Diversity in fungal pathogens

Those of us that attempt to maintain cultures of corn pathogens by growing them in artificial culture quickly become aware that isolates of the same species show considerable variability in growth rate, color, spore production and ability to infect the corn plant.  After several generations on the artificial culture media, some appear to lose the ability to infect plants.
 
Genetic variability in all living things comes from sexual recombination in which uniting of chromosomes from two individuals present new possibilities and form mutations.  A human geneticist once told me that humans have an average of 100 mutations, most of which are inconsequential but are different from their parents.
 
Fungi have sex, in which two ‘sexes’ combine and produce new genotypes that are some random combination of the two original parents.  The sexual stage for many corn pathogens is not easily found and therefore is assumed to not be common.  No problem for many fungi, however, because often their vegetative stage, the filaments (hyphae) of a fungus, often fuse, allowing their nuclei to join in a process similar to sex.  It is called parasexuality (outside of true sex) and can give the same benefit of producing new combinations of genetics.  The third source of variability comes from mutations. 
 
Variability is the source of survival and success of most species of plants, animals and fungi. Most pathogenic leaf fungi that produce enzymes to break down complex carbohydrates in plants basically have to kill the leaf tissue first to obtain nutrition for survival and reproduction.  The northern leaf blight fungus enzymatically drills into the leaf, grows towards the vascular tissue, plugs it up so it wilts at least in a area of a couple inches, killing that tissue. Now the fungus feeds on the dead tissue and produces more spores when moist, allowing it to spread and infect elsewhere on that and other corn plants.
 
If the plant’s initial resistance system cannot prevent infection of the veins, corn breeders have selected for resistance inherited by a single gene (Ht1) that inhibits the fungus from plugging the vein. Consequently the fungus is unable to survive and reproduce.  After being widely used for about 10 years, genetic variants in the fungus increased sufficiently to negate to use of that type of resistance.  The battle continues between genetic diversity in the fungus versus genetic diversity in the corn.