Why do you think that sounds too good to be true? Why would a species become extinct naturally? Nature seems pretty good at maintaining a species once its evolved to be viable. Obviously things do die out naturally, but to me (a non-botanist) it seems like something that would be very unusual without external factors (eg meteorites, global floods, climate change..)
Also, there are diseases that can target specific plants, when unchecked, can wipe out an entire plant population (see Panama disease for bananas for an example)
Extinction level blights similar to these are not often found in the natural world. They're only a thing because we (humans) eliminated genetic diversity from crops through grafts and clones. If the plants reproduced naturally its likely that a range of genetically diverse children would have been resistant to the fungus.