HI William, first I would find out waht fungus it is. several can cause lawn problesm. Often by the time you see the damage it may be too late to use a fundgicide. better otions as you have discovered are imporving the overall helath of the lawn. thtechniques than can make fungus dudiease worse are overfertilizing, compacted soils, lawn grass not appropriate for site. also find out what fungus it is. there are some grass cultivars that are resistant to some diseases and therefore could be overseeded now into existing lawn. samples can be sent to UI plant clinic. here is their website.
http://plantclinic.cropsci.illinois.edu/ this website might help to examine lawn care practices.
http://urbanext.illinois.edu/lawntalk/ hope this helps. sandy