Agriculture Reference
In-Depth Information
is quite different from fungi, producing a jelly-like
structure (plasmodium) inside the cells of the
host plant (see p. 76).
The hyphae in most fungal species are able,
in the appropriate circumstances, to produce
spores. Two soil-borne fungi,
Rhizoctonia
(Thanatephorus) solani
(that causes a damping-
off symptom, and black scurf disease on potatoes)
and
Sclerotium (Stromatinia) cepivorum
(that
causes white rot on onion, see Figure 19.3)
are two notable exceptions, where the hyphae
only very rarely produce spores. Wind-borne
spores are generally very small (about 0.01 mm),
not sticky and often borne by special hyphae
protruding above the leaf surface (e.g. those of
grey mould), so that they catch turbulent wind
currents. Water or rain-borne spores are often
sticky (e.g. those of damping off).
Minute asexual spores produced without fusion
of two hyphae commonly occur in seasons
favourable for disease increase - for example,
humid weather for downy mildews and dry,
hot weather for powdery mildews. Sexual
spores, produced after hyphal fusion, commonly
develop just before unfavourable conditions
occur (e.g. a cold, damp autumn). They are
produced as single spores in the oomycetes. In
the powdery mildews and many other species in
the Ascomycota (see p. 62), spores are produced
in groups within a protective hyphal spore-
case (like a tiny black spherical flask less than 1
mm in size) often observable to the naked eye.
Different genera and species are identified by
microscopic measurement of the shape and size
of the spores, or the spore-bearing spore cases,
or by the appearance of the hyphae (especially
those special hyphae producing spores, see
Figure 19.2).
Figure 19.3
White rot on onion. Note the black
sclerotia which enable this disease to survive long
periods in the soil
Gardeners without microscopes must use the
symptoms ('easily visible features or signs that
are characteristic of the disease') to identify the
particular problem.
produce a further crop of spores (see Figure 19.5).
Leaf diseases such as potato blight often increase
very rapidly when conditions are favourable.
Roots may be infected by spores (e.g. in damping
off), by hyphae (e.g. in wilt diseases), by sclerotia (e.g.
in
Sclerotinia
rot) or by rhizomorphs (e.g. in honey
fungus). Root diseases are generally less affected by
short periods of unfavourable conditions and often
increase at a slower, more constant rate; although in
hydroponic systems (see p. 183), increase is likely to
be much more rapid.
A
plant disease
is an unhealthy condition in a
plant caused by a fungus, bacterium or virus.
While disease-causing or parasitic fungi are the main
concern of this chapter, it should be realized that in
many parts of the garden and environment, there
are useful saprophytic fungi that usefully break down
organic material such as dead roots, leaves, stems
and sometimes decaying tree stumps (see p. 62), and
there are useful symbiotic fungi that may live in close
association with the plant, e.g. mycorrhizal fungi in
fine roots of conifers and many other garden species.
19
Phyllosphere
Infection
On the surface of leaves and stems, there lives a
population of micro-organisms (mainly bacteria)
which occupy a microhabitat commonly called
the phyllosphere. These bacteria may be 'casual'
or 'resident'. Casual organisms such as
Bacillus
spp. mainly arrive from soil, roots and water,
and are more common on leaves closer to the
ground. These species are capable of rapid
The spore of a leaf-infecting fungal parasite, after
landing on the leaf in damp conditions, produces a
germination tube, which being delicate and easily
dried out, must enter through the cuticle or stomata
within a few hours before dry, unfavourable conditions
recur. Within the leaf, the hyphae grow, absorbing
food until, within a period of a few weeks, they
