PP728 Pathogen Profile
A plant pathogen profile written by Lisa M. Kohl
as a requirement for PP 728, Soilborne Plant Pathogens, at NC State University.
Heterobasidion annosum (Fr.) Bref., also known as annosum root rot, is a basidiomycete fungus that is considered to be the most economically important forest pathogen in the Northern Hemisphere. In Europe alone, H. annosum is responsible for the loss of 800 million euros annually (1 billion dollars US), and this pathogen is also widespread in forests in the USA.Heterobasidion annosum was first described by Fries in 1821, under the name Polyporus annosum. The fungus was linked to conifer diseases by Hartig in 1874, and re-named Fomes annosus by H. Karsten. In 1888 Brefeld gave the fungus its current name, Heterobasidion annosum (1) . The anamorph of H. annosum is Spiniger meineckellus (Olson) Stalpers (5) .
Fruiting bodies (basidiocarps) of Heterobasidion annosum
Host Range and Distribution:
Heterobasidion annosum is an economically important pathogen of at least 200 different species in 31 genera of conifers and hardwoods, including Abies, Acer, Larix, Malus, Picea, Pinus, Populus, Prunus, Quercus, Sequoia, and Tsuga (2; 3) . This pathogen is widespread in the Northern Hemisphere, and most frequently occurs on gymnosperms (3) .Previously, all fungi that shared the ecology and morphology of this fungus were classified as Heterobasidion annosum. However, mating experiments determined that many different intersterile groups were present within this genus, and that these intersterile groups showed a great deal of host specificity. The groups are classified as P-type (host preference for Scots pine), S (Norway spruce), and F-type (silver fir), with some intersterile groups having multiple group types, and different populations existing within intersterile groups. Genetic analyses of these populations show that the European and North American Heterobasidion annosum populations form different clades, and new species names of Heterobasidion parviporum and Heterobasidion abietinum have been proposed for the European S and F intersterile groups, respectively (1) .
Heterobasidion annosum can be isolated in many different ways. Water agar has been successfully used with infected host tissue to produce the conidiophores used to identify H. annosum. A semi-selective medium can also be used, such as the BDS (Benomyl dichloran streptomycin) medium that is a mixture of ingredients used by Hunt and Cobb in 1971 and Rizzo and Harrington in 1998. The cultures should be incubated at15-23 oC (5) .
Cultures can also be obtained from a single spore by placing a basidiocarp of the fungus over an empty petri dish overnight, and then transferring a diluted solution of the spores released to a 1.25% malt extract agar (MEA) medium. Cultures can be stored for longer amounts of time on MEA or PDA slants at 4 oC (5) .Thin disks of living sapwood from Picea abies, cut to fit into petri dishes, and placed on moist filter paper, can also be used as a selective method to isolate H. annosum. This technique allows spores to be captured from the air, and will result in colonies of the anamorph stage forming on the disks (5) .
Pure culture of H. annosum.
Fruiting bodies, also called basidiocarps, are formed from small, white, button-shaped pads of mycelium called pustules. The perennial basidiocarps of H. annosum that form at the base of infected trees or stumps are pileate, irregular, and have a reddish to black upper surface with a white margin. The lower surface is white to cream colored, and is made up of visible pores. When sectioned, the stratified layers of tubes can be seen. The basidiocarps also incorporate and envelop needles and debris into the fruiting bodies during growth, which is a characteristic feature of this species (4) .
A diagnostic feature of H. annosum is that basidiocarps envelop and incorporate needles and other debris during growth.These basidiocarps produce the sexual spores, called basidiospores, which are 4-6 x 3-4.5 μm in size. The basidiospores are single-celled, colorless, and oval-shaped (3) . They usually contain two nuclei, have a thin cell wall, nonamyloid, and have an asperulate surface. The asexual stage, Spiniger meineckellus occurs in nature and produces asexual spores, called conidiospores. The conidiospores are smooth, 3.8-6.6 x 2.8-5.0 μm in size, have 1-4 nuclei, and are produced on conidiophores (4) .
Conidiophores of the anamorph Spiniger meineckellus.
Symptoms and Signs:
Trees that have root infections of H. annosum will often show no above ground symptoms, but below ground the roots will have lesions, which can then spread into the butt of the tree (3) . However, up to two-thirds of the root system of the tree may be killed before any symptoms become apparent in the rest of the tree (4) . In resinous tree species, stained and resin-soaked wood will be present in the butt of the tree and the roots will leak resin. In some species, these lesions will form in the sapwood, phloem, and the cambium, girdling the tree and killing it. White mycelium may be found growing underneath infected bark, and white pockets containing black specks may be present in the cambium. These infected pockets will cause the wood to become soft and stringy in texture. Dying trees will have browning foliage, stunted needles or tufted needles, chlorosis, and reddish to purplish bark. Windthrown trees are also characteristic of infection by H. annosum (3) .
Resin leaking from roots damaged by H. annosum will cause sand to clump around the root surface.The most tell-tale sign of infection is the presence of basidiocarps of H. annosum around the base of the stump or butt of the tree. Basidiocarps can appear on an infected stump after 1.5 to 3 years, and can form just a slight layer of tubes over the surface of a log or stump, or can grow up to 30 cm in diameter (3) .
An infected stump. Note the basidiocarps growing at the base.
It is often necessary to clear needles and brush from around trees to detect basidiocarps.
Ecology and Life Cycle:
Basidiospores are the primary infective propagule, and are released from basidiocarps in the summer time. The basidiospores are carried long distances by wind currents, and will fall on and infect freshly cut tree stumps. The fungus then colonizes the stump, and moves into the roots via mycelium (1) .
Within a stand of trees, H. annosum moves short distances from the roots of infected stumps through root grafts with other trees. H. annosum cannot move very far on its own in soil, and relies on growth in tree roots, where it can grow a distance of 0.1-2.0 meters per year. This movement results in stands of infected trees and “disease gaps” in the forest (3) . These stands of dying trees affect the overall ecosystem of the forest, because as the trees die gaps are created in the forest canopy, which affects the moisture and sunlight available, ultimately altering the habitats for plants and animals on the forest floor (1) .The asexual stage, Spiniger meineckellus, is produced on stumps during moist conditions, and the conidiospores produced can survive in the soil for up to 10 months. However, their role in the infection process is unknown, and is not thought to be very significant compared to the role of the basidiospores (3) .
Heterobasidion annosum spreads from infected to healthy trees via root grafts.
Prevention is the best strategy for controlling H annosum. After being cut, stumps should be treated with a compound such as urea, borate, of one of the labeled chemical products registered to prevent the establishment of H annosum. Applications of the biocontrol agent Phlebiopsis gigantea have also proven to be an effective control for annosum root rot. Planting hardwood trees or conifer species that are resistant to H. annosum is also useful at reducing H annosum outbreaks (1) .
Links to Other Sites:
USDA Forest Service -- Annosus Root Rot in Eastern ConifersForestPests.org -- Annosum root disease
1. Asiegbu, F.O., Adomas, A., and Stenlid, J. 2005. Conifer root and butt rot caused by Heterobasidion annosum (Fr.) Bref. s.l. Molecular Plant Pathology 6:395-409.
2. Farr, D.F., Bills, G.F., Chamuris, G.P., and Rossman, A.Y. 1989. Fungi on plants and plant products in the United States. St. Paul, MN: APS Press.
3. Sinclair, W.A., and Lyon, H.H. 2005. Diseases of trees and shrubs. 2nd ed. Ithaca, NY: Comstock Pub. Associattes/Cornell University Press.
4. Woodward, S., Stenlid, J., Karjalainen, R., and Hutterman, A. 1998. Heterobasision annosum Biology, Ecology, Impact and Control. Wallingford, Oxon, UK: CAB International.
5. Worrall, J.J., and Harrington, T.C. 1992. Heterbasidion. In Methods for Research on Soilborne Phytopathogenic Fungi, edited by Singleton, L. L., Mihail, J. D. and Rush, C. M. St. Paul, MN: APS Press.
All pictures are courtesy of the North
Carolina State University Plant Pathology Department Slide Collection and Dr.
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