Fungi and their classification pt 2

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Fungi and their classification pt 2

John Evans, Tredegar, Gwent

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

Glossary of Fungi
Ascus	a microscopic, club shaped cell in which ascospores are produced.
Aseptate	having no septa.
Basidium	a microscopic cylindrical cell at the end of which the basidiomycete fungi produce basidiospores. Carbohydrate - compounds such as sugars, starch and cellulose made from the elements carbon, hydrogen and oxygen.
Cell	the basic unit from which plants are built up. Can be likened to the bricks and other materials making up a house. Just as parts of a house are specialised to perform particular functions, e.g. the plumbing, so some cells in a plant are specialised.
Cellulose	the carbohydrate material that is the basis of cell walls in plants.
Conidia	reproductive cells produced asexually, that is vegetatively, by many fungi, conidiophores are the "stems" on which they are produced. As with vegetative propagation in garden plants such spores germinate to produce fungi identical with the parents.
Gamete	any sexually produced reproductive cell.
Hypha	the hollow threads from which a fungus is made, plural hyphae.
Mycelium	a mass of hyphae making up a whole fungus.
Organic	in the context of chemistry this means a chemical compound containing the element carbon. Not all carbon containing compounds are produced naturally in plants or animals (but they are still organic to the chemist) and not all compounds found in plants or animals contain carbon. Those compounds not containing carbon are, to the chemist, inorganic even if they are present in a plant or animal.
Parasitic	a fungus, or other living organism, that obtains its food from another living organism while that organism is still alive. Parasites of plants can be animals, bacteria, fungi or viruses. Plasmodium - in the context of this article a mass of fungus matter that is not divided up into cells and which can flow over a surface - not unlike a large amoeba which many of us will remember from our school days!
Saprophyte	a fungus, or other living organism, that feeds by digesting dead plant and animal material, normally in the soil. Many fungi are saprophytic and do no harm to plants or animals.
Septate	a fungus thread (hypha) that is divided up by cross walls is said to be septate, each cross wall is a septum.
Sporangium	any structure producing spores, plural sporangia.
Spore	the reproductive part of a fungus. Those spores such as ascospores and basidiospores produced after sexual reproduction can be likened to seeds.
Synthesis	the creation of complex chemicals from simple ones.
Zoospore	a spore that can swim in films of water on plants or in the soil

This article originally appeared in the Members Bulletin, the journal of the National Vegetable Society, which is sent quarterly to members. You can Join the National Vegetable Society here

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