UBC scientists find key to deadly poplar tree fungus

I'm not sure what they mean by "extra genes." Extra to what? Do the fungus carry them around in a briefcase? I'm confused.

The researchers’ study, published this week in the The Proceedings of the National Academy of Sciences, found that the fungus, Mycosphaerella populorum, uses extra genes to produce a toxin that can cause fatal lesions on the leaves, stems and branches of poplar trees.