Approximately 500 million people depend on cassava as a major carbohydrate (energy) source, partly because it yields more energy per hectare than other major crops and also because it is grown in areas where mineral and vitamin deficiencies are widespread, especially in Africa. Despite being the largest producer of cassava in the world, yields continue to remain poor in Nigeria. Regionally, yields are also particularly low across Africa with just 10 t/ha compared with India at 26 t/ha, for example.
Pests and pathogens play a strong role in this current deficit, including plant-parasitic nematodes where they either cause direct damage to their host or act as virus vectors. Most affect crops through feeding on or in plant roots, whilst a minority are aerial feeders. In addition to direct feeding and migration damage, nematode feeding facilitates subsequent infestation by secondary pathogens, such as fungi and bacteria. It has been gathered that nutrients generally decrease where there is a disease pressure, plants respond quickly to severe disease pressure by decrease in the nutrient composition. Traditionally however; nematodes are not viewed as a problem on cassava. So let us see. Consequently efforts are underway to improve yields, in addition to nutritional content. Bio-fortified cultivars have been developed to this cause. African countries are not only faced with problem of food security but that of nutritional insecurity leading to different forms of micronutrient deficiencies in the diet. Bio-fortification of cassava is therefore highly appropriate as this will contribute to the alleviation of diseases associated with vitamin A deficiency (VAD) which is a common dietary health problem, especially in countries where cassava is a major staple crop.
What does our research prove?
Research on the effect of nematode infection on the yield and nutritional content of bio-fortified cassava is still at its early stages. The first report was by Akinsanya et al., (2017) where they reported that Meloidogyne incognita infection suppressed the growth and yield of bio-fortified cassava cultivar TMS-IBA011368. Their subsequent research also shows that bio-fortified cassava cultivars (TMS-IBA011368, TMS-IBA011412, TMS-IBA011371, TMS -IBA070593, TMS-IBA070539 and NR 07/0220) were susceptible to M. incognita infection in their screen house experiment, where there growth and yield were heavily affected. From their results, it is very clear that Meloidogyne incognita cause significant yield loss in cassava. This is as a result of root damage, cell disruption, disintegration and formation of giant cells which transfer nutrients meant for plant growth to nematodes. Plant-parasitic nematodes can seriously undermine cassava growth and productivity and poses a major threat to production, while the proportional nutritional appears to be affected but trivial.
Possible way out
As a research fellow at the International Institute of Tropical Agriculture (IITA) Ibadan, Nigeria, I have taken my time to discuss with the cassava breeder, Dr. Peter Kulakow on the challenges posed by nematodes in cassava production as he was never aware of any significant damage. As a result, we decided to include him in our current research titled ‘The effect of root-knot nematodes and other nematodes on the growth, yield and nutritional quality of bio-fortified cassava cultivars in the screen house and field conditions’ where he was convinced beyond reasonable doubt that nematodes are a major threat amongst others. Currently, he is working on breeding cassava cultivars that will be resistant/tolerant to root-knot nematodes.
I am also conducting periodic seminars to farmers around my locality as well as some extension agents on the impact of nematodes in crop production. The management of nematodes is however difficult since they cannot be seen with the naked eyes, the most conspicuous symptom is the presence of galled roots on the crop, others are wilting, stunting, cracking of fruits/crops etc I intend to introduce to farmers the use of soil amendments like peat, manure and composts for effective management of nematodes as well as planting of nematode-suppressive plants e.g. French marigold in an heavily infested field. Other cultural practices include crop rotation, fallowing, soil solarisation while preventive measures include planting of resistant varieties (which will be recommended) and adequate sanitation. All these are cheaper, highly effective and have no negative effect on the environment.
As a University teacher, several trainings have been conducted to farmers at the farm unit of the University to sensitize them about nematodes where majority of them were intrigued as they have never heard of nematodes. I also took my time to invite them to my laboratory where they were able to see the nematodes under the microscope. My students are not left behind as majority of them are dreaming of building their career in the field of nematology.
Aminat Korede Akinsanya, Nigeria