Dying Grass

Hi everyone, Im posting on behalf of my Dad who is having problems with his lawn. He has patches of grass that are dying. When he dug up a patch of the dead grass there is a white powdery substance in the soil below (he thinks it might be a fungus). He has dug up many patches where the grass has died and it is always present. He has also dug up areas where the grass is healthy and no white powdery substance. He lives in the UK, South East and his garden is very well maintained. All ideas, suggestions and any advice gratefully received.
Thank you!

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