Yesterday, July 25, I began to notice a lot of damaged grass around campus and around Ames. This was not unexpected because of the extreme weather that we have had lately. We are nearing records for days over 90 F, followed by 70+ F nights with sufficient moisture to keep the turf green. This is ideal conditions for several diseases.
The problem appears to be Leaf Spot caused by the fungal organism (Drechslera poae). The old genus name for this organism was Helmintosporium, a name that still in use by much of the industry. This disease is common at this time of year, particularly when the spring has been wet.
So, if you have this problem, what should you do. There are a number of fungicides labeled for this disease, but usually we let it run its course. The area should recover later in the season and through the fall. Notice in the last picture that the new leaves are emerging in a healthy condition. I am expecting full recovery by September.
The first three pictures below shows leaf spot on central campus at Iowa State. It has now gone into the crown and root stage and the damage is quite apparent.
Beardshear hall just off of central campus.
Individual leaf showing lesions.
It is typical of this disease to damage older tissue first. Notice that the newly emerging leaves are health.
Here is an additional picture that I took yesterday afternoon on my own lawn.
In the area where afternoon shade cools the Kentucky bluegrass, there is very little leaf spot. In the area that gets the stress from the afternoon sun, there is considerable damage from leaf spot.