Hi Brian,

There could be some necrotic ring spot (NRS) starting - we're getting to the point of the season where this disease may present itself. It might also be some drought stress on the turf due to inefficient irrigation.

To rule out irrigation as possible cause, I would encourage you to do an irrigation audit and collect water during one of your irrigation cycles. We can't "see" issues (usually) by just watching the sprinklers run. It always looks like water is going everywhere. 

• Put out several (10-12) cups of uniform shape and size in an irrigation zone. You can use tuna cans, yogurt cups, etc. Be sure to place some in the green spots and some in the brown spots.
• Run the system for 15-20 minutes.
• While the system is running, look carefully at the heads. If there are heads that are crooked (even slightly), not popping up fully, or at an angle, flag those heads to adjust later.
• After the system runs, measure the water in the cups and compare the output. Are the thin spots getting less water? Even a slight difference of 0.10" can lead to INCHES over a growing season. Do the thin/brown areas correspond to any heads that are crooked or not popping up fully?
• Adjust your heads. Make sure they are rising above the turf surface, have a good angle, the pattern applies water evenly, and they are nice and straight.
• Set out the cups again after your adjustments and run the system again. See if you have improved outputs that are more even.
• Measuring your output will help determine how long you need to water each week in minutes. If you get 1/4" of irrigation in 20 minutes, then you'll get 3/4" in 60 minutes. So to apply 1.0" of water, you'll need to water a total of 80 minutes during the week. This might mean two, 40-minute cycles or 3, 26-minute cycles. The seasonal needs of typical bluegrass are:
  • April: 0.5"/week (this includes precipitation too!)
  • May: 0.5-1.0"/week
  • June: 1.0"/week
  • July: 1.0-1.5"/week
  • August: 1.5"/week
  • September: 1.0" week

For necrotic ring spot, this is a disease we see following heavy spring irrigation/precipitation and more than 2 pounds/nitrogen applied during the spring. It's a root rotting disease that kills roots in the spring, but doesn't show up until the weather gets hot and the turf starts to dieback.

If you rule out irrigation as a cause, then it could be NRS and your steps would be to adjust cultural practices - try to keep the lawn drier in the spring and push fertilization to the fall - and then overseed with perennial ryegrass, which looks a lot like bluegrass but is resistant to NRS. Use 6-8 pounds of perennial rye/1000 square feet.

Wait to seed until the weather cools, perhaps the end of August into mid-September. Then aerate and make lots and lots and lots and lots of holes = multiple (4-5) passes. Where the problem areas are located, go over these spots multiple times. Then use your push-type fertilizer spreader and add half the seed needed for the total area and go in one direction, then perpendicular with the other half. Rake to work the seed into the holes. Water to keep the seed coat moist.  But seeding will only be successful with changing cultural practices. Try to delay turning on your irrigation system in the spring or run it manually for seasonal adjustments.