You can try isopropyl alcohol but it doesn't always work. It seems to depend on what type of fungus has infected the lens. You might get lucky.
Another good bet is several days of strong sunlight. The UV kills off most fungi.
I've had good success in several cases with isopropyl alcohol; it's not as fast as you would hope but it does the job. It usually takes about 10 minutes slowly cleaning the surfaces to completely eradicate the mold.
thanks, i have some isopropyl alcohol, and also a UV 'light box' - which i think is more powerful than sunlight. Is it possible to overexpose a lens to UV?
Too long in direct sunlight may cause the lens to get hot and damage the coating. I've sat lens in direct sunlight for 6 to 8 hours a day for several days with no damage.
A man made UV light source should not damage a lens unless it gets very hot and the lens is close to it or if its left there for an extreme period of time.
The coating will be damaged if it gets too hot or fade if its in direct UV light for an extremely long time.
I have had occasional luck with smearing the glass with body lotion and letting it sit for about ten minutes before washing off. Some fungi cannot be removed as they are actually etched into the glass, but lotion can sometimes mitigate the badness. - David Lyga
... thanks guys for the suggestions. Isopropyl alcohol and a cotton bud worked OK but the glass is etched - the location is peripheral and small so probably not an issue.