FUNGI – vegetable renegades

The fungi are unique among plants in that they are unable to make their own food by photosynthesis. Early in their evolution they lost their chlorophyll and must therefore, like animals, obtain ready-made organic food, either as saphrophytes feeding on dead organic matter or as parasites.

The fine threadlike cells of fungi, called hyphae, resemble those of other plants in having cell walls and not moving. They are sometimes woven together forming solid structures as in the mushrooms and toadstools which are the ‘fruit-bodies’ of some species, but more usually form a loose weft, or mycelium, spreading through their food. Hyphae usually have dividing cross-walls and each cell contains protoplasm, droplets of stored oil and several nuclei.

Reproductive structures and classification

The basic unit of reproduction in fungi is the spore, a tiny parcel of protoplasm, nuclei and stored oil, sometimes less than 0.00002mm (8xio_7in) in diameter, contained within a protective wall. Spores may be produced sexually following the fusion of two cells, or asexually from the vegetative hyphae. Most fungi can be identified only from their reproductive structures and are therefore classified by these.

In the lower fungi, or phycomycetes, which have hyphae without dividing cross-walls, asexual swimming zoospores with whiplike tails are sometimes produced. More often stalked sporangiophorcs containing many spores are formed. These burst open when ripe, the spores being dispersed by wind or on the bodies of insects. The sexual zygospores of phycomycetes are formed singly and have a thick protective wall. They may survive adverse conditions for several months, eventually germinating to form a sporangiophore liberating asexual spores.

The higher fungi are divided into the ascomy-cetes (the cup and flask fungi and the yeasts) and the basidiomycetes (the mushrooms and toadstools). In all higher fungi, the asexual spores are not enclosed but are produced at the end of special hyphae. These ‘conidia’ range in size from less than 0.00002mm to 0.1mm (8 xio~7in to 4 x io_3in), and may contain one or several cells.

In ascomycetes, the sexual ascospores are formed in groups of eight within a cylindrical ascus which shoots the ripe spores up into the wind. The fungus Pyronema has an ascus which bursts after absorbing water by osmosis, throwing the spores out to a distance of two to three centimetres.

The sexual basidiospores of the mushrooms and toadstools are formed uncontained in groups of four on the spore-bearing hymenium of the fruiting body. This hymenium may take the form of gills, pores or spines and is aligned at right angles to the direction of gravity so that the spores can fall out freely into the air. Consequently a toadstool laid sideways overnight in a collector’s basket can be found in the morning to have turned its cap horizontally. The spores are liberated in an ingenious way: each ripe spore has an attendant drop of water which, as it dries, sets up surface tensions which cause the spore to be shot off the hymenium.

The Fungi Imperfecti is a class with no known sexual reproduction and their classification is therefore uncertain. Spores are produced in huge quantities; the bracket fungus Ganoderma applanatum may produce as many as 30 billion a day, and 5,500 billion in a year. A common mushroom, Agaricus campestris, measuring 10cm (4m) across can produce 1.6 billion spores in the first few days of its existence, whilst the American giant puffball Calvatia gigantea has fruiting bodies which may reach 1.7m (5.5ft) in diameter with correspondingly huge amounts of spores.

To germinate, a spore must have moisture, warmth and suitable food. Many spores are wasted but the vast number produced ensures the continuation of a species. Many spores are found high in the atmosphere and may be carried long distances in the jet stream, which helps explain the worldwide distribution of many species, such as Aspergillus niger which can be found from the arctic to the tropics. In fact, their effective dispersal combined with fast growth, great variety and ability to evolve and adapt quickly make the fungi a ubiquitously successful group, far more common and varied than the sight of the occasional toadstool might lead us to believe.

Feeding and growth

Fungi employ several methods of obtaining the organic food they require. Most species are-saprophytes, feeding on dead plants and animals. Some, such as the black mould Aspergillus and the green mould Penicillium thrive on a wide variety of substrates from old books to dead leaves; others are less catholic. With the bacteria, these fungi dispose of the world’s waste; the all-too-obvious building-up of plastic wastes in our environment is a testament to the efficiency with which other digestible organic wastes are dealt with by these agents of decay.

In nature’s cycle of growth and decay, the fungi are essential in releasing from their food organic salts needed for plant growth. The growth of the fairy ring champignon Marasmius oreades graphically illustrates the results of decay by fungi. Having begun from a single spore, the mycelium of the fungus advances outwards, at a rate of I5~30cm (6-i2in) a year, forming a circular patch. At the outer edge of the ring fungal decay of organic matter increases the mineral salt content of the soil, particularly of nitrates, causing luxuriant grass growth. Inside this is a ring of poor growth where the thriving mycelium clogs the soil reducing aeration and drainage and stunting plant growth. It is here that the fruiting bodies appear. Towards the centre, where the ageing mycelium is dying, releasing minerals to the soil by bacterial decay, is another ring of rich growth.

Saprophytic fungi

Essential as they are to the fertility of our soil, some saprophytic fungi cause great damage to man economically. Many fungi are capable of digesting the resistant lignin and cellulose in wood, as is obvious from the proliferation of bracket and cup fungi on both living trees and dead wood. The dry rot fungus Merulius lachrymans partly avoids the need of most species for damp conditions by transporting water in its long spreading rootlike rhizo-morphs which are capable of penetrating bricks and stone. Storage rots of grains, fruits and vegetables also cause considerable damage to man’s crops. Species of Rhizopus cause soft rot in potatoes, peaches, tomatoes and other crops. Fusariutn caeruleum causes dry rot in potatoes, and various species of Aspergillus cause deterioration in grains, sometimes producing poisons too, such as the aflotoxin which killed IO.OOO turkeys fed on groundnut meal imported into Britain in i960.

Of great use to man in brewing and baking are the yeasts, a uniquely one-celled group of ascomycetes reproducing by asexual budding. Other fungi hold great promise for future food production. Aspergillus niger is already used in the industrial production of citric acid and the making of soy sauce.

Fungal parasites of plants

Many fungi are parasitic, obtaining their food from living organisms, usually plants. Most are facultative parasites being capable also of living saprophytically. Pythium, for example, which causes damping-off in seedlings, usually lives saprophytically in the soil. It has a very wide host specificity, attacking a variety of vulnerable seedlings when the opportunity arises. Obligate parasites such as the black rust of wheat Puccinea graminis tritici can only exist as parasites and are often host-specific, parasitizing only one species. Plant parasites have caused immense damage to crops since the plagues of biblical times, and today demand the expenditure of millions of pounds a year on fungicides and preventative measures. Monoculture, the growing of one crop over large areas, renders crops particularly vulnerable. The great Irish famine of 1845-60 was caused by the potato late blight fungus, Phytophthora infestans, which forced 1.5 million Irish people to emigrate to the USA and starved another million to death. Many plants are attacked by more than one fungal parasite. Besides late blight, the potato suffers early blight, Alternaria solani; black scurf, Rhizoctania solani; common scab, Strep-tomyces scabies; powdery scab, Spongospora Subterranca dry rot, Fusarium caeruleum; and several others attacking through the plant’s life cycle.

The vine Vitis vinifera also suffers several fungal diseases. Among them is downy mildew produced by Plasmopara viticoia, a fungus originally native to America. Around 1885, the aphid Phylloxera was accidentally introduced into France from America, decimating the economically essential vineyards. To combat this, resistant root stocks were imported. This defeated Phylloxera, but introduced downy mildew, which again destroyed vast areas of crops. The French scientist Alexis Millardct noticed that vines near paths sprayed with a mixture of copper sulphate and lime—to deter hungry passers-by—were much healthier. Working from this he developed the first fungicidal spray, which saved the French wine industry, and is used to. this day under the name ‘Bordeaux mixture’.

Cereals form a major part of the world’s food production, and the smuts and rusts that cause such havoc among them are of enormous economic importance. Black rust of wheat parasitizes two hosts during its life cycle. In autumn, resistant overwintering spores are produced, and these germinate in the spring to infect not wheat but the shrub, barberry. After a short period parasitizing barberry, spores are produced which infect the now thriving wheat. Spores produced on the wheat spread the disease rapidly throughout the crop until autumn when overwintering spores are produced again. In America, a campaign to exterminate the barberry and thus cut the rust’s life cycle has had some success in the north, but not in the south where the fungus can miss out this step. The breeding of resistant strains has been more successful, but the rapid evolution of the parasite and the large number of rust strains in existence make this a winning battle between parasite and plant breeders.

The ascomycete ergot, Claviceps purpurea, infects the ovaries of rye, reducing them to a mass of hyphae producing a sticky insectattracting honey-dew. Ergot is extremely poisonous, causing a painful death to man and grazing animals, and serious outbreaks of ergotism, or St Antony’s fire, caused by eating contaminated grain, occurred as recently as 1951 in Pont-St Esprit in France. A muscle-contractant used to control haemorrhaging during childbirth is extracted from ergot, perhaps explaining why farmers have long avoided certain fields where many stillbirths have occurred among their animals.

Fungal parasites may annihilate their host species; the American chestnut has been completely wiped out by Endothia parasitica, and Dutch elm disease, caused by Ceratocystis ulmi and carried by wood-boring beetles, threatens the elm with the same fate.

The common mushroom, Agaricus hortensis, is one of the few larger fungi which have successfully been taken into cultivation. This began in France in the 17th century, following the use of hot-beds of horse- manure which were often found to sprout mushrooms. Commercial mushroom cultivation today is a complicated and very precise operation carried out on a large scale-in light-proof sheds or sometimes under- ground. The rich, manure-based compost used is ‘spawned’ with small lumps made up of the ‘roots’ or hyphae of the mushroom.

Fungal parasites of animals

Fungal diseases of animals are less common. The fly fungus Entomophtbora muscae is one of several fungi which parasitize insects. The mycelium, growing from a single spore landing on the host, grows through the fly’s body, eventually leaving an empty husk. These victims are often seen clinging to dusty window panes, surrounded by a halo of emerging spore-bearing hyphae. Another group, the hyphomycetcs, feed on eelworms, trapping them in looped sticky hyphae.

In man (and other mammals) fungi such as Epidermophyton floccosum produce the skin diseases athlete’s foot and ringworm, which may reach epidemic proportions. Others, such as Cercospora apii, can cause severe disfigurement. Aspergillosis, or farmer’s lung, is caused by the common Aspergillus niger, often found on fruit. Several other fungi produce severe lung diseases such as blastomycosis caused by Blastomyces dermatitidis in America, often misdiagnosed as tuberculosis. Many of these fungi are normally soil-dwelling saprophytes, and soil may be an important source of infection.

Symbiotic fungi

Some fungi form nutritional relationships with other plants which are mutually beneficial to both parties. Such a relationship is called a symbiosis. In lichens the fungus gains nutrients by parasjtizing living algal cells and feeding on dead ones, whilst the algal cells are protected from high light intensities and dehydration. Mycorrhiza, associations between fungi and the roots of higher plants, usually trees, are very common, especially with conifers. A thin layer of mycelium covers the roots, sometimes invading the root cells, and the tree seems to benefit from the efficiency of the fungi in absorbing water and mineral salts from the soil, while it is thought that the fungus gains sugar from the roots. Some mycorrhizal associations are specific, Boletus elcgans for instance only grows with the larch (Larix decidua).

Other fungi produce mycorrhiza with many tree species. The dependence of the fungus on the tree varies, but some seem only to grow in these associations. Members of the Ericaceae, the heather family, seem particularly dependent on mycorrhiza, some species never being found without them. The fungus may even be found in the seeds, as with Phoma radicis. In the orchids, the protocorm which grows from the tiny seed, will not develop until infected by the appropriate mycorrhizal fungus from the soil, which may take two years.

In most symbiotic relationships, the partners are ‘fair weather friends’. If the delicate balance between parasitism and symbiosis is tipped due to conditions favouring one or the other partner, the relationship may cease to be mutually beneficial and break down, causing the parasitic destruction of the host.

Survival mechanisms

The fungi have evolved several very effective survival mechanisms. Many toadstools are distasteful and some are actually poisonous, ensuring that the minimum number are eaten and the maximum number survive to produce spores and propagate the species. The collector avoiding the Death Cap (Amanitaphallaides) ensures its spread. Several saprophytic fungi such as Penicillium and Streptomyces compete directly with bacteria for food and, by producing antibiotics to kill them, eliminate the competition and thus thrive.

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