Because sex, as practiced by fungi, is one of the most potent threats to our ability to suppress crop diseases. Down at the cell level, sex looks the same in fungi and people. Two sex cells, each carrying genetic information from one parent, merge together. The offspring of this merger bring a new blend of genes into the world. For people, shuffling the genetic deck creates unique combinations of everything from eye color to toenail thickness.
Fungi lack eyes and toenails, but sex-induced changes are just as profound.
One fungal trait that can change through sexual deck-shuffling is resistance to fungicides. All the other fungi are still vulnerable to the poison. In a few weeks, these three tough guys have multiplied into zillions of clones, all fungicide-resistant. The next fungicide spray has little effect, and the one after that has no effect. The potato crop turns brown and dies.