Yellow spots may be due to disease as you suspect or environmental influences or differences in grasses species within your lawn or wildlife or possible insect damage. You can contact a lawn care firm that has a strong background disease management to determine if your situation is actually being caused by a fungal problem in the lawn. Lawn diseases in general will need to be prevented, rather than cured. How you manage your lawn will greatly influence the lawns health. Check out
http://urbanext.illinois.edu/lawntalk/