Australian honey fungus

Appearance

Up to 10 cm in diameter, the cap is convex to flattened in shape with a central umbo 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 and is up to 20 cm long by 1.5 cm 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 , non-amyloid , and typically measure 6.5–7.5 by 4.5–5.5 μm. The basidia are thin-walled, hyaline, and lack clamp connections at their bases. They are usually four-spored but occasionally two-spored, with sterigmata up to 4 μm long. The cheilocystidia are mostly club-shaped, thin-walled, hyaline, and measure 15–30 by 6–10 μm.Trees that are infected by ''A. luteobubalina'' show characteristic symptoms both above and below ground. Above the ground, the base of the tree develops inverted V-shaped lesions, and the infected wood undergoes white rot, a fungal wood decay process where the cellulose and lignin of the sapwood are both broken down, leaving the wood stringy. The bark of the stem dies and becomes discoloured up to 3 m above the ground. Clusters of fruit bodies appear at the base of the tree in autumn. Crowns may show gradual deterioration, or tree death may occur suddenly. Below the ground, characteristic symptoms of infections include rotting the ends of tree roots, white-rotted sapwood, and the presence of fan-shaped areas of white mycelium below dead or infected bark.

Naming

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 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.

Distribution

''Armillaria luteobubalina'' has been recorded in southeastern Australia, from the southeastern corner of Queensland through eastern New South Wales and across Victoria into southeastern South Australia. It also occurs in Tasmania and southwestern Western Australia. Those of the karri forests of the southwest have paler and yellower caps than those in the jarrah forests further north. The fruit bodies arise on wood, especially on stumps or around the base of trees, and often in huge numbers. They usually appear between April and July, although most production occurs in the second half of May. Abundant in woodlands, it can invade gardens and orchards, where it can attack many woody plants. The honey fungus infected and killed many plants near tuart trees which had been cut down near Kings Park in suburban Perth. ''Armillaria luteobubalina'' is commonly found in eucalyptus forests in Australia, and is thought to be the most pathogenic and most widespread ''Armillaria'' species in the major western Australian forest types. The mushroom has also been reported from southern South America, in Argentina and Chile. A 2003 study of the molecular phylogenetics and pattern of its distribution in South America and Australia indicate that ''A. luteobubalina'' is an ancient species, originating before the separation of the precursor supercontinent Gondwana. Genetic differences between isolates in the South American and Australian populations indicate a long period of geographical separation, and the authors suggest that they "later might be regarded as independent taxa".

Habitat

''Armillaria luteobubalina'' has been recorded in southeastern Australia, from the southeastern corner of Queensland through eastern New South Wales and across Victoria into southeastern South Australia. It also occurs in Tasmania and southwestern Western Australia. Those of the karri forests of the southwest have paler and yellower caps than those in the jarrah forests further north. The fruit bodies arise on wood, especially on stumps or around the base of trees, and often in huge numbers. They usually appear between April and July, although most production occurs in the second half of May. Abundant in woodlands, it can invade gardens and orchards, where it can attack many woody plants. The honey fungus infected and killed many plants near tuart trees which had been cut down near Kings Park in suburban Perth. ''Armillaria luteobubalina'' is commonly found in eucalyptus forests in Australia, and is thought to be the most pathogenic and most widespread ''Armillaria'' species in the major western Australian forest types. The mushroom has also been reported from southern South America, in Argentina and Chile. A 2003 study of the molecular phylogenetics and pattern of its distribution in South America and Australia indicate that ''A. luteobubalina'' is an ancient species, originating before the separation of the precursor supercontinent Gondwana. Genetic differences between isolates in the South American and Australian populations indicate a long period of geographical separation, and the authors suggest that they "later might be regarded as independent taxa".

Evolution

''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 . 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 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 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'' , ''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.