Judging by the number and content of many emails we get, not many gardeners realise the important and beneficial effect of most fungi - toadstools or mushrooms - that are to be found in the garden! But then, most fungi related queries are concerning Toadstools in the Lawn.
Under many lawns, there are dead and dying tree roots, lawn 'thatch' and the like - or even leaf litter that has been taken down into the ground by earthworms. This is all eventually broken down by beneficial microscopic fungi - and in doing so, they release nutrients into the soil. So far so good!
The problem is, that these microscopic fungi normally produce 'fruiting' growths, by which means they multiply. The fruiting growths emerge in the lawn (and other parts of the garden) as toadstools!
They are the 'flower and seedpod' of the fungi. (That is botanically incorrect, but best describes the appearance of the toadstool above ground. (The microscopic fungi are the roots - the toadstools are the flower. I await the indignant emails from the botanists!)
Insofar as visible fungi - Toadstools - in the lawn are concerned, then it is mainly the toadstools that make up Fairy Rings, that are harmful to the lawn.
Most toadstools - mushrooms - found in lawns are in fact performing a quite natural act - that of further decaying dead and rotting matter under the surface of the lawn. What we actually see - the toadstool or mushroom - is in fact the tip of the iceberg, with most of the activity related to the toadstool going on out of site, under the turf.
So, decaying organic matter under the lawn is basically the cause of the problem known as "Toadstools in my Lawn" which then materialise on the lawn's surface. As with flowers, the toadstools go on to produce spores which act as seeds, but can easily be blown by the wind to other parts of the garden. In most other parts of the garden, the new toadstools which will then appear the following year or so are not a problem, for they are often hidden beneath the cover of shrubs or other garden plants. They can carry on their good (essential) work relatively unnoticed and not create a panic. It is only when they are visible in the lawn that they seem to create the panic emails that flood in!
The lawn Toadstool is in fact not harming the lawn at all. Neither is the network of underground mycellium which is helping nature do its natural tasks. Nonetheless, generally the toadstools are not 'liked' when in the lawn.
The toadstools normally appear in damp periods - Autumn being a favourite time.
The good news is that the toadstools do not harm the lawn (unless they are forming fairy Rings). The bad news is, that there are no chemical treatments that will 'cure' the toadstools - which are often referred to as mushrooms, but they are NOT the normal edible mushrooms of the kitchen.
Simply brush off the toadstools as and when they appear, to prevent them from producing spores (seeds) which will settle in other parts of the garden and produce yet more toadstool growths eventually. They cannot be dug out; they cannot be killed chemically; basically they do no harm.
Regular scarifying of the lawn will prevent thatch. before laying a new lawn, ensure that all dead tree roots etc are removed. (A difficult if not impossible task!)
Whilst these toadstools in the lawn are not harmful to the grass, they could be poisonous if eaten.