Armillaria luteobubalina

Armillaria luteobubalina, commonly known as the Australian honey fungus, is a species of mushroom in the family Physalacriaceae. Widely distributed in southern Australia, the fungus is responsible for a disease known as Armillaria root rot, a primary cause of Eucalyptus tree death and forest dieback. It is the most pathogenic and widespread of the six Armillaria species found in Australia. The fungus has also been collected in Argentina and Chile. Fruit bodies have cream- to tan-coloured caps that grow up to 10 cm (4 in) in diameter and stems that measure up to 20 cm (8 in) long by 1.5 cm (1 in) thick. The fruit bodies, which appear at the base of infected trees and other woody plants in autumn (March–April), are edible, but require cooking to remove the bitter taste. The fungus is dispersed through spores produced on gills on the underside of the caps, and also by growing vegetatively through the root systems of host trees. The ability of the fungus to spread vegetatively is facilitated by an aerating system that allows it to efficiently diffuse oxygen through rhizomorphs—rootlike structures made of dense masses of hyphae. Armillaria luteobubalina was first described in 1978, after having been discovered several years earlier growing in a Eucalyptus plantation in southeastern Australia. It distinguished itself from other known Australian Armillaria species by its aggressive pathogenicity. It may take years for infected trees to show signs of disease, leading to an underestimation of disease prevalence. Studies show that the spread of disease in eucalypt forests is associated with infected stumps left following logging operations. Although several methods have been suggested to control the spread of disease, they are largely economically or environmentally unfeasible. Phylogenetic analyses have determined that A. luteobubalina is closely related to A. montagnei and that both of these species are in turn closely related to the Brazilian species A. paulensis. The distribution of A. luteobubalina suggests that it is an ancient species that originated before the separation of the precursor supercontinent Gondwana. Armillaria luteobubalina was first described in 1978 by mycologists Roy Watling and Glen Kile, who studied its effects on a fast-growing plantation of Eucalyptus regnans near Traralgon, Victoria. The plantation, established in 1963, consisted largely of trees with a mean height of about 25 m (80 ft). A cluster of dead and dying trees discovered in 1973 suggested attack by a virulent primary pathogen, that is, one capable of infecting a host before invasion by other, secondary pathogens. This finding was inconsistent with the pathogenic behaviour of the known Armillaria species in Australia at the time, A. mellea and A. elegans. Further study over the next few years showed that the fungus spread by the growth of underground mycelia in root systems, expanding outward from the initial infected stump at an average of 2.5 m (8.2 ft) per year. Most Australian records of Armillaria infections referred to A. mellea, based on the presence of black rhizomorphs. For over one hundred years, A. mellea was thought to be a pleiomorphic (occurring in various distinct forms) species with a widespread distribution and host range, and variable pathogenicity. which led to great confusion among taxonomists and plant pathologists alike. In 1973, Veikko Hintikka reported a technique to distinguish between Armillaria species by growing them together as single spore isolates on petri dishes and observing changes in the morphology of the cultures. Using similar techniques, mycologists eventually determined that the Armillaria mellea species complex in Europe and North America in fact consisted of five and ten distinct 'biological species', respectively. Watling and Kile compared the macroscopic and microscopic characters of the pathogenic Armillaria with A. polymyces (now known as A. obscura), A. mellea, A. limonea and A. novae-zelandiae and found sufficient differences between them to warrant designating the species as new. Its specific epithet is derived from the Latin lutea 'yellow', and was chosen to highlight an important distinguishing characteristic: the strong yellow colour of the cap and lack of reddish or brown tones in the stem typical of other resident Armillaria. A phylogenetic study of South American Armillaria species concluded that A. luteobubalina is in a lineage that includes A. montagnei, and these are sister to a lineage containing A. paulensis, a species known from a single specimen collected in São Paulo, Brazil. Although they are very similar, specimens of A. luteobubalina have smaller spores than Argentinian specimens of A. montagnei, and their distinctness is well-supported with phylogenetic analysis. Based on analysis of pectic enzymes, A. luteobubalina is closely related to A. limonea, a species found in New Zealand; this result corroborates phylogenetic analyses reported in 2003 and 2006. Molecular analysis of 27 collections of A. luteobubalina from southwest Western Australia and one from Traralgon revealed four distinct polymorphic groups. The genetic variety suggests it is native to Australia. Up to 10 cm (4 in) in diameter, the cap is convex to flattened in shape with a central umbo (a rounded elevation) and is various shades of cream, yellow and tan. The cap surface is covered with darker scales and feels rough to the touch. The cap edge, or margin, is rolled inward in young specimens. The crowded gills are sinuate and white to cream in colour initially, brownish-cream or pinkish brown in maturity, and sometimes with yellow or rust-coloured marks close to the margins. The stem is central (that is, it joins the cap in the centre) and is up to 20 cm (8 in) long by 1.5 cm (1 in) thick. It is slightly thicker at its base than its apex, sometimes almost bulb-like. The stem surface is streaked with fibrils that run up and down its length. It has a floppy yellow wool-like ring which may develop irregular, jagged edges with time. The flesh is white, and in the stem has a woolly or stringy consistency. Although it has a hot-bitter taste, Armillaria luteobubalina is edible, and cooking removes the bitterness. The spore print is white when fresh, but becomes more cream-coloured when dry. The smooth spores are oval to ellipsoid, hyaline (translucent), non-amyloid (meaning they do not absorb iodine from Melzer's reagent), and typically measure 6.5–7.5 by 4.5–5.5 μm. The basidia (spore-bearing cells) are thin-walled, hyaline, and lack clamp connections at their bases. They are usually four-spored but occasionally two-spored, with sterigmata (projections that attach to the spores) up to 4 μm long. The cheilocystidia (cystidia that occur on the edge of a gill) are mostly club-shaped, thin-walled, hyaline, and measure 15–30 by 6–10 μm. Five other Armillaria species are found in Australia. Within the range of A. luteobubalina, A. hinnulea is restricted to gully habitats. A. fumosa is a rarer species found only in poorly drained or seasonally wet locations. A. luteobubalina and A. montagnei share cap features and a similar unpleasant flavour, but the latter species has an olive-tinged cap, larger spores (9.5–11 by 5.5–7 µm compared to 6.5–7.5 by 4.5–5.5 µm) and a more conspicuous annulus than those found in A. luteobubalina. The morphology of the vegetative structures of A. limonea is distinctly different than A. luteobubalina, and can be used to distinguish the two species. A. novae-zelandiae has a sticky more flattened cap and stem below the ring and is found in wet forests, and A. pallidula is a species with cream gills maturing to pale pink found in tropical Australia arising from dead tree stumps or the roots of dead or living trees. A. luteobubalina is the only Armillaria species which occurs in Western Australia. Distinguishing Australian species is economically important, because A. luteobubalina is more pathogenic than the other members of the genus. A molecular diagnostic test, developed in 2002, can accurately identify each species using DNA extracted from its mycelia. Before this, species identification was limited to times when fruit bodies were in season. This technology also revealed a variation in the molecular material of A. luteobubalina that suggested sexual reproduction.