Lecture #15

Inoperculate Discomycetes: Leotiales, Rhytismatales

Apothecium- cup, disk, tongue-like
Ascus- unitunicate, lacks operculum

Rhytismatales

Produce ascocarps in stromata; ascocarps are black externally and may be sphaerical, discoid, or elongated in shape; ascocarps are superficially similar hysterothecia, specialized ascocarps of some loculoascomycetes

Asci that may blue in iodine or not and paraphyses

Asci exposed by the rupture of the stromatal layer covering the hymenium; rupture may be by a single slit or by several radial splits to give a stellate appearance.

Ascospores range in shape from ovoid to filiform and are hyaline or brown, often with a gelatinous sheath (epiplasm); the spores often are septate and asymmetrical with the upper portion being broader

The species of the order are saprobes, endophytes, and plant pathogens, such as Hypoderma, Lophodermium, and Rhabdocline, which cause needle cast diseases of conifers.

Rhytisma

• Most species of Rhytisma produce numerous apothecia in each stoma
• Ascocarps of Rhytisma acerinum overwinter in fallen, dead maple leaves. The flat, circular, black, tar-like stromata, which bear the apothecia within them, give the disease its name, tar spot of maple. The surface of each mature stroma is characterized by radiate fissures along which the stroma splits above the apothecia in the spring
• Apothecia are small, saucer-shaped structures
• When the stroma splits open, long needle-shaped ascospores, each with a gelatinous sheath, are released in great numbers from the apothecia by a puffing action that liberates visible clouds of spores to be picked up by air currents and dispersed.
• After reaching a susceptible host, the ascospores adhere to the underside of the leaf by their sheath, germinate by germ tubes that invade the maple leaf through the stomata and infect the epidermal and mesophyll cells

• Mycelium of Rhytisma acerinum grows within the upper epidermal cells and forms compact hyphal masses, the ascocarp primordia; this results in the infected epidermal cells splitting horizontally and forms a pocket in which the fungus develops its reproductive structures

• The top of this black stroma, which contains the upper half of the split epidermal cells, bulges over the leaf and looks like a spot of tar.

• Structures that resemble acervuli form within the stroma. These appear as small pimples with a minute hole in the center; their function is not known but they have been called spermogonia, and they produce enormous numbers of minute, rod-shaped "spermatia" that exude through the perforations in the stroma above them. No asexual method of reproduction has been found in Rhytisma acerinum, and it is not known how the fungus spreads during the growing season.

• The somatic hyphae continue to grow and spread, the stroma enlarges considerably beyond the spermogonial area and develops radiate wrinkles under which the apothecia begin to develop

• Toward the end of the growing season ascogonia are produced among the apothecial initials, which form profusely within the stroma around the spermogonia and, in some cases, within them.

• Sexual reproduction is not completely understood;
  • each multinucleate ascogonium bears a multinucleate trichogyne;
  • the septum that separates these two parts partially dissolves, and
  • the trichogynal nuclei pass into the basal ascogonial cell and pair with the ascogonial nuclei;
  • the nuclear pairs then pass into the ascogenous hyphae, which grow out of the ascogonium, and
  • the asci and ascospores are formed,
  • spermatia have not been observed to be functional in fertilization.

Rhytisma acerinum passes the winter in the fallen leaves in the immature ascocarp stage, developing slowly during the winter months; in early spring, the asci complete their development; the stroma splits along the preformed, radiate lines, the needle-like ascospores are forcibly ejected

Ostropales

Apothecia are oval to lirrelate (?sp?)

Asci are cylindrical and have thin-thick lateral walls

In some species, a thick apical cap perforated by a narrow pore through which the ascospores are discharged

Ascospores are several septate, and maybe filiform, sigmoid, or muriform in shape.

Paraphyses may be simple or branched, but do not fuse to form an excipulum

Known primarily from temperate regions and at high elevations in the tropics

Some species in the order are saprobes that occur in xeric habitats, including exposed plant stems, tree bark, and wood, and many of these species have tough ascocarps that can be revived upon wetting.

Species of Stictis often can be found on bare exposed fence posts; Other species are biotrophic parasites of lichens (Skytta) and certain plants (including Nothofagus). Some are necrotrophic on mosses (Bryodiscus). The parasitic species usually have very strict host relations. Graphidaceae is one of the families that includes lichen-forming species.

Poorly studied group

Leotiales (Helotiales) The largest of the orders of inoperculate discomycetes

Cup-, disc-, tongue-shaped apothecia

Asci only slightly thickened at the apex

Ascospores may be septate and can be round, elliptical, elongated, or on rarer occassions, thread-like

Many members of Leotiales live saprobically on the soil, on dead wood, on dung, or on other organic matter.

Some of them are parasitic on plants;Sclerotinia sclerotiorum, the cause of lettuce drop and other vegetable diseases

Large order has been divided into about ten families

Sclerotiniaceae

One of the largest families of all inoperculate discomycetes and, from a plant pathogen standpoint, the most important one

Apothecial initials arise from stromata or sclerotia a

Apothecia are of medium size, generally brown, and are most often borne on stalks

Ascospores are generally hyaline, one-celled, oval, or somewhat elongated.

Geoglossaceae

Saprobic species

Ascocarps of which are modified into tongue-, club-, or fan-shaped apothecia with long stalks

Found in forests, growing on soil, decaying leaves, wood, or other organic matter that contains considerable moisture.

Hymenium covers the surface of the upper portion of the mature ascocarp

Asci are elongated, and the ascospores vary from one- to many-celled, and from hyaline to dark-brown.

Ascocarps are club-shaped, spoon-shaped, capitate, or pileate and this character has been important in defining genera.

Geoglossum with black or brown club- to tongue-shaped apothecia

Spathularia with spoon-shaped ascocarps

Leotia with palid yellow or green club-shaped to pileate apothecia

Orbiliaceae

Studies of moist chamber cultures of dung led to the discovery that some species of Orbilia have Arthrobotrys anamorphs; Arthrobotrys has long been the primary example of nematode-trapping fungi, and several species are grown easily in agar culture and when nematodes are added they produce modified branching hyphae called traps that entangle the nematodes

Cyttariales

Sole genus of the order,Cyttaria, is known only from species of Nothofagus, the Southern Hemisphere beech, and is found in southern South America and Australasia (Australia and New Zealand)

Specimens were taken from South America to Britain by Charles Darwin where they first were described by the Rev. Miles Berkeley.

Cyttaria has been the object of biogeographical and phylogenetic interest. The fungus is not found throught the range of Nothofagus and is absent in Papua-New Guinea and New Caledonia. It is believed that there are two lineages within the genus that after the separation of land masses, evolved independently in the two distant regions (Korf, 1983), and coevolution with some fungus-host pairs has been suggested from cladistic analysis (Crisci et al., 1988).

The white, bright orange, or yellow fleshy stromata of Cyttaria are edible, and egg yolk- to golf ball-sized stromata are regularly sold in markets in southern Chile.

Cyttaria is characterized by ascocarps that are embedded in fleshy, globose stromata produced in large clusters that fuse secondarily. Conidia are produced in pycnidia on a stroma and, later, the apothecial initials are formed in several places within the same stroma

The developmental details of Cyttaria that have been reported are contradictory (Gamundia, 1991); the ascus has been described as both operculate and inoperculate. (Mengoni,1986) study of fresh material revealed a wide amyloid pore that became compressed as a ring when pressure was exerted by the mature spores; ascospore discharge ruptured the ring giving unmistakable evidence it was not a functional operculum

Electron microscopy revealed an ascus apex that she compared to to that of Bulgaria, a member of the Helotiaceae; DNA sequence analysis indicated a relationship with Helotiales (Lanvik and Eriksson, 1994).