Sweetpotato improvement and virus control

One of the highest scorers in CIP's 1996 priority-setting exercise was a new project-an effort to increase the dry matter yield of sweetpotato. The strong showing was largely due to the project's potential impact in China, where 85% of the world's sweetpotatoes are grown. But the effects should also be felt in Southeast Asia and sub-Saharan Africa, where the crop is often vitally important to the poorest farming families. Overall, average benefits were estimated at $309 per hectare, spread over the largest geographic area of all CIP projects.

"The idea is not to produce more tons of sweetpotatoes per hectare," explains sweetpotato breeder Dapeng Zhang, the project leader. "It's to produce more usable material in every sweetpotato."

Current breeding efforts are aimed mostly at increasing fresh yields and fortifying plants against stresses such as drought, flooding, insects, and diseases. The dry matter project will focus instead on the postharvest characteristics of the roots themselves. There is plenty of room for improvement. Dry matter content among the 5000 accessions in the sweetpotato gene bank maintained at CIP can be as high as 45%. But the most commonly cultivated varieties in the project areas range from just 20% to 35%. Moist, orange-fleshed varieties are important in places where sweetpotatoes are an occasional part of a varied diet. But in Africa (where sweetpotato is an important energy source) and Asia (where the crop is used in animal feed, starch production, and industrial processes), dry matter counts. The key to CIP's contribution is the genetic diversity available in its gene bank. In China, CIP will provide national breeders with new sources of genetic material-and new breeding techniques-to boost productivity and improve quality. In Southeast Asia, CIP will help national program breeders produce fast-maturing (or "early-bulking") pest-resistant varieties adapted to local conditions. In Africa, CIP breeders and their partners will produce hardy modern varieties that are high in energy and essential vitamins.

"To increase fresh yields, you normally have to increase the use of water and fertilizers," says Zhang. "But high dry matter is highly heritable. If you can breed new varieties with 5% more dry matter, it's like increasing the fresh yield by 15-20% without using more inputs."