Controlling the twin viral cassava diseases in East Africa through breeding—major victories, but fight is far from over

Cassava farmers in Eastern Africa have been watching in despair as two viral diseases―cassava brown streak disease (CBSD) and cassava mosaic disease (CMD)―ravage their valuable crop. Edward Kanju, IITA cassava breeder based in Tanzania, has spent the last 11 years at the Institute leading regional efforts to develop improved cassava varieties with tolerance or resistance to the two diseases. He shared his 11-year journey, progress and challenges at a recent seminar presentation at the IITA Eastern Africa hub in Dar es Salaam, Tanzania.

According to Kanju, IITA started cassava research in the region in 1984 by establishing the East and Southern Africa Research Centre (ESARC) in Uganda. In 1994, the East African Root Crops Research Network (EARRNET) was formed to address the outbreak of the CMD pandemic. The disease, which causes discoloration and curling of the leaves, affecting photosynthesis, drastically reduces yield. It was first reported in Uganda in the 1990s and had rapidly spread to neighboring countries.

“The collaboration of IITA, EARRNET, and the National Agricultural Research Organization (NARO) of Uganda led to the release of 12 improved cassava varieties with good resistance to CMD between 1993 and 2003,” Kanju said.

“Through EARRNET, these varieties and hundreds of promising breeding lines were shared and released in the neighboring countries of Burundi, DR Congo, Rwanda, and Tanzania by their respective national research systems. Planting materials were made readily available to farmers through various initiatives such as the Crop Crisis Control Project (C3P) and the Great Lakes Cassava Initiative (GLCI). We were very successful in combating CMD,” he said.

In 2004–2005, when the researchers were winning the war against CMD, a new disease arrived on the scene in Uganda. This was CBSD. It was actually not a new disease since it had been reported to be endemic in the coastal lowlands of Tanzania, Kenya, and Mozambique in the 1940s. However, this was the first time it was reported to have spread in mid-altitude areas.

The disease was worse than mosaic disease because its symptoms were not obvious until the roots were harvested and the dry brown rot was found when chopping or peeling. The roots then could not be used in any way, not for food for humans or livestock or for any raw material.

The lead cassava breeder at NARO asked IITA for CBSD-resistant germplasm from its breeding program in Tanzania. As a result IITA sent
5,000 botanical seeds derived from CBSD-tolerant cultivars grown in Tanzania. In 2012, out of the 5,000 seeds shared, 14 derived breeding lines were found to be resistant to mosaic disease but only three showed resistance to CBSD.

Out of the three, the Uganda government recently (2015) officially released one (MM 2006/0130–NARO-CASS 2). All three promising clones are now extensively used as parents to generate better performing new varieties in the country. Seeds from these families have also been shared with the national breeding programs of Burundi, DR Congo, Malawi, Rwanda, and Tanzania to broaden their cassava genetic base and be able to develop new improved varieties suitable for their agroecologies.

In searching for other sources of resistance the team introduced germplasm from IITA-Ibadan: “We also requested germplasm from IITA Ibadan, West Africa, to screen for resistance to CMD and CBSD and have identified two genotypes that are doing very well. They have been evaluated under high disease pressure at Sendusu, Uganda, since 2005 to date,” Kanju said.

The breeding efforts have also gone hi-tech and are using advances in biotechnology to reduce the time it takes to deliver new varieties: “We are also working with molecular geneticists to identify the sources of resistance in varieties which are showing good resistance to the diseases. With such information, we will be able to use marker-assisted breeding to reduce the breeding time to less than six years. We will also be able to stack together the different resistance genes in new varieties,” Kanju said.

Five countries (Malawi, Mozambique, Kenya, Uganda, and Tanzania) have also come together to speed up breeding for resistance/tolerance to the two diseases. They have shared their five best materials which are being tried along with local checks in various sites in the five countries. This is under an initiative known as “The New Cassava Varieties and Clean Seed to Combat CBSD and CMD” (5CP) led by IITA.

Therefore, according to Kanju, while a lot has been accomplished in the last 11 years there is still a lot to be done to control the two diseases. For instance, there is a great need to pyramid genes of resistance to the two diseases so as to decrease the probability of resistance breakdown.

“There is also a need to develop truly CBSD resistant varieties. Truly resistant varieties have very low virus concentrations and therefore will not spread the diseases easily as tolerant ones,” he said.