Certain conditions can cause ‘Grey Leaf Spot’ which is a Fungus. These include humidity, rainfall, shade, leaves and other debris.
The Quick Solution
If you see Grey Leaf Spot in your lawn it is best to treat straight away to avoid/limit the spread.
Bumper 625 Fungicide 1 L (Covers 650 m²)
Controls certain fungal diseases of turf.
Spray to fix the problem. In 7-10 Days should see a dramatic improvement.
It is also advisable to rake/remove any dead leaves and debris which will be decreasing the sunlight and holding more water and moisture.