry of Higher Education, Science and Technology, Kenya; and the Government of the Federal Democratic Republic of Ethiopia. icipe’s research on nematodes includes: studies on the potato cyst nematode (PCN), conducted within the Food and Agriculture Organization of the United Nations (FAO) emergency assistance for the control of PCN project, focusing on the training of government technicians and determination of PCN status in soil samples. Ongoing activities on the potential of using the African nightshade as a ‘dead end trap crop’ for nematodes are funded by the Federal Ministry for Economic Cooperation and Development (BMZ), through the German Federal Enterprise for International Cooperation (GIZ). icipe is also involved in the Microbial Uptakes for Sustainable management of major bananA pests and diseases (MUSA) project, funded by the European Union Horizon 2020. The views expressed herein do not reflect those of these donors. For further information, visit: http://www.icipe.org/research/plant-health/research-nematodes