The reproduction by means of small spores is a cornerstone in the ecology of fungi. Although a single spore may have a negligible chance of reaching a suitable substrate, spores may be produced in such quantities that even discrete substrates can be exploited by the species as a whole. Only a few fungi make do without spores, surviving solely by means of mycelium and sclerotia.
Spores may be organs of sexual or asexual reproduction, and they are involved in dispersal and survival. Gregory (1966) distinguished between xenospores (Gr. Xenos= a foreigner) for spores which are dispersed from their place of origin and memnospores (Gr. Memnon =steadfast, to persist), which stay where they were formed.
Some spores are violently discharged from the organs which bear them, energy for dispersal being provided by the spore itself or the structure producing it (Ingold, 1971). However, many spores are dispersed passively by the action of gravity, air or water currents, rain splash, or by animals, especially insects.
Dispersal may also occur by human traffic. Spores may be present in the outdoor air at such high concentrations (e.g. 100 Cladosporium spores l-1) that they can cause allergic respiratory diseases when inhaled (Lacey, 1996). In freshwater, the asexually produced spores (conidia) of aquatic hyphomycetes, which colonize autumn-shed tree leaves, may reach concentrations of 10 000 – 20 000 spores l-1.
Long-range dispersal of air-borne spores over thousands of kilometers is known to occur in nature. For instance, the urediniospores of the coffee rust fungus, Hemileia vastatrix, are thought to have travelled from Africa to South America by wind at high altitudes, and the urediniospores of black stem rust of wheat (Puccinia graminis) undergo an annual migration from states bordering the Gulf of Mexico to the prairies of North America and Canada.
These spores are protected from the deleterious effects of UV irradiation in the upper atmosphere by pigments in the spore wall.
Some spores are not dispersed but survive in situ, e.g. the oospores of many soil-inhabiting Oomycota, the zygospores of Zygomycota, and the chlamydospores of Glomales and other fungi. Fungal spores may remain dormant for many years, especially under dry and cold conditions.
An extreme example of spore survival is shown by the recovery of viable spores of several fungi from glacial ice cores, including those of Cladosporium cladosporioides from ice samples 4500 years old.
The morphology and structure of fungal spores show great variability, from unicellular to multicellular, branched or unbranched or sometimes spirally coiled, thin- or thick-walled with hyaline or pigmented walls, dry or sticky, smooth or ornamented by mucilaginous extensions, spines, folds or reticulations.
A number of general descriptive terms have been applied to characterize spores in relation to the number of cells and septa which they contain. Single-celled spores are termed amerospores (Gr. a = not, meros= a part; i.e. not divided), two-celled spores are didymospores (Gr. didymos = double), spores with more than one transverse septum are phragmospores (Gr. phragmos = a hedge, barricade), and spores with transverse and longitudinal septa are dictyospores (Gr. dictyon = a net).
These terms may be qualified by prefixes indicating spore pigmentation such as hyalo- for colourless (hyaline) spores and phaeo- for spores with dark-colored (melanized) walls.
Special terms have also been used to refer to spore shape. Scolecospores (Gr. skolex = a worm) are worm-shaped, helicospores (Gr. Helix= twisted or wound) are spores with a two- or three-dimensional spiral shape, whilst staurospores (Gr. stauros = a cross) have arms radiating from a central point or axis.
Spore septation, color, and shape, along with other criteria such as the arrangement of structures that bear the spores, have been used in classification and identification, especially in conidial fungi which do not show sexual reproduction. These criteria rarely lead to natural systems of classification, but to ‘form genera’ or ‘anamorph genera’ made up of species unified by having similar spore forms.