Join the RHS today and support our charitable work
The new app packed with trusted gardening know-how
RHS members get reduced ticket prices
RHS members get free access to RHS Gardens
Free entry to RHS members at selected times »
Reduced prices on RHS Garden courses and workshops
Behaviour of severed rhizomorphs of Armillaria mellea and A. gallica in growing media
Armillaria, honey fungus, rhizomorphs
Replanting after the removal of a host of Armillaria can be very difficult, as removal of all sources of inoculum is almost impossible. Some researchers believe Armillaria rhizomorphs to be short-lived, surviving just a few days (Baumgartner et al. 2011). This study aims to determine how long rhizomorphs of two different Armillaria species survive either in loam or peat-free compost.
Our current advice suggests removal of the infected plant and if replanting in the area to use a less susceptible species. However, the hosts of Armillaria tend to be trees and woody shrubs, often used as focal points, which are usually replanted with something similar. It is hoped that this work will enable us to better tailor the advice we provide by understanding how any remaining inoculum may behave in the soil.
The factors being tested are the size of the piece of severed rhizomorph, the species of Armillaria, the growing media, and the length of time. These all relate to practical steps a gardener could take once their infected plant has been removed.
If the gardener was to rotavate the surrounding soil, after plant removal, would this help by reducing the size of the remaining rhizomorphs? To answer this question we have used two lengths of rhizomorph, 2cm and 5cm. An initial experiment showed no difference in survival between 2cm and 5cm pieces, therefore 2cm length rhizomorphs were used for further repeats of the experiment, which was subsequently repeated three times.
This work may change advice given, depending on which Armillaria species is identified in a garden. The rhizomorphs of different species, such as A. gallica, may behave in a very different way to those of other species, such as A. mellea. Research suggests a possible link between rhizomorph production and the pathogenicity of Armillaria species (Redfern and Filip, 1991). Pathogenic species, such as A. mellea tend to produce rhizomorphs less frequently; whereas those of the weak pathogen, A. gallica are seen to be produced much more frequently, and further from the original food source, as these are one of the main infection structures for this species. These differences would affect the treatment of a site after infected plant removal.
The differing experimental growing media would help us understand whether soil enrichment, once the host plant was removed, could help suppress the rhizomorph growth. For this reason a loam is being used to model a quite ordinary soil, and a peat-free compost derived from composted bark, wood and green material to model the compost made and used by gardeners.
Time is obviously an important factor. If the rhizomorphs are severed from their food source but can remain viable indefinitely in the soil, it would be important to never replant that area with a susceptible species, however if after time they ceased to be viable it would allow a gardener to replant with their choice. The rhizomorphs in this experiment are assessed on a regular basis to check for growth and viability.
The results derived from this work will be able to be used directly for advising on the best approach for replanting after a case of Armillaria has been confirmed.
Survival of Armillaria in mulches (834kB pdf)
Latest report: 'Behaviour of servered rhizomorphs of Armillaria mella and A. gallica in growing media (87kB pdf)
More on honey fungus
Baumgartner, K.; Coetzee, M. P. A.; Hoffmeister, D. 2011: Secrets of the subterranean pathosystem of Armillaria. Molecular Plant Pathology 12, 515-534.
Pérez Sierra A and Gorton C (2005). Survival of honey fungus in wood and bark chip mulches. The Plantsman vol. 4, 204-207.
Pérez Sierra A (2003). Systematics, diagnostics and epidemiology of the fungal genus Armillaria. PhD thesis, University of London.
Redfern, D.B. and Filip, G.M. 1991: Inoculum and infection. In Armillaria Root Diease, pp. 76-87. Ed by Shaw III, C. G.; Kile, G. USDA Forest Service Agricultural Handbook number 691.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.