Order: PUCCINIALES   (Rusts)

kingdom: Fungi,  phylum: Basidiomycota


Rusts (Pucciniales, formerly Uredinales or Urediniomycetes) are obligate parasites of vascular plants. Approximately 7,000 species are known, some (e.g. Puccinia graminis) being economically serious crop pathogens, others being minor or major nuisances in horticulture. Infection is usually local, i.e. forming individual colonies in leaves or other aerial parts of the host and dependent on re-infection each year, but infection is sometimes systemic and persistent in the plant.

Puccinia urticae
Puccinia urticae forming gall with abundant aecia, on stem of Stinging Nettle (Urtica dioica), Weardale, Co. Durham, 2006

Life cycles are usually complex, involving up to five distinct types of spore and often two different hosts. Characteristically, rust fungi produce spores in sori (singular: sorus) that appear to be spots, or rarely stripes, on leaf and stem surfaces. Commonly the sori are brown (hence "rust") but can be white, yellow, orange, purple or black, according to species and spore stage.

The spore stages are conventionally represented by Roman numerals (0, I, II, III, IV):

StageSorus typeSpore stage
0pycnium (or spermogonium)spermatium
Iaecium (or aecidium),
aeciospore (or aecidiospore)
IIuredinium (or uredium)urediniospore (or uredospore)
IIIteliumteliospore (or teleutospore)

Basidia develop from teliospores and so have no independent existence as a spore stage on the host.

Rusts commonly omit one or more spore stages in their life cycle. Those with all five stages are described as macrocyclic.

Rusts may be:
          autoecious     completing their life cycle on one host;
          heteroecious     alternating between two different hosts.

Reproduction and spread:
In heteroecious species, basidiospores and aeciospores are the spores that travel between hosts. Urediniospores enable spread and build-up of infection on a single host. Note that urediniospores usually germinate to produce lesions that produce more uredinia. This allows rapid asexual spread during a single growing season and also enables a heteroecious species to persist on one host, even when its other host is not present. Aeciospores are often thicker-walled than the urediniospores of the same species, no doubt an adaptation that gives greater survival potential and more chance of transmission between hosts.

Sexual reproduction: Nuclear fusion and meiosis in the basidium precede basidiospore formation. Fusion of spermatia with receptive hyphae restores the typical dikaryotic state (i.e. the resulting hyphae contain pairs of genetically distinct haploid nuclei). This equates rusts with other basidiomycete fungi.

Host specificity:
Rusts are usually highly host-specific, being restricted to single or closely related hosts at a particular stage of their life-cycle, though the two hosts of a heteroecious species are usually very different. Furthermore, when a rust infects closely related species, it may show further minor morphological and / or physiological specialisation to single hosts without cross infection being possible. When recognisable, such a strain may be distinguished as a "forma specialis" (plural: formae speciales) and given its own name.
     Puccinia graminis f.sp. tritici (uredinial stage) is restricted (more or less) to wheat,
even though the species occurs on a number of other grasses. This means, in this case, that wild grasses do not act as a reservoir of infection even though they may carry the rust. In heteroecious rusts, it is often the case that two or more formae speciales occur on a single host but differ in their alternate host species.

© A.J. Silverside
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