<Li> Kelp (powdered dried kelp is fed to cattle to help prevent fungal infection) </Li> <P> Pathogens respond to the use of fungicides by evolving resistance . In the field several mechanisms of resistance have been identified . The evolution of fungicide resistance can be gradual or sudden . In qualitative or discrete resistance, a mutation (normally to a single gene) produces a race of a fungus with a high degree of resistance . Such resistant varieties also tend to show stability, persisting after the fungicide has been removed from the market . For example, sugar beet leaf blotch remains resistant to azoles years after they were no longer used for control of the disease . This is because such mutations often have a high selection pressure when the fungicide is used, but there is low selection pressure to remove them in the absence of the fungicide . </P> <P> In instances where resistance occurs more gradually, a shift in sensitivity in the pathogen to the fungicide can be seen . Such resistance is polygenic--an accumulation of many mutations in different genes, each having a small additive effect . This type of resistance is known as quantitative or continuous resistance . In this kind of resistance, the pathogen population will revert to a sensitive state if the fungicide is no longer applied . </P> <P> Little is known about how variations in fungicide treatment affect the selection pressure to evolve resistance to that fungicide . Evidence shows that the doses that provide the most control of the disease also provide the largest selection pressure to acquire resistance, and that lower doses decrease the selection pressure . </P>

What two categories of fungi cause human diseases