Question
So what kills the good Fungi, is it the fertilizer or the weed killer? Will an application of chelated iron help?

Answer
Sorry to say, but Beneficial Fungi

in some ways are more resilient than, say, Nematodes (a delicate group), Protozoa and Bacteria.  Fungal Hyphae -- the 'roots' that reach out at a considerable distance through the Soil -- offer advantages over the limited mobility of Bacteria.  The Fungal Hyphae make it possible for a Fungus to invade a Nematode and extract nutrients almost effortlessly.  They can invade something as hard as a rock or porcelain; they can digest cellulose and woody lignin and insect chitin.  They grow with incredible speed.

Then, what ruins a day in the life of a good Fungus?

The USDA posts a page on Garden Fungi, 'The Living Soil: Fungi', authored by soil scientist Elaine Ingham:

http://soils.usda.gov/sqi/concepts/soil_biology/fungi.html

Ingham says there are plenty of problems Fungi face in the 21st century.

'Land management practices affect the formation of mycorrhizae. The number of mycorrhizal Fungi in Soil will decline in fallowed fields or in those planted to crops that do not form mycorrhizae. Frequent tillage may reduce mycorrhizal associations, and broad spectrum fungicides are toxic to mycorrhizal fungi. Very high levels of Nitrogen or Phosphorus fertilizer may reduce inoculation of roots. Some inoculums of mycorrhizal Fungi are commercially available and can be added to the soil at planting time.'

Translation: Beneficial Mycorrhizal Fungi that team up with plant roots pull up water and nutrients, especially Phosphorous, for the plant; in exchange, they get other nutrients from the plant roots.  But these Fungi bite the dust when you disturb the soil.  Each time you slice into the soil with a shovel, you cut into microscopic webs of hyphae -- invisible straws the take vitamins and minerals from all over the soil and bring it to the plant.  Imagine what rototilling an acre will do to the Mycorrhizal Fungi population there.  Fungicides of course that are used to kill Bad Fungi also damage Good Fungi.

Chemical Nitrogen and Phosphorus Fertilizers interfere with the Good Fungi's employment opportunities underground; with those Fertilizers dousing the roots, who needs a Fungus?  Broad spectrum weedkillers and pesticides like Methyl Bromide, which wipes out ALL biological activity, certainly take no prisoners including good Fungi.

I have to say I do not understand your suggestion that Chelated Iron be used to 'help'.  Would you elaborate for me?

If you are concerned about a Yellow Lawn, I need more information beyond name, rank and serial number.  Zipcode, type of Grass if you have that (please tell me you know what the Grass is...), when the Yellow happened, etc.  Thanks for writing, rsvp.