The fungi have a complex and rather confusing taxonomy. For simplicity, let's just consider the main groups relevant to plant pathology.
| Fungi |
| ||||||||||||
The Chytrids (Chytridiomycota) are thought to be sister to almost all other fungi. The pathogen coloquially known as ‘chytrid fungus’ which is devastating amphibian populations worldwide also belongs to this group.
The Ascomycota include the Powdery Mildews, Ergots, and a bunch of less well known groups of pathogens.
The Basidiomycota include the Smuts, Rusts, and Exobasidiales. They also include the Agaricus mushrooms you buy in the shop.
The genus Synchytrium (False Rusts) forms warty pustules on many different hosts. These can be colourless, orange, red, or brown. They produce zoöspores which infect further parts of the plant by swimming through films of water. Synchytrium is the only group of chytrids which is a major pathogen of plants.
They are usually inconspicuous and as a result most are likely very underrecorded. The only species which is commonly recorded in Ireland is Synchytrium taraxaci.
Powdery Mildews are very common in most habitats. They are highly diverse and most are host-specific, but without microscopy they mostly look quite similar. Often they can be distinguished by host alone, as many hosts have only one known powdery mildew.
Ergots form dark brown to black sclerotia in the flowers of grasses and Eleocharis. They all look quite similar, but several different species can be distinguished. The important characters are the host and whether the sclerotia float or sink when placed in water for 2 hours or longer1,2.
Smuts are a polyphyletic group. This means the smut lifestyle has evolved independently several times, so when we consider the smuts as a group we are considering several different branches of the tree of life. What unites them is producing large numbers of teliospores on the plants they parasitise[source?].
Smut fungi can be divided roughly into flower smuts and leaf smuts.
| Basidiomycota |
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Flower smuts classically produce large numbers of teliospores in the anthers of their hosts, replacing the pollen. There are also a number of species which produce teliospores in the ovaries of their hosts, like Urocystis primulae and Tilletia spp. These species generally produce conidia in the anthers of the flower as well.
Most Urocystis species form spores in the leaves and are under Blister Smuts.
Note not all Ustilago species are flower smuts — some, like Ustilago perennans, form spores in the leaves, so I have put them under Blister Smuts.
These form dark brown powdery blisters on the leaves of plants. There seem to be a lot of species on grasses and sedges, and in these hosts they follow the direction of the veins, forming long brown lines that split open like zips along the length of the leaf.
Rusts are a widespread, common, and iconic group of plant pathogens. Many of them have complex lifecycles with multiple stages on different hosts.
These are the ‘typical’ rusts that are often the most conspicuous.
Very variable in form.
Small warty orange uredinia on many different herbaceous and shrubby hosts. They also form telia but these are minute. At least some have an aecial stage on trees.
White uredinia on ferns. They often look like a dusty or dirty residue on the underside of the fronds, sometimes causing brown dead patches as well.