NAIROBI, Kenya, Sept. 8 - Biologists warned Thursday that a virulent new strain of a previously controlled plant disease had emerged in East Africa and could wipe out 10 percent of the world's wheat production if its spread is not halted.

The disease, wheat rust, caused huge grain losses and even famines in the first half of the 20th century. The new strain was discovered in Uganda in 1999 and has since spread to Kenya and Ethiopia, damaging wheat crops there.

The fungus that causes wheat rust, Puccinia graminis, produces a rusty color on the stem of wheat and slowly destroys the plant. It was controlled in the late 1950's and 1960's through the groundbreaking work of Norman Borlaug, an American who won the Nobel Peace Prize in 1970 for developing high-yield grains that led to the green revolution.

Dr. Borlaug, now 91, spoke at a news conference on Thursday in Nairobi and called to draw attention to the new threat.