Some fungi are single celled (such as yeasts) but others are multicellular, such as the moulds and the more familiar mushrooms

Horticulture MushroomDowny Mildew Fruiting Body
Figure 4.10 Fungi showing fruiting bodies

and toadstools. Most are made up of a mycelium, which is a mass of thread-like filaments (hyphae). Their cell walls are made of chitin. Their energy and supply of organic molecules are obtained from other organisms (heterotrophic nutrition). They achieve this by secreting digestive enzymes on to their food source and absorbing the soluble products. They obtain their food directly from other living organisms, possibly causing disease (see Chapter 15), or from dead organic matter, so contributing to its breakdown in the soil (see Chapter 18).

Fungi are classified into three divisions:

  • Zygomycota (mitosporic fungi) have simple asexual and sexual spore forms. Damping off, downy mildew, and potato blight belong to this group.
  • Ascomycota have chitin cell walls, and show, throughout the group, a wide variety of asexual spore forms. The sexual spores are consistently formed within small sacs (asci), numbers of which may themselves be embedded within flask-shaped structures (perithecia), just visible to the naked eye. Black spot of rose, apple canker, powdery mildew, and Dutch elm disease belong to this group.
  • Basidiomycota have chitin cell walls, and may produce, within one fungal species (e.g. cereal rust), as many as five different spore forms, involving more than one plant host. The fungi within this group bear sexual spores (basidiospores) from a microscopic club-shaped structure (basidium). Carnation rust, honey fungus, and silver leaf diseases belong to this group.

An artificially derived fourth grouping of fungi is included in the classification of fungi.

• The Deuteromycota include species of fungi that only very rarely produce a sexual spore stage. As with plants, the sexual structures of fungi form the most reliable basis for classification. But, here, the main basis for naming is the asexual spore, and mycelium structure. Grey mould (Botrytis), Fusarium patch of turf, and Rhizoctonia rot are placed within this group.

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