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Due to the problems in identification of the distinct species of Armillaria many assumptions have been made about conditions needed for their initial infection. It is often assumed that for an infection to occur the host must first have been stressed in some other way; however this is not always the case. It is widely accepted, and research has proved, that A. mellea and A. ostoyae are the most pathogenic Armillaria species, dependent on host, where A. mellea is more often seen on hardwoods, and A. ostoyae on coniferous hosts. However in a garden environment, although the majority of the isolates found through the Armillaria survey were identified as A. mellea (79.4%) it has been shown that A. gallica was the second most common, making up 17.1% of isolates. The remaining 3.5% were made up of A. ostoyae, A. cepistipes and A. borealis. It is assumed that A. gallica is only weakly pathogenic, but throughout the survey it was found on living hosts. Although it is difficult to currently ascertain how much of these infections are due to host stress from other abiotic and biotic factors.
This project has two main constituent experiments: The first aims to look at the response of a host plant under controlled conditions, to two different species of Armillaria: A. mellea, known to be pathogenic to a broad range of woody plants; and A. gallica thought to be weakly pathogenic, affecting only stressed plants. Previous host trials have determined that strawberry is a suitable susceptible host plant with high mortality rates. In addition they are easy to grow and manage, and relatively small in size, making them an ideal plant for these experiments. The experiment will be set up with host plants in pots and with the inoculum source close enough to the host to allow infection if the species is able, but the plants are not artificially inoculated at the start of the experiment. These plants will be watered normally. This will allow us to see which species are able to overcome the hosts natural defences without the aid of external factors, such as water stress (waterlogging). We will also have another set of plants which have been exposed to water stress (waterlogging), mirroring conditions that may be experienced in a garden, and observe whether there is a different pattern of infection in these plants. This will enable us to gather information about the initial infection under controlled conditions, indicating whether water stress alters the ability of Armillaria species, either those known to be aggressive pathogens or those thought to be only weakly pathogenic, to infect a host. Although much work has been done looking at the direct result of differing levels of pathogenicity, such as colonisation of material, the other experiment involved in this project intends to utilise enzyme assays to assess pathogenicity. By looking at specific enzymes it will hopefully be possible to understand the ability of the various species to be pathogenic. By looking at several different enzymes it will be possible to look at discrete areas of the infection process, for example the initial overcoming of the hosts defences, the use of host resources for growth and development, and host cell death. Due to the collection of isolates that are held at RHS Garden Wisley it may also be possible to use both of these experiments, in the future, to test for individual isolate virulence, and see if that differs across a species. This could provide extremely detailed information about the species and their role in the environment.
Armillaria survey Behaviour of severed rhizomorphs of A. mellea and A. gallica in growing media Effect of allicin on the growth of Armillaria mellea and A. gallica Survival of Armillaria in mulches (834kB pdf)
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Baumgartner, K.; Coetzee, M. P. A.; Hoffmeister, D. 2011: Secrets of the subterranean pathosystem of Armillaria. Mol. Plant Pathol. 12, 515-534 Gregory, S.C.; Rishbeth, J.; Shaw III, C.G. 1991: Pathogenicity and virulence. In Armillaria Root Disease, pp. 76-87. Ed by Shaw III, C. G.; Kile, G. USDA Forest Service Agricultural Handbook number 691 Pérez Sierra A, Whitehead, D S & Whitehead, M P (1999). Investigation of a PCR-based method for the routine identification of British Armillaria species. Mycological Research 103 (12), 1631-1636
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