I received this question from a reader called Tom in November 2018. Tom is concerned that his lawn is rotting away and would like to know why. This question arrived in the autumn following the drought and heatwave of 2018.
Over the past 6 weeks my lawn has developed patches of what appears to be rotting grass, almost as though it has been waterlogged but the area does not hold water, and these patches are spreading rapidly and joining up.
Lawn is more than 20 years old but has been regularly treated with nutrients, moss and weedkillers. Area is slightly sloping but is not permanently shaded.
I have attached some photos. Hope you can help!
Here are the photos:
Thanks for the photos and for the question.
An issue like this would need examining on site to be 100% sure of the problem. I would need to look at the soil to confirm my guesses. That isn’t possible in this case so here are my best guesses from the pictures:
Most Likely: Compaction and drought damage left over from summer.
Many compacted lawns have still not recovered from the drought of 2018. They look a lot like your lawn. Yet, the problem should be reducing, not actually getting worse! Check for compaction by trying to drive a stick or fork into the soil at different areas of the lawn. If it is compaction damage, the soil in the affected area will be a lot harder than the green areas of grass. Read more about compaction and how it affects your lawn here.
It is the most common problem for lawn damage which I see on my rounds.
Next Likely: Chafer Grubs.
Chafer grubs are the larvae of chafer beetles. They work their way through the root zone of the soil eating the crowns of the grass. They are most active in autumn, which is why this is also a likely culprit. You would need to have a look under the turf to confirm Chafer damage. Examine the turf and tug at the grass. If the damaged areas lift away from the soil and don’t seem attached, have a dig around at the edges. If it is chafers, then you will find the white, c-shaped grubs.
Damage this severe would mean that you will have a lot of grubs in the soil. Usually they don’t affect lawns too much unless they are there in numbers, so damage like this is bad news. There isn’t any product on the market to kill them. Check out https://thelawnman.co.uk/chafer-grub-identification-control/ for information on what you can do to keep the numbers down.
Also Consider: A fungal infection.
There are a few fungal diseases that it “could” be. But these are quite rare on domestic lawns. The the damage exhibited by fungal infections can be like your photos. Further examination of the grass and damage to confirm. It could be Anthracnose, Rapid Blight or an older Fusarium infection.
The cure for any of these is the same. A fungicide treatment and careful monitoring. Contact your local lawn care technician for these. Effective fungicides are not available to the public.
These are my “best” guesses without being able to look at the lawn. It could be a few other things (physical damage, water leak, animals) but these are not likely in this case. I hope it has given you some clues on what to look for and how to proceed with solving your rotting lawn problem.
Hope this helps. Thanks for reading!
The Lawn Man