Research

Honey fungus (Armillaria)

The disease

Armillaria root rot is one of the most important root diseases distributed worldwide. The disease is cause by the fungus Armillaria and affects a large range of hosts including orchard, amenity and forest trees, vegetables and ornamental plants. The disease is well known among gardeners as honey fungus or 'bootlace' fungus and is at the top of the list of the disease enquiries received at the Members Advisory Service at RHS Garden Wisley.

Common symptoms shown by infected plants

• Yellowing of the leaves
• Early leaf fall
• Premature autumn colour
• Splitting of the bark
• Exudation of resin
• Dieback and eventually the death of the whole plant

The disease is mainly confirmed by the presence of the white mycelial sheets found underneath the bark of the roots and/or collar of infected plants (right).

The fungus spreads in soil by means of black or reddish-brown root-like cords known as rhizomorphs, hence the name of 'bootlace' fungus. These can spread through the soil, sometimes growing more than 1m (39in) a year. In some years, when environmental conditions are suitable, the fungus may produce toadstools during the autumn. These are honey in colour; they appear in clusters at the base or near infected trees; they have a collar-like ring below the cap and the spores are white.

Controlling honey fungus in the garden

The genus Armillaria

Until the 1970s we referred to all Armillaria in the UK as A. mellea but we now know there are about 40 different species of Armillaria worldwide and not all of them cause plant disease. Only seven Armillaria species have been identified in Europe, A. mellea, A. ostoyae, A. gallica, A. cepistipes, A. borealis, A. tabescens and A. ectypa and they differ markedly in pathogenicity. It is not possible to differentiate them by their rhizomorphs or the mycelial sheets. It is possible to identify them from the toadstools but these are not always present during infection and, if they appear, they are seasonal. Distinguishing between the species is important, to be able to give appropriate advice to RHS members, since some species will only grow in decaying material and do not cause plant disease. A rapid and accurate method for identification of Armillaria species has been developed.

Armillaria species in the UK

Polymerase chain reaction (PCR)-based and restriction fragment length polymorphism (RFLP) methods were used for the identification of Armillaria species in the UK. Armillaria was isolated onto media from infected plant material. The seven species recognized in Europe have been identified in the UK, with A. borealis found only in Scotland and only three cases recorded of A. ectypa. The latter was identified on the morphology of the fruiting bodies and was not included in this research due to the lack of material available. The most common Armillaria species found in gardens were A. mellea, A. ostoyae and A. gallica. The first two are mostly identified from dying plants and A. gallica was mainly identified from networks of rhizomorphs found in compost, in the soil or colonizing dead plants.

Armillaria gallica

To continue the work on Armillaria, a range of experiments have been designed to assess the importance of A. gallica. This species is accepted as a saprophyte, but there is some evidence that it is capable of killing plants under stress. To test this further, a field trial has been set up using Rosa laxa plants which will be infected with either A. gallica or A. mellea. 50 percent of the roses will then be defoliated. Furthermore, two different isolates of A. gallica will be used - one from a dying tree and the other from a wooden post. This is to assess whether some isolates act predominantly as saprophytes or pathogens, or whether previous food source has an effect on virulence. The results of this trial could influence the advice which we offer members on dealing with compost which is colonised by A. gallica and the risk it poses to garden plants.

Further research is investigating the survival of A. gallica and A. mellea at various temperatures, using both severed rhizomorphs and mycelium located under tree bark. When combined with other research, this is again intended to shape the advice we offer to members.

Conditions favouring the growth of Armillaria

Work has also begun to investigate differences in the conditions required for growth of A. gallica and A. mellea. The classification of Armillaria has only recently been resolved and previous work has not compared the species to each other. Optimal moisture saturation of soil and pH for growth, and the ability of severed rhizomorphs to survive in sand versus peat will be determined.

Armillaria in mulches

The use of organic mulches in gardening has increased in recent years for water conservation purposes and weed suppression. Mulches can help to conserve soil moisture, to control weeds and other competing vegetation, to replenish organic matter and nutrients to the soil and they also tend to have an insulating effect on the soil. The most common organic mulches used in gardens are bark- and wood-chips. Gardeners’ concerns about honey fungus include the role organic mulches play in spreading this disease, the effect of mulches on the honey fungus infection process and the risk of introducing honey fungus in infected chipped mulch.

Investigating the growth and spread of honey fungus in mulches under controlled conditions of temperature and moisture
Two different types of mulches were used, bark- and wood-chip, and these were placed on the top of a pot of soil to recreate soil conditions of the garden. No plants were used in these experiments. After one year honey fungus grew well in both mulches when a 10cm layer of mulch was used. However, when a thinner layer of mulch (7cm) was used honey fungus grew well in wood-chips but completely dried out in bark-chips.

Investigating the effect of mulches on the honey fungus infection process
This experiment used Rosa laxa as a host plant. Rose is highly susceptible to infection by honey fungus and this rose is used as a rootstock by the rose industry. The plants were artificially infected with honey fungus and four different treatments were applied: bark-chips, wood-chips, gravel or no mulch. There were no statistically significant differences in levels of infection in any of the four treatments. After three years it was concluded that mulching with these three products had no effect on the honey fungus infection process and that the disease killed plants irrespective of their mulch treatment.

Investigating the risk of introducing honey fungus in infected mulch material
Host plants were again Rosa laxa, but they were not infected by honey fungus. Instead honey fungus-infected wood chips were placed in bark-chip mulch at various distances from the plant. After 14 months the plants were harvested and checked for infection. Only 2.5 percent of the plants were infected when the inoculum was placed close (5cm) to the plant. In most cases the infected mulch material dried out without infecting the plant. The results indicate that infection can occur under these conditions but only at a low level.

Publications from this research

Perez Sierra, A. & Whitehead, D. 1997. Identification of Armillaria species in the UK using a PCR-based technique. In: Proceedings of the Ninth International Conference on Root and Butt Rots of Forest Trees, pp.436

Perez Sierra, A., Whitehead, M. & Whitehead, D. 1999. Investigation of a PCR-based method for the routine identification of British Armillaria species. Mycological Research, 103, 1631-1636

Perez Sierra, A., Whitehead, D. & Whitehead, M. 2000. Molecular methods used for the detection and identification of Armillaria. In Armillaria root rot: biology and control of honey fungus (eds. Fox RTV) pp95-110, Intercept, Andover, UK.

Pérez Sierra A, Henricot B. 2002. Identification of fungal species beyond morphology. Mycologist 16, Part 2: 42-46.

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