Dry patch is still relatively poorly understood, and may have a number of different causes. One of the most common, however, is thought to be the deposition of water-repellent substances on soil particles by fungi growing through the thatch layer and amongst the roots. Even if the grass has an extensive root system the water cannot get to the roots and the plant effectively suffers from severe drought, even in wet weather.
These fungi are often not pathogenic species that directly attack the grass, and many may simply be helping to break down the dead roots, stems and leaves in the thatch layer. However, it is known that some of the fungi that cause fairy rings can also leave behind water repellent substances as they move through the soil.
Dry patch is also sometimes thought to occur as a result of chemical or physical changes within the soil itself, leading to reduced water penetration.
Factors that may make a lawn more prone to dry patch include compaction, light soil and a thick accumulation of thatch.