Honey fungus: Improving the advice for gardeners with affected plants 

RHS project team
Dr Jassy Drakulic
Start date
01/09/2017 00:00:00
End date
31/03/2020 00:00:00
Armillaria, honey fungus, species competition, field trials
The problem
Honey fungus infection can be caused by different species of Armillaria.

The most common species found to cause plant disease in gardens is Armillaria mellea which is highly pathogenic.

The second most common species found in gardens is Armillaria gallica; this species is weakly pathogenic and plants are only killed when a secondary cause of stress limits the plant’s ability to keep the fungus in check.

At present, whenever RHS advisors diagnose honey fungus, our advice is the same whether the disease was caused by A. gallica or A. mellea.

The current approach to controlling honey fungus infection in a garden is costly, time-consuming, labour intensive, and is often unsuccessful. Whether this course of action is suitable or worthwhile if the infection was caused by A. gallica is questionable, as promotion of general plant health may be a better course of action. Furthermore, it is plausible that if a garden has A. gallica infection the more virulent species A. mellea may not be able to invade.
Research Question 1: Are there ecological benefits to living with A. gallica in a garden?
This subsection of the project has the aim of determining if Armillaria gallica infected plants are able to exclude a subsequent attack by A. mellea

To achieve this, an outdoor trial using potted privet (Ligustrum) plants is underway. Plants have been infected first with A. gallica, and at two different timings, either three months or six months after, have been exposed to A. mellea. Plants will be monitored for growth and signs of ill-health over one year before harvesting to examine the root systems for signs of Armillaria infection. 

Research Question 2: What is the optimum way to achieve rapid species-specific diagnosis from infected plant samples?

This subsection of the project aims to deliver a molecular test that can distinguish between the species relevant to UK gardeners (A. mellea, A. gallica and A. ostoyae). 

Sample processing will be streamlined to remove the need for culturing the fungi from the samples, and the results of the diagnostic test will be available immediately without the need for further downstream processing.

To achieve this we are developing a real-time PCR assay that will be used to diagnose the causal species of honey fungus infection and display the results of the test on screen in real time during the assay. Real-time PCR also has the capacity to quantify the amount of target DNA, and this application will be of benefit in calculating the amount of infection from A. gallica and A. mellea in the privet pot experiment detailed above.

The aim of this project is to evaluate the need to diagnose garden honey fungus infections to species level and to develop a rapid diagnostic method.

Benefits to gardeners
Delivering a rapid diagnostic test will enable the RHS to develop a service that could provide information to gardeners about the type of honey fungus found in their gardens. This, in addition to determining if there are potential benefits to living with an A. gallica infection, will provide information to gardeners that will help in their decision making about honey fungus disease management.
Summary of results
This experiment will finish in May 2020, as soon as the findings have been analysed we will share them on this project page.
Advisory information
Baumgartner, K., Rizzo, D. M. (2001) Ecology of Armillaria app. In mixed-hardwood forests of California. Plant Disease 85: 947-951.

Baumgartner, K., Bhat, R., Fujiyoshi, P. (2010) A rapid infection assay for Armillaria and real-time PCR quantitation of the fungal biomass in planta. Fungal Biology 114: 107-119.

Drakulic, J., Gorton. C., Perez-Sierra. A., Clover. G., Beal, L. (2017) Associations between Armillaria species and host plants in UK gardens. Plant Disease 101: 1903-1909.

Mulholland V., Mac Askill G. A., Laue, B. E., Steele, H., Kenyon, D., Green, S. (2012) Development and verification of a diagnostic assay based on EF-1 α for the identification of Armillaria species in Northern Europe. Forest Pathology 43: 229-238.

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