are single celled (such as yeasts) but others are
multicellular, such as the moulds and the more familiar mushrooms 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, or from dead organic matter,
so contributing to its breakdown in the soil.
|Figure 4.10 Fungi
showing fruiting bodies
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
- 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.
kingdom includes a very large number of species that have a
significant influence on horticulture mainly as pests or
as contributors to the recycling of organic matter.
Some of the most familiar animals are in the phylum Chordata
includes mammals, birds, fish, reptiles and amphibians. Mammal pest
species include moles, rabbits, deer, rats and
mice. Bird pest species are numerous including pigeons and bullfinches,
but there are very many that are beneficial in that they feed harmful
organisms such as tits that eat greenfly. Less familiar are important
members of the phylum Nematoda
(the round worms) that includes
a very large number of plant disease causing organisms including
Stem and Bulb Eelworm, Root Knot Eelworm,
Chrysanthemum Eelworm and Potato Root Eelworm.
are the most numerous animals on earth and include insects, centipedes, millipedes and spiders; many of these are
dealt with in the topic on plant pests, but it should be
noted that there are many that are beneficial e.g. honey bees
and centipedes, which are carnivorous and many live on insect species
that are harmful. Phylum Annelida
(the segmented worms) includes
earthworms, which are generally considered to be useful organisms
especially when they are helping to decompose organic matter or improving soil structure, but some species cause
problems in fine turf when they produce worm casts. Phylum Mollusca
is best known for the major pests: slugs and snails.
are single-celled organisms sometimes arranged in chains or
groups (colonial). They are autotrophic (can produce their own energy
supply and organic molecules); some photosynthesize, but
others are able to make organic molecules using the energy released from
chemical reactions usually involving simple inorganic compounds. They
have great importance to horticulture by their beneficial activities in the
soil, and as causative organisms of plant diseases.
Algae and lichens
, comprising some 18000 species, are true plants, since
they use chlorophyll to photosynthesize. The division
Chlorophyta (green algae) contains single-celled organisms that require
water for reproduction and can present problems when blocking
irrigation lines and clogging water tanks. Marine algal species in
Phaeophyta (brown algae) and Rhodophyta (red algae) are multicellular,
and have leaf-like structures. They include the seaweeds, which
accumulate mineral nutrients, and are therefore a useful source of
compound fertilizer as a liquid feed. (The blue-green algae, which can
cause problems in water because they produce unsightly blooms but are
also toxic, have been renamed cyano-bacteria and placed in Kingdom
|Figure 4.11 Lichen – a combination of
fungi and algae
Classification is complex, since each lichen consists
of both fungal and algal parts. Both organisms are
mutually beneficial or symbiotic. The significance
of lichens to horticulture is not great. Of the 15000
species, one species is considered a food delicacy
in Japan. However, lichens growing on tree bark or
walls are very sensitive to atmospheric pollution,
particularly to the sulphur dioxide content of the
air. Different lichen species can withstand varying
levels of sulphur dioxide, and a survey of lichen
species can be used to indicate levels of atmospheric
pollution in a particular area. Many contribute to development. Lichens are also used as a natural dye, and can
form an important part of the diet of some deer.