Plant-Fungi Symbioses

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.


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

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 defenses. Many brackets are not parasitic at all, growing only on dead wood. Others can grow on living or dead wood.

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).

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.

Humanity 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?

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