ARMILLARIA ROOT ROT: A DISEASE December 2003 OF NATIVE AND INTRODUCED TREES ISSN 1440-2262 Ian W. Smith & David I. Smith, Forest Science Centre Background Armillaria root rot, unlike some familiar pests and diseases, occurs naturally in many forests of Victoria. This aggressive fungal pathogen has so far seriously damaged about 2000 ha of mixed species eucalypt forest in the Mt. Cole and Wombat State Forests with minor outbreaks in other areas of Victoria. It attacks many species of native trees and shrubs including eucalypts and wattles, and has also been recorded on introduced trees and shrubs growing in parks and botanical and domestic gardens. Symptoms and Signs of disease The fungus can kill trees and shrub species of any age and has a very wide host range. Young trees and seedlings may, over a period of a few weeks, suddenly wilt and the leaves fall. They may die within 3-6 months of the onset of the first symptoms. Larger trees, especially those with a sizeable trunk, die more slowly. The initial symptoms of sparse foliage and branch dieback may continue for up to 3 years before the remaining foliage begins to wilt, the leaves turn brown and the trees die (Figure 1). Other pathogens and environmental disorders can produce similar symptoms, so it is wise to check that Armillaria infection is present by examining a tree showing advanced disease symptoms. To do this, remove some of the inner bark at the base of the tree and look for fan-shaped, creamy-white fungal sheets (Figure 2). Often the bark tissue around the fungal sheets turns chocolate brown in colour. In many cases small white flecks of fungal tissue surrounded by brown discolouration in the bark also Figure 1. Dieback of eucalypts at Mt. Cole caused by appear. Armillaria luteobubalina. Between April and June the fungus produces fruiting A. luteobubalina has a lingering bitter taste. These bodies (mushrooms) at the base of some trees and stumps features distinguish it from another less pathogenic wood (Figures 3 & 4). These can be quite large, with caps decay fungus Gymnopilus spectabilis that has brown between 4 and 15 cm in diameter on stalks up to 25 cm spores and gills. long. Their colour can vary from yellow-brown to Four species of Armillaria have been recorded in Victoria. yellowish olive. Gills on the underside of freshly Some produce specialised shoe-string-like structures called expanded caps are white and are attached to the stalk; the rhizomorphs, dark in colour and up to 3 mm in diameter, ring is also white and has yellow tufts on the under-surface although the main species causing dieback (Armillaria (Figure 4). Spores are white and the flesh of the cap of luteobubalina), rarely develops rhizomorphs. T © State of Victoria, Department of Sustainability and Environment 2003 Page 1 ARMILLARIA ROOT ROT: A DISEASE OF NATIVE AND INTRODUCED TREES Forests Fact Sheet Armillaria luteobubalina is considered a primary pathogen Spread & Infection of the roots of healthy trees, whereas the other three species (A. hinnulea, A. novae-zealandiae and A. fumosa.) Unlike some other root rot fungi, Armillaria does not are considered to be secondary pathogens which attack infest soil. It spreads from a woody food base, which may trees weakened by other causes such as drought, be a tree or a stump, or even a small piece of infected root. waterlogging, low light levels, nutrient deficiency or insect As the root systems of many trees in these affected forests defoliation. These environmental and biological stresses are in contact with each other, the fungus can move from may also play a role in disease development for A. its food base into the roots of a healthy tree. Its spread luteobubalina. within the roots can be up to 1 - 3 metres per year. Rhizomorphs enable the pathogen to move through the soil between trees to attack other healthy tree roots. As the fungus spreads into uninfected roots, the white fungal sheets grow rapidly through the root's cambial zone, between the bark and the wood. After reaching the base of the tree they keep growing until the stem is girdled. In many eucalypts a basal canker (in the shape of an inverted 'V') can form up to 3 metres above ground which may limit fungal infection and promote host survival. The sapwood rots and a tree may respond by weeping kino or gum. The bark at the base often splits. A tree with white sheets under the bark and mushrooms at the base is seriously infected. Control in Forests Control of this pathogen is difficult. By the time symptoms are obvious the disease is well advanced and Figure.2. Fan shaped creamy-white fungal sheets little can be done to save affected trees. Treatment to under the bark of an infected tree. prevent further spread depends on isolation or removal of the food base. In forestry operations overseas, removal of stumps and dead trees, and ripping through the soil to break up connections between root systems have been used with some success. Although expensive, they were used in a trial established at Mt. Cole in 1978. Here conventional clearfelling with and without ripping around stumps was compared with whole tree pushing, again with and without ripping. Pushed trees and stumps were removed from the trial site and seedlings of Messmate (Eucalyptus obliqua), Victorian Blue Gum (E. globulus ssp. bicostata) and Radiata Pine (Pinus radiata) were planted and their survival and health monitored. Figure. 3 A clump of fruiting bodies on an infected stump. The fungus was active from the establishment of the trial and killed a number of seedlings each year on the heavily infested sites. Unfortunately significant differences between treatments were not observed and therefore no recommendations for site preparation to restore infested areas could be made. However, to reduce the chance of contact between healthy trees and the fungus, clearfelling (followed by regeneration burning) rather than selective harvesting techniques, may reduce the effects of Armillaria in areas prone to infestation. Stumps burnt in the process should have less plant tissue left for Armillaria to grow on. The creation of an ashbed should promote dense and healthy seedling regeneration that will allow for disease escape, genetic Figure. 4 Close up of fruiting bodies Armillaria. selection for resistance to infection, and drying of the site, © State of Victoria, Department of Sustainability and Environment 2003 Page 2 ARMILLARIA ROOT ROT: A DISEASE OF NATIVE AND INTRODUCED TREES Forests Fact Sheet thus making conditions less favourable for Armillaria. dig out the smaller root pieces. Soil fumigation is also a Single tree selection, group selection, or shelterwood possibility in high value areas. Biocontrol agents are also harvesting may exacerbate the disease on sites that are being evaluated. prone to infection, because live trees which may support Replanting the area immediately with other tree species is growth of the fungus are left in the harvested areas. not recommended. During a 6-12 month period after Control in Home Gardens treatment, the infested site should be kept well watered, with the addition of compost and other organic matter Once a tree is infected and showing symptoms in a home where possible. This will encourage decomposition of garden or park, there is little that can be done to save that residual root pieces by other saprophytic fungi. If tree. Avoid over-watering that promotes disease required, small herbaceous plants may be grown over the development. Exposure of the root/stem collar region of site occupied by the tree before replanting with another the tree may also aid in drying the trunk and reducing tree. disease development. Trenching around diseased trees has been attempted but is There are also some systemic fungicides that provide rarely successful because by the time the foliage symptoms promise as protectants and curatives. For example, stem appear the disease has spread to adjacent healthy-looking injection with potassium phosphonate appears to promote trees. Hence when attempting to trench around an recovery in stone fruit trees affected by Armillaria infection, a border of healthy trees should be included infection. While research is continuing into the around the diseased trees. The trench should be dug to a development of chemical control, other measures may be depth of 30-60 cm (depending on soil) and a thick, double taken to stop it infecting other trees. In light sandy soils it sheet of black plastic used to form a barrier around the may be possible to cut the tree down, remove the bole and area. The trench can then be filled. Further reading Kile, G.A. 1981 Armillaria luteobubalina: a primary Smith-White, J.L., Summerell, B.A., Burgess, L.W. and cause of tree decline and death in mixed species eucalypt Coulter, H. 2003 The influence of water potential on forests. Aust. For. Res. 11:63-77 colonisation by Armillaria luteobubalina. In Abstracts th Kile, G.A., R O Squire and J.G. Edgar 1982. An from the 8 International Congress of Plant Pathology Armillaria root rot control experiment in the Mount Cole Christchurch, New Zealand, February 2003. State Forest, Victoria. 1. Establishment and progress Washington, W.S. 1999 Armillaria root rot. Agriculture report, Forests Commission Victoria. Research Branch Notes (AG0155), Dept of Natural Resources and Report 197. 30 pp. (unpubl.) Environment, Victoria. Kellas, J.D., Kile, G.A., Jarrett R.G. & Morgan B.J.T., 1987 The occurrence and effects of Armillaria Further information luteobubalina following partial cutting in mixed eucalypt stands in the Wombat Forest, Victoria. Aust. For. Res. If you require any further information please call the 17:263-76 Department of Sustainability and Environment, Customer Service Centre on 136 186.