Parasites of Plants
Above: an oomycete zoospore. The oomycetes (oomycotes, 'egg fungi' or 'water moulds') are
protofungi and most are aquatic. Unlike fungi which produce immotile spores that are dispersed on the
wind, oomycetes produce spores, called zoospores, that require water in which to swim by means of two
flagella. Having two flagella these spores are described as
biflagellate. In the case of those species
which parasitise plants, they may swim in a drop of water on the surface of a leaf. One flagellum is a
tinsel flagellum, a flagella with tinsel-like lateral branches, called mastigonemes, and this flagellum
pulls the cell through the water and so is directed forwards when swimming. The other flagellum is a
smooth
whiplash flagellum, which pushes the cell through the water and so is held backwards. The
flagella motors undulate from side-to-side, with the tip tracing out circles, as waves of movement pass
along their length from the base to the tip. The large spherical structure towards the rear of the
zoospore is a lipid droplet, which may act as a both an aid to buoyancy and a fuel/energy store.
Oomycetes (protofungi)

Prerequisites: fungi.

We begin with a general discussion of the oomycetes. One of the best studied examples,
Saprolegnia,
is not a plant parasite at all, but grows on rotting organic matter, such as dead fish or dead seeds
floating in ponds and the like. Some species are also parasitic, growing on the scale or eggs of fish, and
also on amphibians,
rotifers, nematodes, arthropods and diatoms, depending on species. This water
mould grows as a mycelium of multinucleate threads, called hyphae (see fungi) which are branched
(and lack internal cross-walls so that the protoplasm is continuous throughout their length). The
mycelium absorbs nutrients from the surrounding water and the substrate on which it is growing, that is
it is a
saprobe (or saprotroph).

The plasma membrane of the hyphae is surrounded by a rigid wall, which in at least some oomycetes
contains
cellulose, a polymer of glucose (whereas in true fungi the wall contains chitosan, a chitin-like
polymer). [See carbohydrates for an introduction to sugar-based biopolymers like cellulose and chitin].
The hyphae penetrate the substrate on which the oomycete is feeding, whilst some hyphal branches
stand out into the water. An internal cross-wall may develop in these hyphal branches, separating off a
large portion of the tip as a separate compartment. This terminal compartment enlarges and becomes a
zoosporangium, with zoospores developing inside it. When mature, the zoospores escape through a
pore (ostium) that forms in the tip of the zoosporangium, as shown below:
a Pov-Ray model of an oomycete zoospore
a sporulating germ sporangium
Pov-Ray model of a secondary zoospore
a Pov-Ray model of a sporulating zoosporangium
fertilisation of the oogonium
fertilisation of the oogonium with labels
zoospore
Left: a sporulating zoosporangium.
Note the cross-wall dividing the
zoosporangium from its parent hypha.
The zoospores escape by swimming
backwards, and then disperse by
normal forwards swimming.

Above: close-up of a zoospore.

Click images to enlarge.
This is a process of asexual reproduction. After swimming around for a few minutes (immediately on
leaving the zoosporangium in some species) the zoospores withdraw their flagella, round-up and then
secrete a 'shell' or
cyst around themselves. Presumably they may survive harsh conditions that may
occur whilst so encysted (such as drying of the pond). In good conditions, however, the cyst soon
hatches, and the zoospore re-emerges, only it has metamorphosed into a type which is bean-shaped
and with the two falgella inserted midway along the concave side of the cell body. Such a zoospore is
shown below.
This phenomenon, of having two different zoospore types
is called
diplanetism, and we could call the second type
of zoospore a diplanospore, or secondary zoospore, to
avoid confusion. The reason for this change is uncertain,
though it seems likely that the first arrangement of
flagella assisted the zoospore in reversing out of the
zoosporangium, whilst the second arrangement is
perhaps better suited to dispersal. The second zoospore
type swims around for several hours, so is clearly the
main dispersive stage. However, some oomycetes have
monomorphic zoospores, in which only one type of
zoospore, the secondary bean-shaped type, is produced
directly in the zoosporangium. The more complicated
version,, involving two spore types (dimorphic zoospores)
may be a relic of some past evolutionary form, though
such 'relics' are typically retained when they perform a
useful function.

Eventually the diplanospore withdraws its flagella,
rounds-up and encysts again. This secondary cyst may
emerge as another diplanospore, or it may emerge as a
germinating hypha. In either case, the final cyst will
eventually germinate as a growing hypha, called a germ
tube, which grows from the cyst, feeds and eventually
develops into a new mycelium.

Click image to enlarge.
In addition to this mode of asexual reproduction, oomycetes such as Saprolegnia, at some point
undergo
sexual reproduction. Certain hyphae growing from the substratum produce side-branches
that develop female organs or
oogonia (singular oogonium) at their tips. The oogonium is typically
divided from the hyphal branch supporting it by a cross-wall. The tips of branching male hyphae also
develop end compartments, partitioned off from the rest of the hypha by a cross-wall, called
antheridia (singular antheridium). Oomycetes are oogamous, meaning that the egg cells are immotile
and larger than the sperm. Each oogonium contains several such eggs or ova (singular ovum). Each
antheridium contains several male nuclei. The female hyphae release a hormone called antheridiol,
which stimulates the development of male antheridial branches, and conversely male hyphae release
a hormone called oogonial which induces oogonia development in the female hyphae.

The male hyphae are attracted to the oogonia, and when an antheridium makes contact with the outer
surface of an oogonium, it grows one or more hyphae into the oogonium toward one or more eggs.
Male nuclei then travel down these
fertilisation hyphae to the eggs and one male nucleus fertilises
each egg. (The tip of each fertilisation hypha ruptures when it contacts an egg to allow the sperm to
reach the egg). Typically, several antheridia will fertilise eggs in the same oogonium, though each egg
is only fertilised once. The male and female nuclei fuse to form a diploid nucleus. (Since the parent
mycelia were diploid, it seems that meiosis occurs in the formation of the eggs and male nuclei, as
these germ nuclei are haploid). The fertilised egg then secretes a shell or cyst around itself, becoming
a resting
oospore.
In some species some oospores are also
formed without being fertilised by male nuclei.
In either case, after a period of rest the
oospores, whilst still inside the oogonium,
germinate. A hypha, called a germ tube,
grows out from the oospore, punctures the
oogonium wall and grows out, forming a
club-shaped tip, which eventually becomes
compartmentalised by the formation of a
cross-wall at its base. The end compartment
develops into a
germsporangium, which
produces
germ zoospores (which are
diploid) which are released when ripe and
disperse and eventually encyst and
germinate to produce new mycela.
Above: a sporulating germsporangium. Click image to enlarge.
Oomycetes have helped shape human history!

Some oomycetes have adapted to life on land by parasitising plants, forming so-called
'downy mildews'. One species, incidentally,
Pythium insidiosum, is a parasite of mammals,
causing disease in horses.
Peronospora causes the blue mold disease of tobacco, Albugo
candida
causes the white rust disease of mustard. Plasmopara viticola causes downy mildew
of grapes, which almost destroyed the French wine industry in the 1870's (by accident the
first fungicide, the Bordeaux mixture, consisting of lime and copper sulphate, was found to
treat this disease).

We discuss two groups of oomycetes in this article: the Saprolegniales and the
Peronosporales. The Saprolegniales include
Saprolegnia which we have already considered
in detail. The Peronosporales include
Peronospora and Phytophthora species. These plant
parasites grow inside leaves of the host plant, forming a mycelium in the intercellular air
spaces within the leaf mesophyll (see
leaves). It puts out protuberances, called haustoria,
that penetrate the plant's cells to take nourishment from them. The mycelium then puts-out
aerial hyphae through the stomata of the leaf. These hyphae, called
sporangiophores,
produce air-born spores which may land on an uninfected leaf and germinate in a drop of
water or dew on the leaf surface. When germinating they put out a hyphal thread, called a
germ tube, which locates a stoma (possibly by touch and/or chemosensation, perhaps
detecting carbon dioxide emissions from the stoma) and then enters the leaf through the
stoma pore and infects the leaf tissue, forming a new mycelium. [Bordeaux mixture forms
copper(II) hydroxide in water and this kills the germ tubes which germinate in water].

One of the most influential oomycetes is
Phytophthora infestans, which caused potato blight,
resulting in the Great Irish Potato Famine of 1845-1852. This tragedy killed some one million
people in Ireland and caused a further million to emigrate. Many settled in America, shaping
the culture of the United states of America.
Phytophthora infestans grows by feeding on leaf
tissues. Dead spots appear on the leaves, fringed by growing and feeding mycelium which
spreads out across the leaf, killing it, and puts out sporangiophores through the stomata of
the leaf. These sporangiophores bear sporangia capsules which break away to be dispersed
by the wind. When they land on a new leaf the sporangia will split open in a drop of
rainwater, releasing swimming zoospores which encyst. The cyst germinates, putting out a
growing germ tube. Sporangia may also enter the soil, in which the liberated zoospores can
swim through moisture to plant roots, infected the tubers of the potato plant for example.

A number of species of
Phytophthora continue to cause concern. This oomycete attacks the
cambium, the growing part of tree trunks, roots and branches, responsible for secondary
growth which causes thickening in these structures.
Phytophthora infection produces
rusty-red exudations on the bark and sometimes killing the tree. For example, the 'ink
disease' of sweet-chestnut trees kills the roots and hence sometimes the tree. It favours
waterlogged soils (presumably as this aids zoospore dispersal). A similar disease occurs in
alders;
Phytophthora quercina attacks oaks and another species attacks the horsechestnut.

Fungal diseases of plants

True fungi cause many serious plant diseases. In Britain, in the 1960s, Dutch Elm Disease
killed 90% of elm trees. (The disease is so-called because it wa sfirst reported in Holland in
the 1920s-1930s). This disease is caused by the fungus
Ceratocystis (Ophiostoma) ulmi (an
ascomycete - see
fungi). The spores of this fungus are carried by elm bark beetles. The
disease affects mature trees, but some have survived by growing back from root suckers,
though these become infected again when mature. This cycle of regrowth and infection is
continuing, though the disease, having largely killed its supply of hosts is apparently evolving
to be less virulent, whilst the trees become more resistant. Such destructive diseases are
generally imports and result from an imbalance, with nature tending to restore a balance
over time, in which host and parasite coexist. Wych-elms still survive, as these are among
the few elms that regularly grow from seed in the British isles. Some mature elms survive, for
example in East Sussex where fungicide was applied. The stately elm tree was once iconic of
the English countryside and its sudden demise dramatically altered the landscape. Insects
and fungi that grow largely or exclusively on elm trees were also affected, in short an entire
ecosystem (as tree species are) was decimated.

Oak wilt is caused by another Ceratocystis, Ceratocystis fagacearum, the spores of which
are mainly wind-borne. It has caused serious disease in red oaks and live oaks in Minnesota,
USA, with white oaks being less affected.

Chestnut blight fungus causes bulbous growths or cankers on the trunk of sweet chestnut
trees. These cankers may eventually encircle the infected trunk or branch and kill it. The
spores are wind-dispersed. This disease is also caused by an ascomycete fungus,
Cryphonectria (Endothia) parasitica.

The ergots,
Claviceps, also ascomycetes. Claviceps purpurea attacks rye and makes the  
seeds of this plant toxic for consumption.

Basidiomycete fungi also cause a number of serious plant diseases. The
rust fungi or
Uredinales, of which there are over 5000 species, are exclusively plant parasites. The
life-cycles of rusts are complex. For example,
'black rust' or 'stem rust' disease of wheat is
called by
Puccinea graminis. This fungus alternates between two plant hosts: wheat
(infecting the stem) and barberry (infecting the leaves). This alternating between two hosts is
quite common in animal parasites (such as
tapeworms) but among the fungi it only occurs in
the rusts. This complex life-cycle involves the production of five different types of spore (!) of
which one type are the sexual basidiospores characteristic of basidiomycetes.
Some negative images for printing.
Animal parasites of plants

Fungi and protofungi are the most serious diseases of plants. Many animals parasitise plants,
such as foliage eating insects, and while these may completely destroy herbaceous plants,
they rarely kill trees. Many trees can survive being totally defoliated in bad years.  Oak trees
support a wide range of animal parasites and grazers without any major adverse effect. One
curious example is the acorn weevil,
Curculio, an extraordinary and delightful looking insect
which uses its long curved snout to bore a hole in an acorn, into which it lays its eggs, the
grubs eating the acorn when they hatch. Acorn weevils are one example of the larger family
of nut weevils.

The gypsy moth,
Lymantia dispar, can totally defoliate trees, but oaks and elms are
'designed' to withstand defoliation, for example the oak has lammas shoots, new growth that
appears around mid-sumer and thus replaces any lost foliage. Defoliation occurs in a few
years only and causes decreased tree growth, but generally does not kill trees unless the
trees are weakened by additional stressors like several years of severe drought.

Nematode worms occasionally cause very serious plant diseases, such as the nematode
disease of Japanese pines, which is killing red pines in Japan.

Plant galls. Many insect and mite parasites stimulate the tumour-like growth of plant tissues
in order to nourish and sometimes protect the developing larvae. One example is oak apple
gall. These galls are fairly large (2-5 cm diameter) round apple-like structures that develop
when a gall wasp lays a single egg in a developing leaf bud. The developing larva secretes
plant-hormone mimicking substances which stimulates development of the gall tissue which
nourishes, houses and protects the wasp larva. Similar are the marble galls, hard round balls
about the size of a marble, which also develop when a wasp lays an egg in a leaf bud. The
range of plant galls is astonishing, many occur on fruit, leaf and bud. The complex life-cycles
of gall wasps is also extraordinary and maybe covered in a separate article in future.

Bacterial diseases of plants

One well-known bacterial infection of trees is the crown gall disease, caused by infection by
the bacterium
Agrobacterium tumefaciens, which genetically reprograms neighbouring plant
cells to produce large masses of nourishing tissue. These galls are often visible as large
bulbous outgrowths on tree trunks. The parasitical process is remarkable and may also be
the subject of a future article.

Plant parasites of plants

A number of plants are parasitic or semi-parasitic on other plants. These are covered in a
separate article:
parasitic plants.

Plant diseases in the news

Chalara ash dieback disease, caused by the ascomycete fungus Chalara fraxinea
(
Hymenoscyphus pseudoalbidus) is spreading across Europe. It has entered the British isles
and it is feared that it could kill the majority of ash trees, as most seem susceptible. This
could cause a major landscape transformation similar to that caused by Dutch Elm Disease.

Furthermore, trees are reported to be dying at ten-times their normal rate on a global scale!
As trees die, ecosystems die with them. Britain has all but lost its elms, now its ash trees face
imminent decline and its oak trees are showing increasing signs of stress from disease and
drought. If these three species go, then most of Britain's terrestrial biodiversity will go with
them.

Why are these drastic changes happening? Many comfort themselves that such diseases are
natural and that the ecosystem will thus re-establish itself. Elm trees in England were on the
edge of their range as climate warms in the current interglacial period, and this made it hard
for the English elm to reproduce by seed, most cloning themselves by suckers instead. Such
asexual reproduction produces trees of the same genetic makeup (bar mutations) and so if
one was susceptible, then all likely are. Such genetic clones can be rapidly destroyed by
disease.

Climate warming (whether natural or anthropogenic) stresses some tree species in certain
parts of their range. As climate changes, tree species come and go. The glacial periods of
the current Ice Age wiped out trees in the most northern latitudes, and when the ice retreated,
hardy pioneer species, such as birch, repopulated first, forming entire forests, before less
cold-hardy and slower-colonising species arrived.

However, the current change is not entirely natural. To what extent human beings have
accelerated or increased global warming is still uncertain, but perhaps more worrying are the
other changes human urbanisation has wrought. Many areas are deprived of water, as cities
suck-up huge quantities, lowering the water table, pollution stresses and weakens many
trees, forestry and other human activities can suddenly introduce a pathogen to a naive
population that has never encountered the disease before and so have no resistance -
diseases are unlikely to spread so rapidly in nature. Fragmentation has created isolated
packets of woodland, with reduced opportunities to sustain biodiversity. The reduction of tree
coverage (from about 90% to 15% in the British isles) means that any tree population has
less genetic diversity to counter disease. Forestry itself has tended to replace the natural
diversity of trees with certain strains that produce desirable timber or grow fast, further
reducing genetic diversity quite drastically. Forestry and other forms of agriculture also create
large stands of a single plant species, which makes it very easy for parasites to spread from
plant to plant. With these changes in mind, it should be noted that the Earth has never
experienced a change quite like the current one. Such large-scale diseases are symptomatic
of ecological instability.

The Earth may have survived mass extinctions from meteorite impacts, climate change and
supervolcanic eruptions in the past, but it takes many thousands of years for biodiversity to
be restored after such events, humanity might not last such a prolonged trauma; not to
mention the psychological and other health impacts of humans living in a treeless, urbanised
world. That is why, although life on earth will probably continue, it seems wise to me to take
such threats seriously. It seems certain to me that the planet earth is in ill-health, and since
you humans have been so slow in exploring and colonising space, you really ought to take
better care of the only life-bearing planet you have access to. You should be the wardens of
life on Earth, and not the displacers, exploiters and destroyers of that life. It is worrying to see
so few steps taken to rest and nurture back to health your ailing planet, instead increasing
population pressures and commercial exploitation are inflicting further damage at a relentless
rate. Humans
may survive, and they may even preserve enough genetic material in seed
banks and DNA databases to be able to reconstitute a future Earth, but what joy is there in
living in a sterile, dirty and barren world?
beech log with infection
Above: a felled beech tree trunk, showing blackening due to infection (fungal or bacterial,
probably fungal). The tree responded in life by forming a
barrier zone to hamper the
spread of the disease from the dead heartwood into the functional sapwood. (External link to
more about tree resistance to infection:
http://home.ccil.org/~treeman/shigo/SURVIVE.html).
Smuts (Ustilaginales) are another (smaller) group of plant parasites, also basidiomycete
fungi. An example is
Ustilago avenae which infects oats. Smuts are problematic for spoiling
the flowers and seeds of infected plants.

Some large basidiomycetes are also plant parasites.
Armillariella mellea, the honey-fungus,
forms large clumps of toadstools on or near tree stumps. The mycelium grows and feeds on
the tree stump, and can be seen by peeling away the bark. This fungus puts out long black
bootlace-like mycelial strands (bundles of hyphae) called
rhizomorphs, which can be seen
growing out from the stump just beneath the leaf litter or topsoil. These rhizomorphs search
for a new tree, such as a beech tree, to infect and when they find one they infect the tree's
roots. The mycelium grows beneath the bark, in the cambium, growing along the root and up
the trunk of the tree, killing the bark. If it encircles the tree then the tree dies. The fungus
continues to feed on the dead tree, producing toadstools to disperse its spores.

Some large
bracket fungi, growing on the sides of tree trunks and branches, may be
parasitic, feeding on living wood, such as
Phellinus ignarius, which grows on deciduous trees,
especially willows. Often the damage caused by these bracket fungi is slow and may be
contained by the tree's defences. Many brackets are not parasitic at all, growing only on
dead wood. Others can grow on living or dead wood.
Puccinia rust
Above: Puccinia (rust) growing on a leaf of wheat. The fungus has erupted onto the surface of
the leaf as a mass of hyphae (called a uredinium) which produces reddish-brown binucleate
uredinospores which give the fungus its 'rusty' appearance. The uredinium can be seen as
the mass of black-staining hyphae on the right.
clubroot 1
clubroot 2
clubroot 3
Clubroot

Clubroot is a disease of brassicas (cabbage and mustard family) caused by an amoeba,
Plasmodiophora brassicae, which was once classed as a slime mould. It forms multinucleate
plasmodia inside infected parenchyma cells of the infected root, and results in the formation of
root galls (hence the name).
Above: an infected cell in an early stage, containing a plasmodium.
Above and below: plasmodia inside infected cells beginning to divide into
a mass of resting spores.
The resting spores are visible as dark-walled spheres. When the root
dies and disintegrates, the spores will be liberated and in favourable
conditions they will germinate into zoospores (biflagellate amoebae).
Article last updated: 10/8/14