Only a very small proportion of the thousands of species of fungi in the world can cause disease in plants or animals – these are the pathogenic fungi. The vast majority of fungi are saprophytic, feeding on dead organic material, and as such are harmless and often beneficial. Just occasionally, however, the growth of saprophytic fungi can be a nuisance to the gardener.
What are saprophytic fungi?
Saprophytic fungi feed on dead plant and animal remains. Many are extremely beneficial, breaking down this organic material into humus, minerals and nutrients that can be utilised by plants. Without these fungi we would also disappear under a mountain of unrotted dead leaves and logs!
The fungi usually exist in a microscopic form, but occasionally they may produce either an unusually prolific amount of growth, or fruiting bodies (e.g. mushrooms, brackets, etc). At this point they become noticeable and may cause a few (usually transient) problems for the gardener.
Saprophytic fungi are usually spotted in the garden in a couple of ways:
- White fungal growth (mycelium) in the soil, bark mulches or in compost
- Mushrooms or toadstools (fungal fruiting bodies) in lawns, on the soil surface, on woody mulches or on woody plants
Thankfully, the often extensive growth of mycelium in the soil or in compost is usually harmless. However, it sometimes has water-repellent properties, preventing water from reaching the roots of plants. In turf this type of growth is one of the causes of a problem known as ‘dry patch’.
The appearance of fungal fruiting bodies sometimes causes concern, but again in the vast majority of cases these are harmless. There may occasionally be a problem if the fruiting body is:
- Damaging – the fruiting bodies of a number of fungi can exert a great deal of pressure as they expand, and they have been known to push through tarmac and lift paving slabs
- Smothering – occasionally, prolific production of fruiting bodies in compost can smother young seedlings. A fungus called Peziza, which produces brown, disc- or cup-shaped fruiting bodies, is often responsible
- Offensive – the usual culprit here is the stinkhorn (Phallus impudicus). The offensive smell produced by the mushrooms is usually noticed well before the fruiting bodies themselves are spotted!
- In the wrong place (for the gardener!) – for example, a large crop of mushrooms can adversely affect the appearance of a well-manicured lawn
- Poisonous – this can be of concern if the garden is used by young children or pets
In the vast majority of cases where the growth of saprophytic fungi occurs no action is necessary. However, where there are problems as described above the following actions may help:
- If the presence or continual appearance of fruiting bodies is a nuisance, their production can sometimes be prevented by removing the food source of the fungus. Digging down and removing pieces of dead wood or dead tree roots, or removing and replacing the top few inches of soil, may help
- Water repellent fungal growth in soil can be broken up with a fork or hoe. This type of growth in soil or compost often disappears of its own accord if the food source of the fungus runs out, or if environmental conditions (e.g. temperature and moisture levels) change
- If required, fruiting bodies appearing on lawns can be swept up and disposed of, ideally before the caps open to release spores
Please note that only those fungi causing plant disease are identified at the RHS Members’ Advisory Service. Gardeners with serious enquiries regarding the identification of saprophytic fungi (e.g. concerns about their poisonous nature) should contact the Mycology Department at the Royal Botanic Gardens, Kew (telephone: 0208 332 5000).
Chemical controls for saprophytic fungi are neither available nor required.
Many fungi spend much of the year as extremely thin, elongated tubes known as hyphae. Collectively, the hyphae make up the mycelium of the fungus. Hyphae are often invisible to the naked eye, but sometimes become visible if they grow prolifically or become interwoven to form cords.
Fruiting bodies are produced to enable the formation and release of spores, thus allowing the dispersal of the fungus over greater distances than would be possible by vegetative growth alone. They usually enlarge very rapidly, becoming engorged with water in just a few hours. This hydraulic pressure is designed to push the fruiting body through the soil so that it can be exposed to the air to release its spores, but in some fungi (e.g. puffballs) it so great that it damage tarmac and lift paving slabs.
Most of the fungi that produce fruiting bodies on dead wood are saprophytic species. However, the pathogenic fungi that attack and kill live wood also produce their fruiting bodies on dead wood and stumps. These pathogens include honey fungus (Armillaria spp.), silver leaf (Chondrostereum purpureum) and Ganoderma species bracket fungi. In there is any concern that a fruiting body on a tree stump or dead branch might be that of a pathogenic fungus it would be prudent to have it identified.
Finally, a specialised group of organisms called mycorrhizal fungi live in a close, beneficial association with plant roots (mycorrhiza translates as ‘fungus-root’). These fungi enhance the plant’s uptake of water and nutrients, in turn obtaining some nutrition from the plant. Mycorrhizal fungi will often produce fruiting bodies in the vicinity of the roots.