Class three - the Ascomycetes
Fungi with septate hyphae. Most species produce asexual spores known as conidia, which always germinate by means of a germ tube, never by the production of zoospores. Sexual reproduction results in the formation of ascospores in minute structures known as asci. The asci normally aggregate in a fruiting body but in the fungus causing peach leaf curl, Saphrina deformans, the asci are borne freely on the leaf surface. The yeasts are members of this class. The Ascomycetes are divided into three groups of significance to gardeners:
1. Plectomycetes - asci contained in a closed perithecium which normally bears long hair-like appendages, the blue and green moulds.
Example: Erysiphe polygari- powdery mildew of peas
2. Discomycetes - asci borne in a cup shaped perithecium (apothecium), the cup fungi
Example: Sclerotinia fructigena - brown rot of apples and pears
3. Pyrenomycetes - asci borne in a flask-shaped perithecium. In parasitic fungi these perithecia are frequently embedded in the host tissue.
Examples: Venturia inaequalis - apple scab Nectria galligena - apple canker
Class four - Basidiomycetes
Probably the best known group of fungi as it includes the mushrooms, toadstools and bracket fungi. Fungi with septate hyphae. Asexual spores are lacking in some members of this group, but are present in others. The sexual stage consists of a basidium at the end of which basidiospores are produced. Normally each basidium produces four spores, an important feature to note is that the spores are never borne within the basidium as the ascospores are within the ascus of the ascomyctes fungi. The class is divided into three main groups of significance to gardeners:
1. Ustilaginales - spores typically dark (giving them the common name of smut fungi), thick walled and many celled. On germination a basidium and basidiospores are produced, no asexual spores are produced. All members of this group are parasitic.
Example: Urocystis cepulae - onion smut
2. Uredinales - all members of this group are parasitic. Life cycles are very complex frequently involving two hosts in which case the fungus is said to be heteroecious. Both asexual and sexual spores are produced, the basidium resulting from the germination of a dark, thick walled, one or many celled teleutospore. The spores, borne in groups on leaf or stem surfaces are often orange coloured giving the group the name rust fungi.
Example: Uromycetes fabae - bean rust
3. Agaricales-mushrooms, toadstools and bracketfungi belong here. A large group of which many members are saprophytes living in humus rich soils, or on dead wood etc. Asexual spores usually lacking. Basidia borne either on the gills of typical mushroom like fruiting body or on a bracket-shaped fruiting body. In some cases the basidia may line pores or be borne on spikes on the fruiting body instead of being borne on gills.
Examples: Armillaria mellea - honey fungus
Merulius lacrimans - dry rot of timber
Psalliota campestris - the field mushroom
Stereum purpureum - silver leaf disease of plums and related species
Class five - Fungi imperfect!
A large and varied group with septate hyphae in which no fertile (sexual or "perfect") stage is known. Many fungi once placed in this group have since been found with fruiting bodies and been renamed and placed elsewhere - although sometimes the name of the imperfect stage is retained for convenience. For example the snow mould of turf, Fusarium nivale, is the imperfect (infertile) stage of the Ascomycete Calonectria graminicola.The correct name is that of the rare fertile form but the better known name of the commoner, imperfect, form is still used. There are three groups of importance to gardeners:
1. Moniliales - conidia borne freely on short branches of the hyphae.
Examples: Botrytis cinerea - grey mould
Verticilium alboatrum - tomato wilt Cladosporium fulvum - tomato leaf mould
2. Melancoliales-conidiophores grouped into pustules oracervuli.Conidia usually in tendrils embedded in mucilage.
Example: Colletotrichum lindemuthianum - anthracnose of French beans
3. Sphaeropsidales - conidiophores contained within flask shaped pycnidia, which in parasitic species are embedded in the host tissues and conidia in mucilagenous tendrils.
Examples: Septoria appi-graveolentis - leaf spot of celery Ascochyta pisi- leaf and pod spot of peas
Class six - Mycelia sterilia
A small group of fungi in which the formation of spores is unknown. The hyphae overwinter by means of sclerotia - a hard thick walled mass of hyphae.
Example: Sclerotinia ceptorum - white rot of onions
Further reading:
The Penguin dictionary of biology
Collin's guide to pests diseases and disorders of garden plants
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