Breeding wheat for horizontal resistance is becoming an urgent necessity for the world's food supply. Almost all wheat breeding during the last century has worked with single-gene vertical resistances. These can provide protection for some time, but they almost inevitably break down to new strains of diseases and parasites.
One of the worst diseases of wheat is stem rust, caused by the Puccinia graminis fungus. Most of the world’s wheat is protected against this fungus by a resistance gene called Sr24, and this gene has allowed wheat to be grown in relative safety from stem rust. But in 1999, a new strain of Puccinia graminis was discovered in Uganda which breaks down the Sr24 resistance. And this new strain of fungus is already spreading.