Question
We live in Central WI.  I have a great looking lawn except for light green patches that seem to be spreading.  It started last year during the month of September with a few small areas and this year about the same time it has spread over most of lawn.  The shapes are irregular but many form small circles.  I could send a pretty clear picture if there was a way to att. to this e-mail.

Thanks, RA Hedtke

Answer
My guess is that either your lawn has become infested by a weedy grass, like rough bluegrass (poa trivialis). This grass would be very clearly seen in summer as it browns before regular lawn grasses.

It is also possible that it a number of lawn diseases. The lawn disease which is closest to your description is: Yellow Patch (Rhizoctonia). It typically show up in early fall as cool temperatures comes around. Hard compact soils (which is often the case on suburbian lawns) and poor drainage are often triggers for this disease.

Look carefully at the individual leaf blades:
If they have small purplish discolorations along the margin (edge) of the grass starting at the leaf tip and spreading downwards, then it is a good sign that Rhizoctonia is present. Also look for black strands of fungus between the leaves. If you see these, then it is probably Necrotic Ring Spot which is another fungus (Leptosphaeria korrae).

Both diseases can be controlled by not overwatering. Water infrequently and deeply when you do water, such as once per week for 1 hour i/o 20 minutes daily. Also water early in the morning so the lawn has time to dry out during the day. If drainage is poor (water stands for a long time after raing) you may have to correct same.

Core aerating the lawn in fall and fertilizing properly will also generally improve chances these diseases not coming back. Fertilize once in late spring (May) and then twice in fall: September and October. Do not fertilize too heavily in spring and avoid fertilizing during hot summer months.

I suspect either of above since you noticed these in september (fall) and due to the appearence of light green patches. The fact that the patches spread in round circles are usual tell tale signs of disease.