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Class Dothideomycetes

The Dothideomycetes are the largest Ascomycota class, with great diversity of forms and habits. This class consists of 45 orders, 231 families, 2,309 genera, and more than 32,000 species. Most of its members produce very small spore-bearing structures known as ascomata. The asci develop within locules of fungal tissues and are double walled (bitunicate). During ascospore release, the inner ascus layer bursts through the thicker outer wall layer like a jack-in-the-box. 

Seven orders of Dothideomycetes have species within this database that are relevant to forest pathology: Asterinales, Botryosphaeriales, Capnodiales, Dothideales, Mycosphaerellales, Pleosporales, and Venturiales.

Order Asterinales

The Asterinales consist of 7 families, 120 genera, and more than 1,560 species. These fungi are parasitic, deriving nutrients from living plant foliage, stems, and fruit by penetrating cell walls with specialized hyphae known as haustoria. Their ascomata are often flattened, less than 1 millimetre in diameter, and dark. Ascospores are usually dark and multiseptate.

Family Asterinaceae

This family of species consists of leaf spot fungi, found on foliar surfaces of conifers and broadleaf plants. They have dark mycelium and produce tiny dark, flattened ascomata that release dark, multicelled ascospores from pores or longitudinal fissures.

Order Botryosphaeriales

The Botryosphaeriales consist of 10 families, 90 genera, and more than 4,100 species. Many are pathogens, causing leaf spots, fruit and root rots, dieback, or cankers on woody host plants, while others are saprophytes or endophytes. Asci are produced in perithecia, and ascospores are usually single-celled, and colourless or yellow. Many genera also form pycnidia, which are microscopic flask-like structures producing conidia (asexual spores) that are initially single-celled and then become dark with a single septum.

Family Botryosphaeriaceae

This family consists of species that have perithecial ascomata, sometimes aggregated, usually associated with woody plant tissues, either as a parasite or a saprophyte. Ascospores are usually single-celled, and colourless or yellow. Many genera also form pycnidia, which are microscopic flask-like structures producing conidia that are initially single-celled and then become dark with a single septum. 

Family Phyllostictaceae

This family consists of species that have dark ascomata erupting from leaf spots, producing ellipsoid to lemon-shaped ascospores. Pycnidia produce single-celled colourless conidia with a mucilaginous sheath and a single appendage. 

Order Capnodiales

The Capnodiales consist of 18 families, 144 genera, and more than 980 species. Most of the genera are sooty mould fungi, forming crusts on the surface of leaves and stems of their plant hosts. They are often associated with insect excretions. Their mycelium and tiny ascomata are dark, sometimes ornamented with setae, and covered with mucus. Ascospores are brown and septate. Some genera produce little surface mycelium and are associated with needle cast of conifers.

Family Capnodiaceae

This family of species consists mostly of sooty mould fungi, but a few of its genera (e.g., Phaeocryptopus) have little or no visible surface mycelium and are pathogens of conifer foliage, producing near-microscopic black ascomata that erupt from the stomates (pores) on the undersurface of conifer needles.

Order Dothideales

The Dothideales are characterized by vegetative hyphae that form dark chains of cells, often referred to as black yeasts. Ascomata, when present, are small, often with a single central locule opening with a central pore. Many species are known only by their asexual conidial state. Some genera in the Dothideales are so morphologically similar that they are currently only distinguished by genomic sequences. This order consists of 4 families, 69 genera, and more than 760 species.

Family Dothioraceae

This family consists of species that have small black ascomata immersed or breaking out of host tissues. Some species produce conidia in pycnidia, and black yeast-like vegetative hyphae. Members are parasitic or saprophytic on woody plants, including conifer shoots and needles. Asci are club-shaped, and ascospores are single-celled to multiseptate.

Family Saccotheciaceae

This family of species consists of “black yeasts” occurring on foliage and many other substrates, producing hyphae that appear like chains of beads, at first colourless then becoming black. Many produce only asexual spores (conidia), which bud from the hyphae. A very common black yeast, Aureobasidium, belongs in this order despite its misleading name, which suggests that it is a Basidiomycete when it is an Ascomycete.

Order Mycosphaerellales

The Mycosphaerellales include many species that cause severe foliar blights and stem cankers on broadleaf trees, and needle casts on conifers. Most species in this order produce ascospores, which are usually colourless and have one or more transverse septa, and which are produced in tiny flask-shaped ascomata called perithecia. Conidia (asexual spores), when present, are produced from clusters of conidiogenous cells, or in pycnidia, which extrude cylindrical septate conidia. The Mycosphaerellales consist of 5 families, 232 genera, and more than 6,700 species.

Family Mycosphaerellaceae

Many species in this family cause serious foliar blights and cankers on broadleaf trees, and needle casts on conifers. Ascospores are usually colourless, have one or more transverse septa and are produced in tiny flask-shaped ascomata called perithecia. Conidia are produced from clusters of conidiogenous cells, or extruded from pycnidia, and are cylindrical and septate.

Order Pleosporales

The Pleosporales consist of 94 families, 726 genera, and more than 11,000 species. This order is the largest in the Dothideomycetes. Some Pleosporales are plant pathogens, but the majority are saprophytic on decaying plant matter. Most sexually reproducing species produce asci in perithecia, and the asci are interspersed with sterile strands of hyphae called pseudoparaphyses.

Family Didymosphaeriaceae

This family consists of species that have dark perithecial ascomata with central openings that are immersed or erupting from woody or herbaceous host tissue. Some species are associated with cankers on broadleaf trees. Asci are cylindrical, bearing ascospores that are usually brown with one cross wall.

Family Melanommataceae

This family consists of species that have dark perithecial ascomata that erupt from wood and bark, and are clustered and appear superficial when fully mature, usually opening with a central pore. Ascomata are sometimes nestled in woolly superficial mycelia that form thick coatings on the host substrate. Ascospores are brown, often with multiple crosswise and longitudinal walls (muriform).

Order Venturiales

The Venturiales consist of 3 families, 64 genera, and more than 530 species. Most of this order’s members occur in the family Venturiaceae, with perithecial ascomata, irregularly septate darkly pigmented ascospores and a mould-like conidial state that produces dark irregularly shaped masses of conidia (asexual spores) on the surfaces of leaves and stems of mainly broadleaf hosts.

Family Venturiaceae

Making up more than 80% of the order Venturiales, this family of species is characterized by dark perithecial ascomata that are either smooth or hairy (setose), open by a central pore, and have cylindrical asci with two-celled ascospores. The ascospores are usually asymmetrical with one cell larger than the other. Conidial states occur in dark masses of conidiogenous cells and conidia on the host substrate. Conidia are darkly pigmented, sometimes septate, and irregular in shape. Many of the species are associated with leaf spots, scabs, cankers, and shoot dieback.