Question
I live in Northern Illinois.  We have several dead patchs of grass in our back yard.  We can not figure out what is wrong.  We have woods that line the back.  A lot of black walnut trees.  The area that we have problems with receive at least 6 hours of sun each day.  Last fall we re-planted (for the 2nd time) grass.  We used a mixture of fescue and Kentucky.  We made sure to plant what was needed for the amount of sun the area received.  The grass grew and came up beautiful in the spring.  It didn't come up thick but very sparse for the amount of seed we put down.  It continued to do well until about July where it appeared to just die overnight.  The pattern is what we find odd.  We have an area of about 3 x 7 dead and then it skips and we have a patch of grass.  Then we have a dead area right off our deck and then another area of dead grass. I guess I find it odd because the areas all receive the same amount of sun and water.  I've looked for grubs and have seen a few in the areas that are dead but there were not a lot of them in the areas I dug up. We have alot of earthworms in lawn also.    Any thoughts.  I really appreciated your time.  

Answer
I can not rule out that you have had a problem with grubs or that there is something else preventing grass from growing adequately, such as excessively compacted soil, excessive shade, etc.

However, one "culprit" which stands out immediately is the black walnut trees.

I have had to learn a new word for this reply: allelopathy.

Allelopathy is a name for a plant's secretion of biochemical materials into the environment to inhibit germination or growth of surrounding vegetation, i.e. such as grass seeds not sprouting, or established grass or other vegetation graudally declining and dying.

This decline occurs because the walnut tree produces a non-toxic, colorless, chemical called hydrojuglone. Hydrojuglone is found in leaves, fruits, bark and roots. When exposed to air or soil compounds, hydrojuglone is oxidized into the allelochemical juglone, which is highly toxic.

The leaves gives out a lot of this chemical but decomposing leaves and fruits also produce it.
This means that next to a black walnut tree it can be very challenging to grow grass.

Unfortunately, the effects of allelopathy appear to be greatest in lawns which are compact, generally poor in organic matter, and which drains relatively poorly. This is a very common sight in most suburban areas as the roots of the tree tends to make the soil compact (reduces airation and drainage). The lack of gracing animals, such as cows or horses, to add "organic matter" to the soil and fact that most plots are put down on rather infertile soils during construction, makes a residental lawn the typical trouble spots when grown near black walnut. You may find that a tree growing under natural conditions may have vegetation which does better.

You can reduce the allelopathic effects by reguarily cleaning up leaves, fruits, and branches which fall from the tree. Maintain high organic matter, such as adding 3-4" of composted cowmanure or forest compost to the soil and mix it into the top 8-12" of root zone before planting new grass again.

Kentucky bluegrass and tall fescue will do better under black walnut than fine fescues and perennial ryegrass.

Just a general note:
- while the black walnut is notorious for causing allelopathy, grass can generally also have big problems growing under any kind of shallow rooted shade tree.

Grass is by definition sun lovers (even shade tolorant cultivars do better in full sun than full shade). Tree roots compact the soil and compete heavily for moisture and nutrients under the soil in summer; especially when it has not rained regularily. You may need to water regularily (twice weekly) but deeply, when you do water, in summer months to ensure that grass stays green. Fine fesuces (hard, chewing, creeping red) while good for shade, do not stand up to draught conditions under trees. Mature canopies of leaved out trees can prevent a lot of moisture from reaching the soil surface so I like to water with a (plastic garden hose) sprinkler below the canopy to ensure enough water gets to the lawn even if it has rained.

When you establish grass (from seed) make sure you keep the soil damp at all times (never allowing it to dry out even slightly). If the seeds dry, they die. You must water twice daily for 30 days or  more if you plant kentucky bluegrass, because it takes this long to germinate.

Also, in your area, fertilize lightly in spring and twice - more heavily - in fall. Do not fertilize during hot summer months.

Good luck.