Honey fungus: managing outbreaks
Honey fungus infection can be deadly for plants. If it’s present in your garden soil there are several options you can use to control the disease and prevent future infections from emerging.
- Different species can be responsible
- Spreads through soil, up to 1 meter per year
- A wide range of plants are susceptible
- Fungus feeds on plant roots and stems
- To treat, remove as much food (wood) as possible
Record the spread of disease in the garden
There are different species of honey fungus which cannot be distinguished by the mushrooms they produce, but which differ in how deadly they are to plants. It is safe to assume that an aggressive species is present when honey fungus has killed off otherwise healthy plants, and action needs to be taken to prevent spread of the fungus (as described below). Noting the positions of affected plants (past and present) will also be useful in working out which area of soil is likely to contain the fungus and require treatment.
If an already weakened plant succumbs to honey fungus but no others seem affected, infection may have been caused by a less aggressive species. In such cases it may not be worthwhile to go to great lengths to eradicate the fungus from the garden, but instead it may be enough to maintain good plant health to prevent further plant deaths.
Choosing your strategy (use a combination that suits your situation)
- Maintain good plant health. Healthy plants are less likely to succumb to honey fungus. Ensure plants have access to sufficient water and nutrients, are not stressed from pest damage and not over-pruned
- Remove soil build up around the root collar (the base of the stem) of plants close to those showing signs of infection. Plants that are too deeply planted are more likely to be affected by the disease
Break the cycle and spread
- Remove sources of infection. Remove as much of the infected root system, stem/trunk material and associated soil as possible. Do not compost this, dispose of it by landfill or burning. It is important to note that if trees are affected, cutting them down to stumps will not remove the source of infection as the fungus can gain nutrients from dead wood and roots for many years
- Consider surrounding plants. In a hedge, remove one healthy plant to either side of the infected area as well. Plants showing no signs of infection but whose roots would be in contact with those of visibly infected plants are likely to have infected roots
- Cultivate the soil. If any infected roots or rhizomorphs (aka bootlaces) are still present, the smaller the infectious pieces are the shorter the time the fungus is likely to survive. Make sure to chop them up when digging out the soil
- Install a plastic sheet barrier. Encircle the affected area with a vertical barrier made from an impermeable membrane, e.g. pond liner, to a depth of 45 cm (18”) which extends to 2-3 cm (1”) above the soil surface. This will prevent any fungal fragments from growing into other areas of the garden. If you are not confident of the area that the honey fungus is present in then this step is unlikely to help. This approach can also be used to exclude honey fungus and 'ringfence' high value areas of a garden
- Leave the bed fallow. A period of 6 months to 1 year without a food source will cause any remaining fungus to die off. Cultivating the soil regularly throughout this period will cut any growing fungus off from food sources and reduce its chances of survival
- Cover the bed with woven ground cover fabric. Potted plants surrounded by gravel could be used to prevent the space looking bare. Avoid using a woodchip mulch directly onto the soil for weed suppression, as any remaining fungus in the bed could use this to feed on and survive beyond the fallow period
- Temporarily replant with grass. Replanting with turf could be considered as an alternative to a fallow period. These plants are expected to be less likely to support survival of the fungus, but this approach is not as risk-free as leaving the bed fallow
- Replace with less susceptible woody plants. Once the bed is ready to be restocked, consult the honey fungus host list. Select replacement plants that are less commonly reported to be infected by honey fungus. Honey fungus has an enormous host range, so when consulting this list some caution should be exercised as the number of reports on a particular plant is influenced by the likelihood of gardeners to ask for advice on the problem from the RHS. Whichever plants you choose, ensure they are suitable for the growing conditions of the bed, are well fertilised and watered and that roots are not damaged during planting
- Redesign the planting scheme. Instead of replanting with trees or shrubs, consider using shorter-lived plants. Herbaceous annual plants are more rarely infected with honey fungus. Specific plants to avoid are strawberry and potato which are very susceptible to honey fungus
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