Pathogen Profile: Phellinus noxius (Corner) G. H. Cunningam

Introduction

Host Range and Distribution

Isolation

Identification

Symptoms

Ecology and Life Cycle

Links to Other Sites

Selected References

Introduction

Phellinus noxius was first described as Fomes noxius by Corner in 1932, who was investigating the cause of a brown root rot disease of trees in Singapore . It was reclassified as Phellinus noxius by Cunningham in 1965. This organism is a member of the family Hymenochaetaceae, order Aphyllophorales, and phylum Basidiomycota of the kingdom Fungi. The devastating brown root rot disease affects a wide variety of important agricultural and forest plant species, mostly woody but some herbaceous plant hosts. Many early reports of root rot caused by P. noxius were made without demonstration of pathogenicity. In 1984, Bolland was the first to fulfill Koch’s postulates for the disease on hoop pine. An inoculation technique for pathogenicity tests was described by Ann et al, 2002.

Figure 1: Phellinus noxius growing on the trunk of a rainforest tree

Courtesy F. Brooks. Reproduced with permission from Brooks, F. E. 2002. Brown root rot. The Plant Health Instructor. DOI:10.1094/PHI-I-2002-0923-01.

Host Range and Distribution:

Phellinus noxius has a pan-tropical/subtropical distribution, and has been found in Africa, Asia, Australia and Oceania, Central America, and the Caribbean. The pathogen has an astoundingly wide host range, spanning over 100 genera in Gymnospermae and both classes (Monocotyledones and Dicotyledones) within Angiospermae. Cross-inoculation studies have shown a lack of host specificity for various isolates of P. noxius, although varying degrees of resistance are seen in different hosts. The USDA-ARS Systematic Botany and Mycology Laboratory maintains a website with an updated list of hosts and information on geographical distribution. It currently lists 153 host species. Some of the most notable include mahogany, teak, rubber, oil palm, tea, coffee, and cacao as well as a variety of fruit, nut, and ornamental trees.

Isolation:

A stick trapping technique involves inserting sticks of a woody host plant such as rubber into the soil, usually at the collar region of a potentially infected host, and examining for mycelial development three weeks later. To obtain a pure culture, soil diluted in water or plant tissue cut into 3mm cubes is plated on a selective medium developed by Chang in 1995. The formulation of the selective medium is detailed in Table 1 below. The malt and agar base medium should be autoclaved and cooled to 40-60^oC before adding filter-sterile solutions of all remaining components. Cultures are incubated in the dark from 25-30^oC. From the selective medium, plugs can be transferred to 2% water agar to allow for single hyphal tips to be cut and maintained on malt agar (20g/L malt extract and 20g/L agar).

Table 1: Selective Medium for Isolation of Phellinus noxius from Soil or Plant Tissue

To induce the formation of basidiocarps in culture, isolates are grown on a hardwood sawdust medium. The formulation of the sawdust medium is detailed in Table 2 below. After incubation in the sawdust medium at 30^oC for one month, the medium is spread on moist sand in the

Table 2: Medium to Induce Formation of Phellinus noxius Basidiocarps

Identification:

Colony morphology in culture:

Raised white and brown plaques are characteristic of Phellinus noxius in malt culture. Colonies on potato dextrose agar (PDA) are white, but with age becoming brown with irregular lines or patches of darker tissue. See Figure 2. Growth rate on PDA incubated at 30^oC varies for different isolates from 8 to 35mm per day. Hyphae have clamp connections, but they are not commonly produced in culture. Arthrospores and trichocysts are not observed in the field, but are produced abundantly in culture. See Figures 3 and 4.

Figure 2: Phellinus noxius colony morphology on potato dextrose agar

Reproduced with permission from Ann, P.-J., Chang, T.-T., and Ko, W.-H. 2002. Phellinus noxius brown root rot of fruit and ornamental trees in Taiwan. Plant Disease 86:820-826.

Figure 3: Arthrospores of Phellinus noxius on potato dextrose agar

Reproduced with permission from Ann, P.-J., Chang, T.-T., and Ko, W.-H.
2002. Phellinus noxius brown root rot of fruit and ornamental trees in
Taiwan. Plant Disease 86:820-826.

Figure 4: Trichocysts of Phellinus noxius on potato dextrose agar

Reproduced with permission from Ann, P.-J., Chang, T.-T., and Ko, W.-H. 2002. Phellinus noxius brown root rot of fruit and ornamental trees in Taiwan. Plant Disease 86:820-826.

Figure 5: Immature Phellinus noxius basidiocarp

Reproduced with permission from Ann, P.-J., Chang, T.-T., and Ko, W.-H. 2002. Phellinus noxius brown root rot of fruit and ornamental trees in Taiwan. Plant Disease 86:820-826.

Appearance in nature:

Basidiocarps are formed on trunks of infected trees or in culture on sawdust medium. They are perennial and can be single or overlapping. They do not have a stalk, but are attached to substrate by a broad base like a shelf, or commonly are resupinate (appressed to the surface of the substrate). When a pileus is formed, it may appear as a flattened half circle, with a dark reddish brown upper surface that darkens with age. The upper surface appears hairy at first, but gradually losing the wooly appearance with age and developing a thick crust. The margin is white and blunt or rounded at the apex. See Figures 5 and 6.

Slicing the basidiocarp open will reveal context tissue up to 1cm thick. The context is golden brown but blackens upon application of potassium hydroxide. The texture is silky with a fibrous or woody consistency. The context contains both branching, simple-sptate generative hyphae and thick walled aseptate skeletal hyphae. Setal hyphae are radially arranged and can be up to 600 x 4-13μm.

The underside, or pore surface of the basidiocarp, is grayish brown. There are 6-8 irregular polygonal pores per mm, each with a diameter or 75-175μm. Partitions 25-100μm thick separate 2-4 pore layers that are 1-4mm each. The pores are darker than the context tissue.

Figure 6: Mature Phellinus noxius basidiocarp

Reproduced with permission from Ann, P.-J., Chang, T.-T., and Ko, W.-H. 2002. Phellinus noxius brown root rot of fruit and ornamental trees in Taiwan. Plant Disease 86:820-826. 

Basidia are formed inside the pores. When viewed with a compound microscope, they are 12-16 x 4-16 x 4-5μm and club-shaped, bearing four spores each. See the pointer labeled “b” in Figure 7.

Basidiospores are 3.5-6 x 3-4 (average 4.2 x 3.2)μm, ovoid or ellipsoid with a slightly thickened, smooth wall. They are colorless and appear to contain irregular oil droplets. See the pointer labeled “s” in Figure 7.

Figure 7: Crossection of pore showing basidia (b) and basidiospores (s) (600x)

Figure 8: Hymenial setae within the pores of Phellinus lamaensis (400x)

Courtesy F. Brooks. Reproduced with permission from Brooks, F. E. 2002. Brown root rot. The Plant Health Instructor. DOI:10.1094/PHI-I-2002-0923-01.

.

Figure 9: Pores of Phellinus noxius without hymenial setae (400x)

Courtesy F. Brooks. Reproduced with permission from Brooks, F. E. 2002. Brown root rot. The Plant Health Instructor. DOI:10.1094/PHI-I-2002-0923-01.

Symptoms:

The symptoms of and signs of infection by Phellinus noxius are similar for its varied hosts, so a general description is provided here.

Root rot disease usually begins in the roots and spreads to the collar. Phellinus noxius colonizes both lateral and tap roots, and is usually found near the collar. The fungus is described as having an ectotrophic extension habit. Basidiomata form a sleeve which grows along the root surface a few centimeters ahead of internal root tissue colonization. With age, this fungal sheath around the roots becomes encrusted with soil particles that adhere to the roots. The interior root tissue browns at first, then turns white and soft with a network of brown lines running throughout. See Figure 10. Although the organism was originally classified as causing a brown rot, it is now known to be a white rot fungus capable of degrading lignin in plant tissues. Therefore, P. noxius colonizes and degrades both the cortex and the lignified xylem tissues

Foliar symptoms are most readily observed during the spring flush of vegetative growth. Thinning or wilting of leaves is followed by chlorosis and then browning and defoliation. Above ground symptoms are usually not seen until extensive root damage has occurred. Typically the entire root system will be girdled at the butt before foliar symptoms develop. Infected trees with highly decayed root system lose support and may blow over in strong winds.

Development of a mycelial mat or fruiting bodies above ground is variable and not correlated with the extent of below ground colonization. This may accompany a reduction in trunk diameter at the collar, which is easily spotted from a distance. See Figure 1.

Figure 10: Network of Phellinus noxius hyphae within colonized wood

Courtesy F. Brooks. Reproduced with permission from Brooks, F. E. 2002. Brown root rot. The Plant Health Instructor. DOI:10.1094/PHI-I-2002-0923-01.

Ecology and Life Cycle:

Survival in the soil:

Phellinus noxius can survive for many years on infected host plant debris, and has been recovered from infected tissue 10 years after host death. The viability of the fungus declines quickly in soil without host plant debris, with no recovery after five months. Flooding also reduces the viability of this organism. A study by Tun-Tschu Cahng in 1996 found that no P. noxius was recovered from soils containing infested root debris after one month of flooding. This may explain the organism’s apparent preference for sandy soils, which are generally well drained. P. noxius has been reported to grow at temperatures ranging from 10-35^oC, with optimum growth between 25-30^oC. Growth has been observed from pH 3.1-7.5 (and the organism has been isolated from soils up to pH 9), with optimum growth between pH 5.2-6.3. The organism has not been found at elevations above 1000m.

Disease cycle:

Infected host plant debris in soil is the most common source of primary inoculum in newly established orchards or plantations. Seedlings infected in the nursery can also serve as the initial source of inoculum in a field. The fungus spreads primarily via mycelial contact with roots. Phellinus noxius obtains nutrients from host plant tissue by secreting a laccase to degrade lignin as well as enzymes (such as cellulose, pectinase, xylanase) to degrade polysaccharides. The fungus colonizes the root system and moved to the collar, and may form basidiocarps on the trunk of the host. Fruiting bodies form layers of spore-bearing pores during rains. These continue to develop and liberate spores until the end of the rainy season, when a layer of sterile tissue seals the pores. See Figure 11.

Structures involved in infection:

Basidiospores can germinate and infect newly exposed tree stumps. However, it is not believed that this process contributes significantly to epidemic development. The pattern and rate of expansion of disease centers and the timing of basidiospore formation (when rains will deposit them close to where they were produced) suggest that there is not much long distance dispersal via airborne basidiospores. Although arthrospores are produced abundantly in culture, they have not been observed in the field and are not considered to contribute to disease.

Epidemic Expansion:

The structural and biological diversity in natural forest systems restricts the expansion of small, scattered root rot disease centers. Once the natural forest system is disturbed, initial inoculum on infected debris in the soil can spread rapidly through monoculture plantations of susceptible hosts, usually within rows rather than between them. The rate of mycelial growth along roots has been estimated at 0.7 meters per year. Bolland reported in 1984 that patches of 200-400 square meters are common in Queensland, with the largest encountered disease center covered 707 square meters. A survey of dead and infected trees by Hodges in the Mariana Islands revealed infection centers up to 0.1 ha each which coalesced to cover 1 ha or more.

Figure 11: Brown root rot disease cycle and epidemiology

Courtesy V. Brewster. Reproduced with permission from Brooks, F. E. 2002. Brown root rot. The Plant Health Instructor. DOI:10.1094/PHI-I-2002-0923-01.

Suggested disease management tactics:

Forest clearing and ground preparation should be performed to remove as much infested plant residue as possible. Residual inoculum in the small root debris that remains after clearing can be eliminated by flooding the soil for a month or fumigation of soil by ammonia (released in alkaline soils after 3000 p.p.m. urea amendment). When land is cleared and ready to be planted, care should be taken not to use infected transplants. More research on host resistance is needed, since variation in susceptibility has been observed among different species or cultivars within a species. Digging of trenches between rows may minimize disease spread within the field. The systemic fungicides triadimefon, prochloraz, and mepronil were shown to reduce disease incidence without phytotoxicity, but more research is needed on their economics and efficacy.

Links to Other Sites:

American Phytopathological Society’s Plant Disease Lesson on Brown Root Rot:

http://www.apsnet.org/education/LessonsPlantPath/BrownRootRot/default.htm

Systematic Botany and Mycology Laboratory site with nomenclature, host and distribution information, and references. Type Phellinus noxius into the first search box.

http://nt.ars-grin.gov/fungaldatabases/index.cfm

Global Invasive Species Database with detailed information on distribution:

http://www.issg.org/database/species/ecology.asp?si=1007&fr=1&sts=

Selected References:

Ann PJ, Chang TT, Ko WH, 2002. Phellinus noxius brown root rot of fruit and ornamental trees in Taiwan . Plant Disease 86, 820-6.

Bolland L, 1984. Phellinus noxius: cause of a significant root-rot in Queensland hoop pine plantations. Australian Forestry 47, 2-10.

Chang TT, 1995. A Selective medium for Phellinus noxius. European Journal of Forest Pathology 25, 185-90.

The Commonwealth Mycological Institute. 1968. Phellinus noxius. C.M.I. Descriptions of Pathogenic Fungi and Bacteria No. 195. The Eastern Press Ltd., London .

Corner, E. J. H., 1932. The identification of the brown-root fungus. The Gdns. Bull. Straits Settlem. 5, 317-50.

Cunningham GH, 1965. Polyporaceae of New Zealand . New Zealand Department of Science and Industry Research Bulletin 164, 221-2.

Nandris D, Nicole M, Geiger JP, 1987. Root rot disease of rubber trees. Plant Disease 71, 298-306.

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