Symptoms
A network of white fungal threads is visible in the surface layers of soil. This may occur at various times of the year, often in the autumn but also at other times if soil moisture and temperatures are favourable for the rapid growth of a particular fungus. Fungal fruiting bodies or toadstools may be associated with this growth (again most commonly in the autumn) but more often these are not present.
Cause
A wide range of these fungi are found in soil. They feed from the remains of dead plants and animals, hence the term saprophytic is applied to this group of fungi. They decompose organic matter, and the formation of leaf mould is a stage in this process.
This type of fungal growth is more likely to occur:
1. In nutrient-rich soils for example after adding organic
matter such as compost, bark mulch or other manure.
2. After removing plants and leaving dead roots in the soil, which can
stimulate fungal growth.
3. Following soil disturbance or soil fumigation, which may affect the delicate balance between micro-organisms. Prolific growth of a particular fungus may occur, for example, if the population of its natural anatagonists has been reduced or eliminated.
Control
Fungal growth of this type in soil is not harmful - indeed it is beneficial, improving the fertility of the soil. No control is necessary because the fungi will subside naturally as the material on which they are feeding breaks down. The fungal threads can be broken up with a rake if they are forming a dense mat, which can be water repellent.