While growing up in Rajasthan, Deepak Bhatnagar often saw his parents using neem leaves to keep insects out of the wheat they stored in their home. He also saw how well the leaves worked against skin infections when they cured a persistent ulcer on his leg -- one that had baffled the best doctors.

Today, Bhatnagar works at the US Department of Agriculture and has taken up the study of neem's effects on certain fungi. In tests in his laboratory in New Orleans, he ground up neem leaves in water and applied the resulting solutions to Aspergillus flavus, one of the most deadly fungi on earth. It grows on various foods and produces chemicals called alfatoxins that are highly carcinogenic. When Bhatnagar looked at the fungal cultures four days later, they seemed normal. But when he tested them chemically, he could find only 2 per cent of the alfatoxin that the fungus normally would have produced. Neem had left the microbe alive, but had switched off its ability to produce alfatoxins.

Experiments are now under way to determine the components in neem leaves responsible for the bioactivity. Once they are identified, cost-effective systems to control alfatoxin synthesis by the fungus can probably be developed.