Honey fungus are fungi of the genus Armillaria, and are quite common in throughout the UK. Most species are saprophytic and feed on dead wood and hence are not usually a concern for the gardener. However some are aggressively parasitic – in other words, they feed on living plants and trees.
Of particular concern to fruit tree growers is Armillaria mellea, which attacks the root system of fruit trees, eventually killing them. The infection spreads either by direct contact of fungal spores, or through long-distance underground root-like structures called rhizomorphs. There is currently no chemical control available for honey fungus infection.
If you wish to grow fruit trees in soil where honey fungus may be present there are two strategies, and you may need to follow both.
Firstly, you can dig over the soil, removing all infected material, and then place a physical barrier. Heavy duty plastic sheeting, e.g. pond liner, buried to a depth of about 50cm should be effective.
Secondly, you can plant fruit tree varieties which are resistant to infection. To be more precise, since fruit trees are usually grafted, the key here is to plant resistant rootstocks, since it is the root system that is attacked by the fungus.
Fruit trees which are resistant to honey fungus infection
- Quince trees
- Pear trees – when grown on Quince rootstocks
- Mulberry trees
- Walnuts (Juglans regia) when grown on their own roots (which is unusual)
- Medlar trees (particularly when grafted on Quince rootstocks)
Fruit trees that are susceptible to honey fungus infection
- Almond trees - since the plum-related rootstocks used for almond trees are susceptible
- Apple trees – since all apple rootstocks are susceptible
- Apricot trees - since all rootstocks used for apricots are susceptible
- Cherry trees - since all rootstocks used for cherry trees are susceptible
- Medlar trees - when grafted on hawthorn rootstocks, which are susceptible
- Nectarine trees - since all rootstocks used for nectarine trees are susceptible
- Peach trees - since all rootstocks used for nectarine trees are susceptible
- Plum trees - since all rootstocks used for nectarine trees are susceptible
- Pear trees on Pyrus seedling or Pyrodwarf rootstocks
- Walnuts (when grown on Juglans nigra rootstock – the most commonly used rootstock for fruiting walnuts)
It seems surprising that so few fruit tree rootstocks confer any resistance, but this is because honey fungus is not an issue for commercial orchards.
It is also worth noting that whilst honey fungus has a fearsome reputation, it is less likely to be able to take hold on a tree that is well cared for and growing healthily in good soils.