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Class Pucciniomycetes

Known collectively as rust fungi, the Pucciniomycetes do not produce basidiocarps (fruiting bodies). They have complicated life cycles, producing up to five morphologically different spore stages that may alternate between two unrelated plant hosts. They are obligate parasites, requiring a living host to complete their life cycles. Resting spores (teliospores) germinate to produce a four-celled basidium. Each cell of the basidium develops a single basidiospore. Many rust species have one or more rusty, orange-coloured spore types, which is the reason for the common name of this class.

The Pucciniomycetes consist of 5 orders, 31 families, 229 genera, and more than 8,400 species. Only one order, the Pucciniales, contains families that cause damaging foliar and cone blights, stem galls, cankers, and witches’ brooms of conifers and broadleaf trees.

Order Pucciniales

The Pucciniales are one of the largest orders of fungi, with 26 families, 205 genera, and more than 8,000 species. They have up to five different spore stages: spermatia, aeciospores, urediniospores, teliospores, and basidiospores. Often the aeciospores and spermatia are produced on one host (e.g., conifer needles), and the remaining spore stages on another unrelated host (e.g., on leaves of poplar, Populus). When all spore stages are produced, rusts are called macrocyclic. When the rusts alternate between two different hosts, they are called heteroecious. Most rust genera are host-specific to particular plant families, and this characteristic is also used to delimit the rust families.

Family Coleosporiaceae

This family consists of species that have aecia encased in a periderm and occur on spruce (Picea) or pine (Pinus) foliage, young branches, or cones. Uredinia are either diffuse or cylindrical, and teliospores often develop in columns on the alternate hosts. Most are heteroecious and macrocyclic (with exceptions such as Chrysomyxa weirii, which is microcyclic on spruce). Alternate (uredinial and telial) host plant families include Asteraceae (aster) and genera of Ericaceae, including Labrador tea and various rhododendrons (Rhododendron), , blueberry (Vaccinium), raspberry (Rubus) and others. Forest diseases caused by these rusts include witches’ broom of spruce, spruce cone rusts, spruce needle rusts, and needle rusts of pine.

Family Cronartiaceae

This family consists of species that have aecia covered with a periderm and occur on pine (Pinus), causing cankers or galls. Aeciospore masses are orange and interspersed with white filaments. Uredinia open with a central pore, and teliospores form in hair-like columns on foliage of alternate hosts, including paintbrush (Castilleja), toad flax (Comandra), sweet gale (Myrica), oak (Quercus), currant (Ribes), and other herbaceous or shrubby plant genera. Some species have reduced life cycles and occur only on pine (e.g., Cronartium harknessii).

Family Gymnosporangiaceae

This family consists of species that have aecia that occur on leaf spots of rose family (Rosaceae) genera, such as Saskatoon berry (Amelanchier), hawthorn (Crataegus), cotoneaster (Cotoneaster), and pear (Pyrus). The aecia are horn-shaped, with the periderm rupturing longitudinally to release the spores. Telia, which are orange and gelatinous, form on branches of juniper (Juniperus). Infections cause galls on juniper branches.

Family Melampsoraceae

This family consists of species whose aecia usually lack a periderm, exposing spores by rupturing through needle epidermis of conifers in Pinaceae, including fir (Abies), larch (Larix), Douglas-fir (Pseudotsuga), pine (Pinus), and hemlock (Tsuga). Uredinia and telia form on the alternate hosts, poplar (Populus) and willow (Salix), causing a leaf blight.

Family Milesinaceae

This rust family consists of species that produce colourless urediniospores on ferns (Polypodiaceae), except for Naohidemyces, whose urediniospores are orange and occur on Ericaceae (predominately blueberry [Vaccinium]). In Canada, aecial hosts are on needles of true fir (Abies) and hemlock (Tsuga). Aecia are colourless and tubular, bearing colourless verrucose aeciospores. Uredinia open with a central pore. Telia, which undergo dormancy before germinating externally, are formed either in epidermal cells or in a crust under the epidermis.

Family Pucciniastraceae

This family consists of species that have aecia with orange aeciospores covered by a tubular periderm and occur on the foliage of conifers, predominately fir (Abies), larch (Larix), spruce (Picea), and hemlock (Tsuga). Uredinia and telia form on a wide variety of herbaceous and shrubby plants. In Canada, this list includes bearberry (Arctostaphylos), chickweed (family Caryophyllaceae), fireweed (Epilobium), and blueberry (Vaccinium). Uredinia open with a central pore. Telia, which undergo dormancy before germinating externally, are formed either in epidermal cells or in a crust under the epidermis.