This topic was briefly mentioned in the November 1994 issue of our Newsletter, however I thought that it was timely to expand and update the previous article since one of the most frequent questions that I am asked is about Honey fungus. Generally people are concerned about shrubs or trees suddenly dying in their gardens. It is important to establish whether or not Honey fungus is causing the problem since methods of control can be time consuming and costly.
Honey fungus used to be considered one species, Armillaria mellea, however recent work has indicated that several distinct species may be found in the UK which vary in their pathogenicity. The fungus lives inside live or dead wood and can form darkly pigmented root-like strands (rhizomorphs), commonly called "bootlaces", which spread the disease to the roots of nearby trees or shrubs. However, the least harmful species produce the most rhizomorphs and the slender rhizomorphs of more aggressive species can be difficult to find.
Symptoms of attack by Honey fungus include:
1. Yellowish-brown (honey) coloured mushrooms, usually in clumps, on or near tree stumps or recently killed plants. The mushrooms may not appear every year but when they do the spore print should be white - mushrooms with differently coloured spores are not Honey fungus.
2. Occasional death over the years of previously vigorous woody plants in a relatively small area - note that death of numerous shrubs or trees within a short time usually does not indicate attack by Honey fungus [1].
3. The best indication of attack by Honey fungus is the presence of white fungal growth beneath the bark on roots and the collar portion of a dead or dying tree [9]. Peel back a section of the bark from the lower trunk or upper roots. Honey fungus mycelium forms white or cream paper-like sheets sandwiched between the dead bark and underlying wood. The sheets have a strong mushroom smell. If dead inner bark is found without any trace of the fungal mycelium, another root disorder may be involved and expert advice should be sought.
Susceptible plants
Honey fungus is particularly damaging to lilac, privet, apple, many flowering cherries, willow, birch, walnut, cedars, cypresses, Monkey puzzle and Wellingtonia [1].
Resistant plants
Annual plants and lawns are not affected (although mushrooms could sprout through lawns from rhizomorphs distant from tree roots or from the dead underground roots of infected trees). Herbaceous perennials appear to be rarely attacked. Box elder, Californian black walnut and yew seem to be virtually immune. Other resistant species include Grand and Noble fir, bamboos, hornbeam, beech, ash, Common ivy, junipers, larch, Cherry laurel, oaks, False acacia and whitebeam [1].
Control of Honey fungus
Some chemicals such as Armillatox [2] are claimed by the manufacturers to kill honey fungus in soil or small fragments of wood, but these will not cure infected plants or kill the fungus in stumps or large roots. The stumps and roots of dead trees are ideal breeding grounds for the fungus, therefore the most effective way to prevent the spread of the disease is to remove all dead stumps and roots from your garden. Stumps which cannot be dug out can be chipped or ground out with a machine designed for the purpose - methods which claim to burn, dissolve or rot stumps away are ineffective. Large unchipped pieces of roots often remain and need to be dug out. After all trace of the dead trees or shrubs have been removed Armillatox solution can be applied around the collar of nearby healthy plants two or three times a year as a precautionary measure (as described in the green booklet supplied with this product).
In the case of hedges and rows of closely spaced trees, remove at least one apparently healthy tree from each side of an affected individual.
Do not replant on the site for one year and then replant with resistant species. Annuals may be safely grown meanwhile. In parkland, plant replacement trees at least 10 yards and preferably 30 yards or more away from the site of any stumps and known sources of infection [1].
Armillaria species found in the UK
Geographical distribution studies indicate that in the Northwest of England we are much more likely to find A. gallica, A. ostoyae and A. mellea sensu stricto than the other three species listed in Table 1 although A. cepistipes has been recorded in Cumbria. John Taylor has informed me that there are North West Fungus Group records for A. gallica, A. ostoyae and, less often, A. mellea found locally and he has checked their descriptions against Flora Agaricina Neerlandica.
The most aggressive pathogens are A. mellea and A. ostoyae with the latter mainly restricted to conifers. A. mellea can act as a primary parasite in gardens, parks, fruit orchards and vineyards [4]. On the other hand A. gallica and A. cepistipes are relatively benign playing an important role in the recycling of nutrients from dead wood.
A seventh species, Armillaria ectypa, has only been recorded from one site in the UK where it was known to be extant in 1995 [6]. The habitat of this rare species is peatbogs where it grows amongst mosses. The history of A. ectypa is poorly understood and it is unclear whether it has significantly declined following the drainage of its peatbog habitats.
| Species | Distribution | Habitat | Pathogenicity |
|---|---|---|---|
| A. borealis | Scotland (rare), not reported from England. | Saprophytic on deciduous trees and conifers. Mainly recorded on birch in Scotland. | Weak parasite on birch and wild cherry. |
| A. cepistipes | Scotland (common), rare in England. | Decomposer of dead wood in hardwood and mixed forest. | Relatively benign. |
| A. gallica | Common low altitude species. | Usually on soil near hardwoods. | Relatively benign. Weak or secondary parasite of hardwoods. |
| A. mellea sensu stricto | Common especially SE England. | Usually with hardwoods, ornamental and orchard trees. | Aggressive pathogen of deciduous trees. Cherry trees and grape vines very susceptible to attack. |
| A. ostoyae | Common. | Mostly restricted to conifers. | Serious parasite of conifers particularly in monospecific forest plantations. |
| A. tabescens | Restricted to SE England. | Mostly saprophytic with Quercus stumps. | Reported to be aggressive towards Eucalyptus species. |
* Source: From Refs. 3 and 4.
Some field identification characteristics for the more common species are given in Table 2. Microscopically A. mellea is the only species that lacks clamp-connections at the base of the basidia [8]. A. cepistipes has robust rhizomorphs like A.gallica and it is reported that the two species are very difficult to distinguish even in the fresh condition [5].
| Species | Pileus | Annulus | Stipes | Habit | Rhizomorphs |
|---|---|---|---|---|---|
| A.gallica | Tan to pinkish brown, hairy/scaly. | White to yellow, partial veil cortinaceous leaving white remnants on stipe. | Base often swollen, may bruise yellow. | Solitary to gregarious. | Robust. Fruit bodies directly attached to them. |
| A. mellea sensu stricto | Honey coloured, smooth with pale scales. | White to yellow, thick wooly. | Pointed at base. | Clustered in tufts. | Sparse, rare to find in soil. |
| A. ostoyae | Brown with dark scales. | Thick woolly with dark scales. | Sometimes pointed and with scales. | Clustered in tufts. | Intermediate. |
| A. tabescens | Not present. | Clustered in tufts. | Not found. |
* Source: From Refs. 3, 7, 8 and information provided by John Taylor.
Note: A. gallica is also known as A. lutea or A. bulbosa.
Acknowledgement
My thanks to John Taylor for checking the article for accuracy and suggesting additional information.
References
1. Anon (1994). Honey fungus in ornamental plantings. Forestry Commission Research Division.
2. Armillatox Web site: http://www.armillatox.co.uk/
3. Guillaumin, J.J. et al. (1993). Geographical distribution and ecology of the Armillaria species in western Europe. Eur. J. For. Path. 23 321-341.
4. Kile, G.A. et al. (1993). Biogeography and pathology of Armillaria. Proc. 8th Int. Conf. Root and Butt Rots (eds. Johansson and Stenlid), Uppsala, Swed. Univ. Agric. Sci. pp 411-436.
5. Marxmüller, H. (1992). Some notes on the taxonomy and nomenclature of five European Armillaria species. Mycotaxon XLIV(2) 267-274.
6. UK Biodiversity Group Tranche 2 Action Plans - Volume III (Plants and fungi): http://www.ukbap.org.uk/Library/Tranche2_Vol3.pdf
7. Volk, T.J. Key to the North American Armillaria species: http://botit.botany.wisc.edu/toms_fungi/armkey.html
8. Watling, R. et al. (1982). The genus Armillaria - nomenclature, typification, the identity of Armallaria mellea and species differentiation. Trans. Br. Mycol. Soc. 78(2) 271-285.
9. Whitehead, D.S. & Sierra, A.P. (1997). Danger underground. The Garden, 790-792.
Further Information
The Royal Horticultural Society provides advice on the use of organic mulches and honey fungus.
