Sometimes, however, resistance fails, because the fungus causing the disease was able to adapt to the resistance. Learning how fungi adapt will help plant breeders develop resistance that is less prone to this kind of failure. The key is knowledge of the genetic diversity in the fungi that cause diseases. Many fungi can produce identical clones of themselves, which allows them to build up rapidly and cause severe epidemics in crops. But clones cannot adapt to new varieties of crops, because they are identical to their parents.
Many fungi also have a sexual stage in their life cycle.
When fungus individuals mate, they produce unique progeny genetically different from themselves. This creates diversity in the fungus population that helps it adapt to new crop varieties. Knowing how much sex occurs in a fungus is important. It helps us anticipate when resistance may fail and to have other resistant varieties ready before the epidemic can occur. We tested two methods for mathematically analyzing diversity in populations of a fungus that causes northern leaf spot of corn.