The problem with patenting the genes is that they aren't new genes. They're bred into the crop by breeding a resistant species found in the wild with one of their own breeds. This sounds rather easy, but to get these new breeds up to the quality of the old one (think bigger fruits, faster growth, w/e) can be quite a challenge, as ideally you'll want to find that one plant who kept 99.99% of the positive qualities of your own breed but also has the resistancy.
Finding this one plant is quite a task as it involves you basically breeding the plants over and over again and having them checked automatically. We're talking hundreds of thousands of plants here, and that's just the initial stage of selection. Once you get a few that show resistancy you then have to field trial them for their rate of growth, how they look, how the fruit or w/e looks, how big said fruit is, etc. etc. etc. That's pretty expensive.
Let's say you finally find the plant you're looking for after spending literally millions on it, why should somebody else just be able to buy a package of your seeds that they can then use to breed further?
It's ridiculous to patent genes on their own, but having a patent on a breed of plant you've spent a fortune on to create should be possible.