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Pathogen Biology

Phellinus noxius, in the class Basidiomycetes, is a facultative parasite. It obtains nutrients from dead or dying plant tissue, unlike rust or powdery mildew fungi, obligate parasites that require a living hosts for their nutrient supply. As a tropical plant pathogen its mycelium grows best at 25-30°C (77-86°F); it does not grow at temperatures below 4°C (39°F) or above 40°C (104°F). Colonies grown in the laboratory have distinctive raised brown and white plaques and occasionally produce arthrospores, asexual spores formed by division of special hyphae into one-celled segments. Many fungi produce asexual spores that allow them to move within the same plant or from plant to plant. Arthrospores of P. noxius have not been observed in nature, however, and their importance remains unknown.

The mycelium of P. noxius produces enzymes that break down the middle lamella and cell walls of the plant into simple sugars. This supplies nutrients to the mycelium and allows it to move deeper into the wood. It is sometimes called a white rot fungus (Figure 4) because it degrades lignin, a complex molecule that gives wood much of its strength and brown color. Like brown rot fungi, P. noxius can also break down colorless polysaccharides, but neither as quickly nor as efficiently. Brown rot fungi, however, cannot dissolve lignin and the wood retains its brown color. The mycelium of P. noxius also forms microhyphae and hyphae with extracellular sheaths. These structures are found where wood is being destroyed but their exact function is not yet known.

Sexual Reproduction
Basidiospores of P. noxius contain one nucleus (monokaryotic) and are haploid. When haploid hyphae (n) of different mating types touch each other, they can fuse (plasmogamy) to form a dikaryotic mycelium with cells containing two haploid nuclei (n + n). This dikaryotic mycelium is the most common condition in nature. The hyphae can be modified—swelling, thickening, or sticking together—to produce a mycelial crust or sporocarps. The latter often start as small round patches on stems of dead trees (Figure 11). Patches may continue to grow flat against the wood (effused, Figure 12), grow out into a shelf-like conk (reflexed, Figure 13), or a combination of both (effused-reflexed, Figure 14). The upper surface of reflexed and effused-reflexed sporocarps is sterile, the lower fertile surface covered with small tubes, or pores. These pores are lined with basidia, whose two nuclei fuse (karyogamy) and undergo meiosis, producing four haploid basidiospores (Figure 15). During wet weather basidiospores are released and spread by the wind.


Figure 11		Figure 12


Figure 13		Figure 14

Figure 15

Phellinus noxius sporocarps are sometimes confused with those of P. lamaensis, another tropical Phellinus species. Phellinus lamaensis sporocarps have short, reddish-brown, cone-shaped cells called hymenial setae growing into their pores (Figure 16), however, and P. noxius does not (Figure 17).


Figure 16		Figure 17