- Lead scientist
- Dr Ana Pérez Sierra
- Start date
- 2000
- End date
- 2002
- Keywords
mulch, Armillaria, bark-chips, wood-chips, honey fungus
- Benefits to gardeners
This project aims to provide information about survival and growth of Armillaria mellea in material often used for mulching in gardens.
- The problem
Armillaria root rot is one of the most important diseases affecting woody plants in the UK and spreads to new plants by root contact and rhizomorphs. The use of organic mulches for water conservation and weed suppression in the garden has increased in recent years.
It is possible that Armillaria-infected material may be used to produce mulching material. Mulch that is available commercially would have been composted on an industrial scale reaching temperatures of 70oC, which is high enough to kill most pathogens including Armillaria. Material generated and used in the garden has the greatest potential to contain the disease.
Rhizomorph fragments may be contained within the mulch material allowing them to spread to woody plants growing in close proximity to infected mulch material.
- Approach
Greenhouse experiments were designed to study the growth and behaviour of A. mellea in wood and bark chips of Pinus sylvestris (Scots pine). Further field experiments were carried out to study the effect of mulching on A.mellea-infected plants and also to discover if infected mulch could pass on the disease to healthy plants.
Two greenhouse experiments were set up to study the growth and spread of A.mellea. Containers were filled with soil and covered with either a 7cm or 10cm mulch and A.mellea was introduced to the mulch by the addition of artificially inoculated wood pieces (1.5, 2.5 and 5cm long by 1.5cm diameter).
From the first experiment it was concluded that there was no significant difference between wood and bark colonisation within the 10cm thick mulch, however A.mellea was able to survive successfully in both.
The second experiment was set up to determine whether rhizomorphs of A. mellea could survive and grow in the mulch after being severed from their inoculum. After 12 months it was noted that the severed rhizomorphs continued growing despite removal from their food source. Interestingly when a thinner (7cm) layer of mulch was used there was a significant difference in rhizomorph growth. They grew well within wood chips but completely dried out in bark chips.
Two further experiments were undertaken in the field, the first to study mulching effects on previously A.mellea infected plants. Rosa laxa plants in pots were artificially infected with A.mellea before four different mulch treatments were applied: pine bark-chips, pine wood-chips, gravel or no mulch as a control. After three years, there was no difference in the infection process between each mulching treatment and the controls. If honey fungus is already present in the garden, the risk of infection is not increased by using clean commercial mulch, because the fungus spreads no more readily in the mulch than it does in the soil.
The second study to examine the risk of A.mellea-contaminated mulch to uninfected plants. Rosa laxa plants in pots were mulched either 7cm or 10cm deep with bark-chip inoculated with A.mellea. Inoculum was either 1cm or 5cm long by 1.5cm and was buried in the bark-chip either at a distance of 5cm from the plant or at the edge of the pot (15cm). After 14 months the fungus infected only 2.5% of plants. These infected plants had all been inoculated with the 5cm inoculum placed close to the plant.
This demonstrates a small risk of Armillaria contamination if gardeners choose to chip their own material to produce mulch.
- References
Pérez Sierra A (2003). Systematics, diagnostics and epidemiology of the fungal genus Armillaria. PhD. thesis, University of London
Pérez Sierra, A and Gorton, C (2005). Survival of honey fungus in wood and bark chip mulches. The Plantsman, Vol 4: 204- 207 (PDF)
- Advisory information
Read more about honey fungus
Read the studies on the pathogenicity of Armillaria spp
Read about the identification of Armillaria spp using molecular methods
Read about the behaviour of severed rhizomorphs of Armillaria mellea and A. gallica in growing media
Read about the effect of abiotic factors on Armillaria growth
To view the Armillaria survey