In the past, scientists were able to develop strains of wheat that were resistant to a fungus called stem rust.
But, stem rust has now appeared in a new form.
Ug99, a strain of black stem rust fungus, could destroy most of the world's wheat crops.
It has already spread from Africa to Iran. (Fears over wheat prices after disease spreads)
Ug99 stem rust began in Uganda in 1999.
It has already destroyed harvests in Kenya (2001) and Ethiopia (2003).
Stem rust spores have blown across the Red Sea into the Arabian peninsula and infected wheat fields in Yemen (2007) and Iran (2008). Spores have also blown northwards into Sudan.
Experts believe Egypt, Turkey, the Middle East, India and Pakistan could be affected.
Millions of people could face starvation. (Millions face famine as crop disease rages Science The Observer)
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