Waceke Wanjohi

Waceke Wanjohi

Dean and Professor of Plant Nematology at School of Agriculture and Enterprise Development, Kenyatta University, Nairobi

When we refer to somebody as a worm, we are not exactly celebrating his or her relevance in our world. Learning something about nematodes could change this. With approximately 1,000,000 species, nematodes, aka roundworms, are the most numerous multicellular animals on earth, having successfully adapted to almost every ecosystem, where they play the most diverse roles. From an agricultural perspective, a nematode may be beneficial or detrimental: the pest nematodes attack plants and spread viruses, causing a global crop yield loss of $125 billion annually. Waceke Wanjohi since 20 years teaches nematology in universities, develops academic programs and labs, publishes internationally, establishes and leads projects like the Nematology Initiative of East and Southern Africa (NIESA), that is effectively improving crop yields in smallholder farming systems in Kenya, Uganda, Tanzania, Malawi and Zimbambwe by effectively using environmental friendly strategies to control the worms. At the Falling Walls Conference, Waceke Wanjohi will present the benefits of her approach, that allows less pesticide use and misuse, reduces food risks and environmental damages, and increases the food harvest and ensures food security.  

Breaking the Wall of Food Insecurity. How Agricultural Science Minimizes Nematode Damage in Sub-Saharan Africa.


Good afternoon everyone. It is really a pleasure to be at this meeting and to address such a great audience and share with you about our efforts to break the wall of food insecurity by minimizing the damage of nematodes on crops. First, I would like to put agriculture in sub-Saharan Africa in context. It accounts for 45% of Gross Domestic Product and 60% of export earnings. It also employs 76% of the population. In fact, for most of the countries in the region, agriculture is a backbone of our economy. 80% of this agricultural production is subsistent, and it is also largely dependent on rain. There is very little irrigation that goes on.

This is what I will be talking about largely: that is a nematode, basically a worm. Its size ranges from 0.2 to 12 mm in length. What you see in the upper picture is the feeding structure called the stylet. In the next picture you see the stylet inserted into the plant cells as the nematode feeds.

Nematodes impact greatly our crop production in that part of the world, as it does in other parts of the world. One: they are obligate parasites, meaning that they depend on crops – specifically for their food and reproduction, they have a very wide host range, and they infect virtually all organs: the roots, the shoots, the flowers, the fruits and the seeds. They cause up to 60% yield losses, particularly in areas where the infestation is very high. The tricky thing about the nematodes is that they are very difficult to detect, because the symptoms they cause particularly above the ground are those that are associated with mineral and water deficiencies and more often than not nematodes are referred to as hidden enemies. They also reduce quality and market value of the produce, especially the root crops, as we will be seeing later. They transmit very important plant viruses, and they also predispose crops to infection by other pathogens and pests.These are roots of maize plants that are being penetrated by the nematodes. You can see the penetration is taking place right at the root tip. The root tip in the second picture is completely destroyed. The nematodes have managed in the third picture to get into the root’s system, and they are already feeding as they migrate. That is a nematode that has positioned itself inside the root system and is actually feeding on the cells of the plants.

These are some of the symptoms that the nematodes cause on the roots. You can see in the picture on this end, you have the tomatoes. On the diseased root systems, you can see the roots have been completely transformed into galls, and compared to the healthy ones, the fibrous root system is greatly reduced. This means that the function of the root system is greatly affected. Looking at the other affects on the roots, like the yam, you can see that the market value is greatly affected due to the deformation by the nematodes. For those who love kartoffel, that is, potato, you have the surface affected and changed into a wart-like structure. If you do a cross-section, that is the damage that you see on the upper epidermis.

These are the symptoms on carrots. You can see on this end is the healthy one, and they become deformed as you move to the other side because of the infection by the nematodes. You can see that there is also the reduction in size of the carrot. These are mature carrots on the lower picture. The tap roots are completely reduced, and their market value is also reduced.

I’ve worked on nematodes of maize and you can see that is a young root system of maize – the healthy one has much more root mass compared to the diseased one. The diseased one is necrotic (brown). This is what you find in older root systems, particularly just before harvest. That is nematode effects on a banana. Because of weakened root systems, the banana simply topples over when it is producing the banana banches. This is at a very critical stage, because as you note, the banana is actually unripe at this stage, immature, so the farmer loses the entire banana crop.

I said that nematodes affect all plant organs. These are the effects on the leaves of rice. You can see that they are starting to lose their colour, their chlorophyll, right from the leaf tips, all the way down, and they start to dry from the tips. On the picture below you find that the ears of the rice are starting to dry up prematurely.

These are the effects on the wheat kernels. The ones on that end are completely transformed into seed galls. If you crush them you will get nothing else but nematodes compared to the healthy ones.

In nematode management, in this particular region, there are a few considerations that we need to make. One is that the majority of the farmers are subsistence farmers. What does that mean for us? It means that nematicide use is therefore impractical because of the high cost. The small farm sizes also means that no meaningful fallowing can be done. It also means that they practice intense continuous cropping. There is also limited awareness of nematodes as important pests because of what I had said earlier: that the symptoms they cause above ground are very, very similar to those of mineral and water deficiencies. We also do not have a nematode specific resistant breeding program and of course, the tropical climate does not make the situation any better, because we have no natural checks, like the winter, and it also means that the growing seasons are usually long and continuous. Then there are other critical crop production constraints, like soil infertility, other pests and pathogens, and of course drought.

So, in trying to come up with strategies to manage this pest, we have adopted an integrated holistic and multipronged approach. In this meeting, I will be just sharing with you four of those strategies that we have used. One, we use integrated strategies to actually control the pests themselves. We have also realised the need to collaborate in doing research, and so we have established a network. It is also important to properly diagnose the nematodes so that you can be able to manage them. Then, of course, there is an issue of human capacity building – and I will be sharing briefly on each one of these.

Integrated strategies: I will be sharing with you two examples of the strategies that we use on managing two nematodes that I have worked on largely. This is a lesion nematode on maize. Lesion nematodes cause up to 60% huge losses on maize and I am sure that you are aware, that maize is a staple crop in that part of the country. So, in managing this, we have combined intercropping, resistance and green manure. What we do is that in season one, we intercrop moderately resistant maize with legumes that are already being used by the farmers to manage soil fertility. Some of the legumes that we have used are the sunn hemp, the Jack bean, the velvet beans and so forth. After we have harvested the maize, in the second season, we actually incorporate the green manure of the legumes, and we allow up to a minimum of 14 days before we plant the maize in the second season. Just by doing this, we have been able to achieve yield increases of up to 54% and nematode damage reduction of up to 30%.

The second example is the nematode called root knot nematode on tomato. This we have virtually applied on all the vegetables. I showed you its effect on the tomato roots in my previous slide. In this, we use soil solarization – or what we call soil sterilisation using solar heat. We also use it in combination with organic amendments, and we also combine it with rotation.

Now in sterilising the soil, we emphasise sterilising the nursery beds and we use the solar energy. We tap the solar energy, using the polyethylene mulch, which we leave on the nursery bed for up to about between 21 and 30 days. After we remove the mulch, we now sow the seed. When the seedlings are ready for transplanting, we prepare the holes and mix the soil in the hole with a little bit of organic amendments and then we can now put in our tomato.

Then after that, after we have grown tomato in that season, then the next season we grow sweet corn and cabbage. These are some of the organic amendments that we have used. We use animal manure, we use crop residues, and we also use agro- industrial wastes, like Pymarc, green tea residues and rejects. We also use green manure, particularly the Mexican sunflower. This is simply because this is also used in the management of P – the phosphorous in the soil. By doing this, we have achieved a yield increase of up to 63% and nematode damage reduction of up to 37%.

We have also established a network, NIESA. That is the logo of the network. I co- founded this in 2005, and that is our website. This is basically a platform for building capacity, creating awareness among key players, conducting collaborative research, and providing peer support. It comprises of nematologists from Uganda, Kenya, Malawi, Tanzania and Zimbabwe. We have also established partnerships with scientists in developed countries for benchmarking and backstopping.

We have also established laboratories, because like I said, it is important that proper diagnosis is being done. This is a laboratory that I established in my university. So, in the region we have a total of six. We have two in Kenya, one in Malawi, one in Zimbabwe, one in Uganda and another one in Tanzania. These are technicians working on farmer’s samples that have been brought to that lab.

We also conduct trainings. So far we have trained about 60 research scientists, plant inspectors, extension officers and farmer groups from 2007. They are from different countries, including the post-conflict countries like Southern Sudan and Rwanda. To help us do this, we have also developed a training manual that helps us do the training. We have also trained several PhDs and MScs.

What is our current thrust? We want to strengthen vocational training for extension officers, advisors, NGOs and farmer groups. We have also initiated an advocacy program that targets specifically the decision and policy makers, so that they can invest more in soil health. We are also engaging the private firms with an aim of forming sustainable partnerships. We also want to expand the coverage of NIESA, and also expand diagnostic labs to include the diagnosis of other pathogens and pests. Thank you very much for your attention.