Armillaria root rot (honey fungus)

Symptoms

The fungus attacks the roots and stem base of trees and shrubs, causing their slow decline – the plants grow weakly, the crown thins, the leaves become smaller, and eventually they dry up. The diagnostic sign is white, fan-shaped sheets of fungal mycelium forming a dense layer under the bark at the stem base, with a distinct mushroom smell. In the soil and under the bark there are black or dark-brown, bootlace-like rhizomorphs, by which the fungus spreads to further plants. In autumn, clusters of fruiting bodies – honey mushrooms – grow at the base of infected trees. The above-ground symptoms are non-specific and easily confused with drought or overfeeding: some plants die suddenly (the whole tree turns brown within a single season, especially in hot weather), while others weaken over years – producing short growth, discolouring prematurely, and just before death setting an exceptionally heavy crop of flowers or fruit ('stress flowering'). In conifers, resin bleeding at the stem base is common. When the bark is exposed at the root collar, the wood of the roots and stem turns white and softly rots – honey fungus causes a white rot of the wood. The mycelial layer can be cream-coloured and leathery, spreading radially from beneath the dead bark. A biological curiosity, rather than a sign useful in garden diagnosis, is that the mycelium in moist, infected wood can glow very faintly in the dark (so-called 'cold light') – the effect is too weak and requires complete darkness, so in practice it cannot be used to identify the disease. The disease advances in foci – neighbouring plants die off one after another around an infected one.

Treatment

The disease is difficult to control and there are no effective chemical products against it. Heavily infected trees and shrubs are removed together with their roots and stumps, which are a reservoir of mycelium and rhizomorphs, and the soil is cleared of woody debris where possible. On a threatened site, isolation helps – digging a trench that cuts off diseased plants from healthy ones to limit the movement of rhizomorphs. The trench is dug deep (about 60–100 cm, down to the reach of the main mass of rhizomorphs) and can be lined with a geomembrane or thick sheeting to permanently cut off the fungus's movement to healthy plants. On valuable, early-infected trees the root collar is exposed – digging away the soil around the stem base, which dries and aerates this zone and slows the fungus; this is purely a supporting measure and will not cure extensive infection. Grinding the stump alone is not enough, because the mycelium remains in the roots – they must be grubbed out as completely as possible. On a cleared site it is worth taking a break of several years from planting trees and shrubs (sowing grass or herbaceous plants) until the woody remnants decompose and the fungus loses its food base. There is no fungicide capable of destroying the mycelium and rhizomorphs in the soil – do not count on chemical protection.

Prevention

The basis is removing stumps and root remnants after felling trees, because the fungus survives on them for years. Plant healthy material and keep plants in good condition (watering in drought, balanced fertilisation), as the fungus more easily takes over weakened specimens. On sites with a history of honey fungus it is worth choosing less susceptible species and avoiding planting where infected trees have just been removed. Also crucial is avoiding wounds to the roots and root collar – damage from a mower, strimmer or earthworks is a gateway for infection, so it is worth keeping the stem base in a grass-free basin and mulching it (mulch not touching the bark). Good condition also means proper soil aeration and drainage and avoiding compaction and waterlogging. When planting on burdened sites, remember that the most susceptible include stone-fruit trees (plums, cherries), apple trees, roses, currants, rhododendrons, cedar and hedging conifers (false cypresses, arborvitae) – it is better to choose species regarded as less sensitive in their place. Do not carry new material or soil from infected sites on tools or footwear, so as not to introduce rhizomorphs into a clean spot.

How to tell it apart from look-alikes

Control — organic and chemical methods

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