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Double resistance: Scientists closer to developing cassava with resistance to two deadly viral diseases

Scientists are a step closer to making a break-through in developing cassava varieties with resistance to both Cassava Brown Streak Disease (CBSD) and Cassava Mosaic Disease (CMD) for Eastern and Central Africa. The two diseases are the greatest threats to cassava production in the region putting at risk the food and livelihood security for over 200 million people who depend of the crop for food and income.

According to Dr Edward Kanju, a cassava breeder at the International Institute of Tropical Agriculture (IITA), there are 14 types of the crop under research that are very promising. Kanju’s team has just finished harvesting an advanced trial of such cassava in Uganda.

“I am very excited with the results of the trials,” he said. “Out of the 40 types of cassava under trial, 8 did not show any disease symptoms and 6 had very mild symptoms, either on the roots or leaves or both.”

The next step, he said, will be to challenge these varieties under even more pressure by deliberately grafting them with diseased plants. “This is a very severe test for establishing disease resistance and if afterwards they will still not show any signs of the diseases, then we can confidently, and for the first time say we have cassava varieties that are resistant to both CBSD and CMD,” he says. “These can then be used for breeding for resistance in other countries in the Great Lakes region such as Rwanda, Tanzania, Kenya, and DR Congo.”

According to Kanju, this is the fourth year of trials for dual-resistant cassava for mid-altitudes in Uganda. The trials are being conducted at Mukono, near Kampala, an area regarded as a hot spot for CBSD and CMD. The breeding work started with over 5,000 true seeds of parents with tolerance to CBSD from Tanzania that were sent to Uganda for crossing with IITA varieties that are resistant to CMD.

Before 2004, CBSD was found only in the coastal low altitude areas of Eastern Africa and around Lake Malawi. However, the disease, which causes a dry rot in the roots rendering them inedible, started spreading to mid-altitude areas. The disease has drastically affected cassava production in Central and South-Western Uganda, North Western Tanzania, Western Kenya, and North Western DR Congo. Tolerant varieties for the coastal low altitudes were developed by IITA and its partners, and released in Zanzibar in 2007.

CMD, on the other hand, first appeared in Uganda in the mid-1980s, spreading fast across the region and almost bringing cassava production to a halt. The disease causes annual yield losses of 34 million tonnes in Africa, approximately a third of total production; losses from CBSD are less well documented but the disease has been known to cause 20- 80 percent yield reduction as it affects the roots and leaves, the most useful parts of the crop.

The cassava grown from the Tanzanian seeds have been repeatedly tested under high disease pressure alongside susceptible varieties for comparison. From each growing season, only 10 percent of the cassava were selected to proceed to the next stage. “And after four growing seasons,” Kanju says, “we are down to 14 promising varieties.”

Farmers were also involved in the selection process to ensure the cassava meet farmers preferences on cooking taste, texture, and yield. Breeding for dual-resistant cassava is also being conducted in Tanzania and DR Congo. - IITA.

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Article courtesy IITA: www.iita.org 

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Africa has complex problems that plague agriculture and people's lives. We develop agricultural solutions with our partners to tackle hunger and poverty. Our award winning research for development (R4D) is based on focused, authoritative thinking anchored on the development needs of sub-Saharan Africa. We work with partners in Africa and beyond to reduce producer and consumer risks, enhance crop quality and productivity, and generate wealth from agriculture. IITA is an international non-profit R4D organization since 1967, governed by a Board of Trustees, and supported primarily by the CGIAR.

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