Why “Namikonga” farmers’ variety can withstand devastating effects of deadly viral cassava disease

Scientists have now shed more light on how a farmers’ variety in Tanzania is able to tolerate the deadly viral Cassava Brown Streak Disease (CBSD) disease after infection, showing limited symptoms and impact on yield.

The team have established that ‘Namikonga’ plants have a complex line of defense involving many biochemical pathways and genes that are able to limit the virus multiplication in their bodies once infected. This in turn limits disease progression, restricting infection symptoms to the leaves only while allowing the normal root growth without the necrosis—the dry brown rot associated with CBSD.

They have shared their findings in a paper “A time series transcriptome analysis of cassava (Manihot esculenta Crantz) varieties challenged with Ugandan cassava brown streak virus,” published this week in Scientific Reports, a journal of the Nature group.

CBSD is currently a major threat to the production of cassava in East, Central, and Southern Africa as it results in a dry brown rot in the roots making them unsuitable for any use and can cause yield losses of nearly 100%.

“Our findings show that Namikonga resists the virus by restricting its multiplication in the plant. This gives us new hope in our efforts to control the disease and revive cassava production,” says Teddy Amuge, a Plant Scientist, the lead researcher who worked in collaboration with the International Institute of Tropical Agriculture (IITA), the Ugandan National Crop Resources Research Institute (NaCRRI), New York University, and the University of Pretoria, South Africa.

The team has for years been studying two Tanzanian cassava varieties—Namikonga and Albert, that are resistant and susceptible to cassava brown streak disease (CBSD), respectively.  In Albert, the viral loads were higher and infection with the virus caused substantial leaf chlorosis and root necrosis. The study generated massive data that is now being mined further by several other research groups.

“We are very excited by these results, and are now working on approaches to allow plant breeders to quickly transfer this resistance into other, high yielding cassava varieties,” says Morag Ferguson, corresponding author and principle investigator of this research.

This is the first large-scale study of how some cassava varieties are able to resist pests and disease, and this will help scientists to better control these pathogens.

“A better understanding of this effect may help scientists plan more successful strategies for breeding CBSD resistant cassava in the future,” says Dave Berger, a vital collaborator from the University of Pretoria.

First reported in Tanzania, CBSD previously occurred at low levels primarily in coastal East Africa, Mozambique, and around Lake Malawi and was thought to be restricted by altitude. However in the early 2000s, CBSD began to spread around Lake Victoria, and by 2004, typical CBSD symptoms were widespread in farmers’ fields in central Uganda. The disease has spread steadily since then as far as DR Congo and South Sudan and now, together with cassava mosaic disease, causes over US$1 billion losses in production annually in Africa.

The findings complement a similar study that identified markers associated with genes of resistance to both CBSD and CMD in Namikonga and Albert varieties, the findings of which were published in the journal Theoretical and Applied Genetics.

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