Scientific Name:(Peltigera canina
Other name: Felt lichen
Dog Lichen appears as a greyish-black scale called a thallus, and is often encountered growing on the lawn. The thallus is papery in dry conditions and plumps up when wet. The lower surface has white root-like structures called rhizines. It is an indication that the drainage is poor and the grass is in poor condition. Usually there will already be Moss present for the same reason, so dealing with the latter should remove the Lichen as well.
A Lichen is made up of a Fungus and an Algae living together in a symbiotic relationship. The fungus provides a structure taking up moisture and nutrients, but it does not have the means to convert them to sugars. The algae can photosynthesise and produces enough nourishment for both using light as the source of energy required. Lichens are known to grow well when pollution levels are low. If the air quality is poor or where acid rain falls, there will be fewer present.
The common name arose because of the shape of the fruiting bodies which resemble a dogs' teeth. In the middle ages when such resemblances were thought to be significant, as indicated in the Doctrine of Signatures, it was used as a remedy for rabies.
Treatment is the same as for Moss, so scarifying, aerating and feeding should remove the problem from a lawn. The same chemicals which kill Moss will work on the Lichen. An article on Grass Maintenance describing treatment techniques.
See also Liverwort, Blue-green Algae and Moss which can be found in similar locations.
Follow these links for further details on Weeds, Weed Removal and Weed