Explainer on cassava diseases in Kenya

Dr Catherine Taracha,Center Director,KALRO Biotechnology Center

 “There are two major diseases that challenge cassava production. They include the Cassava Mosaic Disease where you get crinkling of the leaves and Cassava Brown Streak Disease where you get streaks of yellow on the leaves and when you cut the roots you get a brown coloration inside.

These two diseases occur together and they are transmitted by whiteflies and by farmers sharing cuttings. Farmers normally plant cassava using cuttings. So, when these cuttings have these diseases, they get transferred to the next farm when they are shared.

The Cassava Mosaic Disease can be managed through traditional breeding methods. But for Cassava Brown Streak Disease, there is no known source of natural resistance. Thus, non-conventional means are needed to control the disease. The decrease can cost yield losses of susceptible cultivars of up to 70%.”

Expert opinion on the difference between the improved cassava varieties and the conventional cassava

“The virus resistant Cassava with the farmer preferred qualities will be resistant to two diseases; Cassava mosaic disease and Cassava brown streak disease.

From our research, we found out that this Cassava brown streak resistance Cassava is actually very safe and this was based on International Food, Feed and Environmental Safety guidelines. We were able to take the leaves and the roots of our improved Cassava that is resistant to Cassava brown streak to a laboratory in the US for safety evaluation and we found that the Cassava just remains the same as the conventional Cassava that has not been improved.

Also, we looked at the agronomic characteristics of the improved cassava and the plant’s characteristics remained the same. Nutritional composition was not altered. Also, there were no negative effects on the environment. There will be no special farming practices needed. The farmer will continue planting Cassava using the usual agronomic practices and it will be distributed in Kenya through existing national seed delivery systems and can be further developed by breeders.

So, this cassava will be beneficial to breeders, farmers, processors and consumers because it is a source of Cassava brown streak resistance for breeders. There will be availability of good quality planting materials both for the breeders and the farmers. They will increase cassava production from reducing losses to cassava brown streak disease and they will get income when there is increased productivity and therefore the livelihoods of the farmers will be improved.”

Explainer on the process of developing and approval of the new cassava resistant disease varieties

“The Virca Plus Project has been carrying out trials for a while to develop Cassava that is resistant to Cassava brown streak and Cassava mosaic disease. The goal has been to enhance livelihoods of smallholder farmers by delivering Cassava varieties that have enhanced resistance to virus diseases.

This journey started in 2008 with trait discovery that was where there was product concept and gene discovery and the proof-of-concept. And this has moved on to product development where there has been various field evaluations and variety development.

So, modern biotechnology has been used to produce virus resistant crops and what normally happens is a part of the DNA of the virus is integrated into plants genome and the plant’s defense activated to recognize the part of the DNA that is integrated in the plants genome and it is then integrated   to recognize, target and degrade the virus pathogen. This technology has been commonly used to produce virus resistant crops all the way from the 1990s.

There are various crops that are already using this technology for example, Papaya, Plum, Beans and Squash and they have been approved for commercialization in other parts of the world. So, this just shows how we produce the Cassava and this is a multi-step laboratory progress.

We start off with the leaf explant and it goes through laboratory processes until you come up with a transgenic plant. This transgenic plant goes thorough molecular screening again in the laboratory. The various selected lines are then advanced for virus and performance testing in the green house. This just shows what happens in the development of the resistant Cassava brown streak cassava.

The local scientists have been able to develop a cassava brown streak resistant trait that protects Cassava against cassava brown streak disease using a natural plant defense mechanism. So, you can see it all starts in the laboratory then moves to the green house then to field testing.

Now, from the laboratory the Cassava brown streak resistance is then evaluated for performance in the green house. Here in the green house, we take a resistant plant which we bind it with. It is infected using a Cassava brown streak bud, so the bud actually has the virus and by grafting.

You graft it on to your resistance plant and you can see the difference that there is. In the Cassava that does not have the brown streak disease resistance, you get the brown coloration and the Cassava that has the resistance you have no brown coloration and that means the Cassava does not have the disease.

25 improved Cassava lines were then tested and included in the field trial. We call them confined field trials because this is a regulatory field research, they had to be carried out in confined trial so that the experimental material does not leave the site and such confined field trials normally have to have security 24hours and the personnel working in there need to have special training so that none of the research material actually leaves the site.

And these trials were actually set up in areas where there is high whiteflies population therefore high disease incidence and these were planted both in Kenya and Uganda. You remember one of our partners is research institute from Uganda and the best lines were then selected for what is called regulatory field trial both in Kenya and Uganda. The regulatory field trial in Kenya was in Kandara and in Uganda they were set up in Serere.

In summary, we find that disease resistance actually increases the usable storage root use twenty times, that means more than 95% of roots bond to brown disease that did not have the resistant but more than 98% of the roots that had the resistance they improved cassava were disease free.

Now, from that we were able to come up with the improved line which we called event 4046 and we advanced this for safety evaluation and put it in our breeding program. This just shows that we were able to prove the Cassava brown streak resistant cassava sustained resistance across multiple generations.

That means we were able to plant it over several generations and of course they sustained the resistance over multiple generations and in different occasions because we were able to plant in Uganda as well as in Kenya.

Now the virus resistance plants project went through a regulatory phase. Regulatory field trial was performed in Kenya and Uganda and this were for two seasons. As you know, one season of cassava is between nine to twelve months, we are talking about two years. The data was collected for food and feed safety, and for environmental safety.

KALRO submitted an application to the National Biosafety Authority (NBA)for environmental release and placing on the market of Cassava that is Cassava brown streak disease and this happened on 9th March in 2020.

The NBA, which regulates biotech in Kenya, put up a notice for it in the Kenya gazette and two widely read newspapers and this was for public participation to get the public’s opinion on this cassava that is resistant to Cassava brown streak disease.

The public participation was held on 10th June 2020 and this was via Zoom and Facebook live and this was because of COVID and this was the first time that the public participation was held virtually and we actually got 1192 participants and the total comments that came from the public were 3342.

Now there was need to develop Cassava that is both resistant to Cassava mosaic disease and Cassava brown streak disease. We were able to get both resistance of the Cassava mosaic disease and Cassava brown streak diseases resistance in our cassava by doing conventional cross breeding. That means we took our improved cassava with the resistant Cassava brown streak disease and crossed it with farmer preferred variety that had latent Cassava Mosaic disease resistance.

We did this process several times, we were able to get seeds, these seeds were then germinated and we got seedlings and we took them through various evaluation to ascertain that both the Cassava mosaic disease resistance and Cassava brown streak resistance were in our plants.

So, these progenies were then evaluated for trait selection, again these progenies were evaluated in two sites in KALRO Mtwapa and KALRO Alupe both in Kenya and they were also evaluated in Uganda at Serere and Namulonge.

The evaluation in KALRO Mtwapa showed that the Cassava brown resistance was sustained across seasons and at different locations. The evaluation of both the cassava which is resistant to Cassava mosaic disease and Cassava brown streak disease was done between 2019 and 2020 and they were various things that we looked at.

We looked at the harvesting index, we looked at the marketable roots, number of unknown marketable roots, we were also interested in the biomass and we also calculated the harvest index for us to select the best performing lines that will advance to the national trial after improvement by the NBA and Kenya Plant Health Inspectorate Service (KEPHIS).

We have also gone ahead and selected the sites for national trial and these are across different agroecological zones and the sites.  

From western Kenya we have Alupe, Kakamega, Kibos, Homabay and Oyani. These are sites which have high population of whiteflies and can be able to translate the virus and are high in virus disease incidence. In the coast we have Mpeketoni, Msabaha, Mtwapa, Matugga and Kikoneni. So, from these we were going to select three sites from western and three sites from the coast to carry out our national performance trial.

We gave  an application for environmental release placing on the market of the cassava brown streak disease resistant cassava on 9th March 2020 to the Kenya National Biosafety Authority (NBA) and we then got an approval for environmental release to conduct national performances trials(NPTs) and this approval was received on 21st June 2021 that is last month.

So, what is the way forward? We have now gotten approval from the NBA to carry out our NPTs and we hope to do this with the supervision of KEPHIS and these will be carried out for two seasons. The NPTs will involve looking at the distinctness, uniformity and stability (DUS) of this Cassava.

After the two seasons, the report and the data will be sent to the national variety release committee. This committee comprises of KEPHIS and cassava breeders and they will then recommend the release of the cassava which is the Cassava brown streak disease and Cassava mosaic disease resistant to be released to the farmer and for general distribution.

What I didn’t say is that this cassava has no intellectual property rights (IP). We don’t have to pay for IP, so, it will be free to the small-scale farmers. This cassava will follow the usual Cassava distribution stem system that we have already in Kenya.

There are certain people who actually sell cassava cuttings. They are the ones who will sell the cassava to the small-scale farmers. We are going to distribute from KALRO but there are those who sell cuttings as a business, so those ones will sell. But the ones we distribute from KALRO will be free.”

Expert opinion on Cyanide in the improved cassava variety:

“You are aware that cassava does have some qualities of anti-nutrients, such as hydrogen cyanide and there have been incidences in Kenya whereby people have died from them and what I was saying when we did compositional analysis, we found that actually the anti-nutrients were lower in our improved cassava compared to the conventional cassava.

So, all cassava actually contains this poisonous hydrogen cyanide but is normally broken-down during processing through cooking, boiling, frying or drying, especially in Kenya. It is not advisable to eat certain cassava varieties while still raw, although some communities do it. The new variety is safer compared to the conventional counterparts.”

Expert opinion on whether there is market for the improved cassava varieties in East Africa

“There will be market due to the potential that cassava has. There is a company already making various products (biodegradable products) from cassava. The company is making Personal Protective Equipment (PPE), that is normally used in hospitals. It is making masks, and there is a potential of course using cassava to make hand sanitizers especially now during COVID.

In Zambia they are using cassava to produce ethanol and using that to make hand sanitizers.

Cassava can also be used for industries, especially textile industries, for making starch for the textile. You know very well Bt Cotton is now being planted by farmers in Kenya that is expected to lead to higher production of cotton hence more textile manufacturing activities I the country. So, the textile industries will need more starch and that is where cassava comes in handy.

Cassava can also be used to blend with other cereals like maize and wheat. There has been a low production of maize because of climate change and diseases.

So that if we were able to mix the Maize and the Wheat materials with Cassava, we are able you know to fill in the requirements that we need for cereals and bread.

And the Ministry of Agriculture has come up with the blending policy, which, though not yet implemented, requires millers to blend maize and wheat with 30% cassava flour confectionary and baking. So, there will be a lot of market for this cassava which is improved. “

Expert opinion on whether the cassava will receive a green light to go to the market or farmer in Kenya

“We have done actually more than five years of trials on these products and we have done the safety assessment and that’s why the National Biosafety Authority gave us the approval. So, what we are going through for national performance trial is a requirement for all crops, not necessarily just cassava. Even maize, cotton, it must go through the national performance trials (NPTs) and the distinctness, uniformity and stability (DUS) before it is released to the farmer.

We are now in the process of which every crop goes through before it is released to the farmer for distribution.

We also have Bt Maize that has been given a go ahead and I think are in final stages of their NPTs which are also a food crop and from the compositional analysis done on the maize, which looks at all components of the Cassava, be it starch, amino acids, fiber, hydrogen cyanide, the composition of the Bt maize turned out to be substantially equivalent to the conventional one. So, I am very optimistic that it is exactly the same and therefore will get the regulatory greenlight for final deregulation. But first, the NBA and other regulatory agencies have to make sure that the product we are introducing to the market is very safe.”



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