Partnership for the 21st Century.
He says it is hard to see signs of the disease in the plant itself.
Instead, it becomes visible in the roots when the plant is harvested.
He says scientists and organizations have to find a way to offer farmers virus-free plant cuttings.
Claude Fauquet says scientists are experimenting with a virus-resistant cassava plant in Tanzania.
Experts warn that brown streak disease could reduce cassava production in Africa by 50 percent.