Fungi As A Cause Of Disease In Plants
( Originally Published 1915 )
Common fungi, mushrooms, toadstools and moulds. How they live. Saprophytes and Parasites. The "Damping off" disease of Seedlings caused by the fungus Pythium. Life Story, means of spreading. and living in the soil, precautionary measures.
In preceding chapters you have become acquainted with the normal uses of the parts of healthy green plants. We are now to consider plants in disease and especially the disturbances of the structure and functions of plants produced by parasitic fungi. It is obviously impossible within the limits of a few chapters to deal with the whole subject of diseases of plants caused by fungi, but by a consideration of some of the diseases most common in fields, gardens and greenhouses, the main features of the subject may be illustrated. By this treatment you will not only become more familiar with the causes of those particular diseases, but also with the sort of precautions and remedial measures it is worth while to adopt In relation to the various types of plant disease.
Practically all parts of plants are subject to disease.
In some cases we have immediate and complete destruction of the plant or part attacked, but in others the death may he delayed or the plant may support the parasite indefinitely. We have diseases of seedlings, diseases of roots, of stems and of leaves, and a very large class of plant diseases comprises those which attack fruits. In this lecture we shall consider the most common disease of seedlings which is known as the " Damping off " disease.
Before dealing in detail with this, however, let me recall some of the more essential features in the structure, nutrition and life history of the more common fungi, facts with which you are no doubt, to some extent, already familiar. In any of the fields and lanes around our large towns, especially in autumn, it is possible to find examples of the larger fungi such as mushrooms and toadstools. Whilst the colour and form of the part which we see above ground are often striking, in reality this is only the reproductive part of the toadstool or mushroom plant, just as much as the flower and fruit are the reproductive parts of the higher plant. It is true that the mushroom as we generally see it, appears above ground, sheds its spores and decays all within a few weeks; but the vegetative part of the plant lives in the ground for a very considerable time before it enters. on the reproductive phase.
The bricks of so-called mushroom spawn contain quantities of fine interlacing threads of fungus in manure which are really the vegetative part of the mushroom. Under the microscope these filaments are seen to consist of long branched tubes. These tubes are divided up into chambers or cells by cross partitions, and each cell is lined with the jelly-like semi-transparent living substance, protoplasm. Within this are drops of water and oil as well as certain denser granules, but filaments of fungi never contain the green colouring matter found in higher plants. The protoplasm is the living part of the cell, and food material is taken in from the soil or manure through the protecting cell membrane. Such food material often appears stored in the tubes as drops of oil.
Although the filaments of the vegetating mushroom plant are not bound together into a complex plant body like the cells of flowering plants, yet the loosely inter-woven threads behave as a whole, and after weeks or months of vegetative growth give rise to the definite fruit bodies consisting of numberless aggregated filaments. These reproductive bodies usually possess a stalk hearing an expanded cap from the under side of which project, the radiating gills. Upon the surface of the gills are borne large numbers of minute round or oval spores. Each spore is really a single tiny cell so small that it can only be seen if very highly magnified. If, however, the stalk is removed from a not over-ripe mushroom, and the latter placed on a sheet of white paper, in the course of a few years so many spores are shed from the gills that a print of the gills is produced on the papier. This print consists of myriads of spores which have fallen like dust from the gills. The spores are the reproductive cells of the fungus and, falling to the ground, they grow out to produce tiny filaments. They soon begin to absorb water and food substances from decaying vegetable matter in the soil and manure, and so the vegetable life of the plant is carried on.
Many fungi are much more minute than the mushroom, indeed are so small that they can only be properly studied under the microscopic. As an example of such a fungus we have Mucor, the common white mould which often appears on damp bread or dung. If a piece of bread is kept moist for a few days under a hell-jar in a warm place the mould soon appears as a dense growth of fungus covering the surface of the bread. Erect silky threads stand up, from the surface like a miniature forest. Microscopic examination shows that the, fungus consists of two sorts of filaments : fine ones which branch and ramify in all directions forming a felt on and in the substratum, and the coarser erect ones which stand free.
The finer network on the substratum is the vegetative part of this fungus, while the 'erect coarser aerial threads are the reproductive organs. The individual branching filaments are very similar to those described for the mushroom, except that in this case there are no cross walls dividing up the tubes. When the aerial filaments have grown for a few clays there appears at the extreme tip of each a minute round swelling like an inflated ball about the size of a pinhead. The protoplasmic contents of this globular body or spore-case soon become divided up to form a, large number of spores, and then it has the appearance of a miniature ball full of shot. As the wall of the spore-case becomes dry it breaks, scattering its spores into the air as a fine dust all directions; the spores of Mucor and, indeed, of many fungi, are very light and easily carried by the slightest current of air. When placed in water the spores germinate in a few hours, the protoplasm within absorbs water, the spore wall bulges in one place and grows out forming a fine filament which, given suitable food material, rapidly grows and brandies to form a dense felt such as we saw on the moist bread. We have seen that suitable food material for this fungus is provided by bread, dung, etc; that is to say by dead or decaying plant or animal matter, just as the mushroom or toadstool lives on similar dead organic remains in the ground. Fungi, which in this way draw all their nutriment from the rotting remains of plants or animals, are known as saprophytes, and although they are unable to injure living plants they readily feed on their remains after death.
Fungi, unlike green plants, possess no chlorophyll and are therefore unable to construct their own carbon compounds such as starch and sugar from the carbonic acid of the air. They, however, take in such complex organic substances ready-made from the remains of plants which have previously manufactured them, and saprophytic fungi play an important part In Nature in living upon and decomposing the dead organic remains of plants and animals. The saprophytes as a rule cannot attack living plants, and therefore do not give rise to plant diseases.
A large number of fungi, however, are unable to live even upon decaying plant remains, and derive nutriment from the cells of living plants. Such fungi are parasites, and not only do they require to take in the carbon compounds of their food material ready-made, but they can only take their food substances from living cells. Now it is clear that: in obtaining substances forcibly from plants while still alive, such parasitic fungi rob the plants attacked of materials which otherwise would have been used for their own life and growth, and may harm them more directly in doing so. A plant which harbours a parasitic fungus is spoken of as the host plant, and in most cases the host suffers injury not only because it is robbed of substance by the fungus, luit also because the work of the particular parts or the plant infected are seriously interfered with. We shall see for example how the stores of food material in roots like turnips, or tubers like potatoes, are raided by fungi, and on the other hand how mildews and rusts prevent leaves from manufacturing food supplies.
Most of the larger and more prominent fungi are saprophytes and live, as we have seen, on decaying organic matter. Some of them, however, are parasites. The bracket fungi and the honey agaric, for instance, live as parasites on trees. The vegetative filaments of these fungi ramify in the tissues, of the tree, gaining entrance often through wounds, and only the reproductive bodies are produced outside. But very many diseases of plants are caused by fungi which, like the saprophytic mould Mucor, are so small that they can only be studied with the aid of the microscope. Such a minute parasitic fungus called Pythium de Baryanum often causes the " Dampingoff " of seedlings.
The " Dampingoff " disease very frequently appears when the seeds of many plants are sown too thickly and grown under conditions which are too warm and moist.
Young seedlings begin to die off in patches and soon present a very characteristic rotten appearance, The disease is very commonly met with in gardens and greenhouses, occurring in seed-beds of all kinds a few days after the germination of the seeds. It is most abundant in very wet weather and when the beds are kept too shaded and badly ventilated; crowding of seedlings also favours the progress of the disease. At first a few individual seedlings are attacked at or near the surface of the ground, the tissues in this region having a water-soaked appearance. Soon the cells collapse, and being unable to support the weight of the cotyledons the seedlings fall prostrate. Those immediately around are similarly affected, the disease spreading through the seed-bed in ever widening circles until practically the whole may be destroyed. When the plants fall over they become pale and rotten, and soon the whole bed is reduced to a moist mass of decaying seedlings. This mass is seen, on closer examination, to be covered with the very delicate threads of a fungus somewhat like the mould on bread. Often the filaments can be seen to have spread from the first diseased seedlings to the outer parts of the circle. These filaments moist often belong to the fungus Pythium, which is so called because of its ability to produce rotting. The fungus continues to grow in the dying seedlings, and the filaments may form a dense felt over the whole seed-bed.
If a seedling is examined under the microscope shortly after it is attacked, the collapse of the tissues just above the ground is seen to be due to the destruction of the cells at that spot. This destruction is caused by the filaments of the fungus which at this stage are seldom visible to the naked eye. They are, however, almost exactly like those found on the mouldy bread, and they run both between and within the dead and dying cells. They can also be traced into and between adjoining living cells, and when the seedling falls prostrate the remainder of the plant is soon invaded, for it is kept moist by contact with the damp soil. As the tissues further decay the filaments of the fungus spread out over the soil reaching across to other seedlings which then succumb in the same sort of way. The fungus attacks the cells of the seedlings by first extracting water, then boring through the cell walls and finally killing the living protoplasm and feeding upon the cell contents. Since the fungus grows very rapidly a seedling may be reduced to a putrid mass in a few hours.
The life story of Pythium is typical of the most thorough and destructive of plant parasites. The filaments at first grow in the air spaces between the cells, but later they enter and kill the living protoplasm. The spread of the fungus at first is due to the growth over the soil from its earlier victims to healthy seedlings which are attacked in turn; but soon a more rapid means of spreading comes into play. When a seedling has become thoroughly infected by the filaments of the fungus the ends of many of the branches ,of the latter begin to swell out into globular bodies very like the spore-cases of Mucor, only much smaller. These swollen bodies are full of protoplasm and serve a somewhat similar purpose to those in Mucor_ They are really special spore cases, but in further development they differ considerably from those of Mucor. In that fungus the spore-cases were borne on filaments standing erect from the surface, the spores being shed into the air. In Pythium, however, the spore-cases arc submerged in the film of moisture around the decaying seedling, and the different behaviour probably depends on this. When the spore case is mature a short tube grows from the side and swells to form a globular body with a very thin wall. Into this the whole of the protoplasm passes from the spore-case; it then rapidly divides into or 10 small rather oval masses of protoplasm, which begin to writhe and wriggle within the thin vesicle. This soon bursts, liberating the minute writhing spores which swim about in the slight film of water on the soil or surface of the seedling. When examined under high magnification each of the colourless swimming spores is seen to be furnished with two excessively fine hair-like threads of protoplasm which, by lashing the water incessantly, bring about the movement ,of the spore. The active movements continue for twenty minutes or half an hour, then the spore comes to rest, rounds off and withdraws the whips of protoplasm. If favourably situated on a seedling it sends out a fine filament which bores its way through the outer wall of one of the cells and grows into the interior. It has been proved that the tip of the filament is able to do this, because it secretes a substance which enables it. to digest its way through cell walls pretty much as gastric juice renders our food materials soluble. Once within the cells of the victim the fungus branches and grows rapidly from cell to cell, spreading destruction as it goes and deriving nourishment from the product of this destruction. In a diseased seed-bed these processes are going on so continuously that the fungus soon produces many thousands of the motile spores which are able to attack new seedlings, thus accelerating the progress of the disease. In addition, as we have already seen, even the threads of the fungus grow across the soil between the seedlings and directly produce new infection.
It is well known that "Damping off " will recur with even greater virulence in seed-beds which showed the disease the previous season. This is explained by the existence of yet. another chapter in the life history of Pythium, by which resting spores are formed that can pass the winter in the soil. At a late stage in the decay of diseased seedlings many threads of the fungus give rise to the resting spores. These possess the power of lying dormant over a long, period, and in this resemble the seeds of higher plants, though they only consist of a single thick-walled cell. When, however, the temperature is favourable and the conditions are sufficiently moist, the thick coat bursts and a fungal filament grows out which soon attacks any seedling that may be near, producing spore-cases and motile spores as before. The resting spores are produced in myriads in a diseased seed-bed; in fact, on one occasion, I estimated the presence of upwards of half a million in a single diseased seedling observed under the microscope. As the seedlings rot these spores all find their way into the soil where they spend the winter.
In considering methods for preventing this disease it is necessary to make use of the facts which are known of the means of spreading of the fungus causing the disease. We have seen that once a seed-bed is infected the fungus rapidly spreads, first by an abundant growth of filaments over the soil from seedling to seedling, and secondly by means of spores which swim in the water in the soil. Any treatment to be successful must check the vegetative growth of fungus in the soil and also the production of spores. It has been shown that the fungus only grows in this way in warm, very moist, badly ventilated seed-beds. Moreover, the effect of excessive moisture and lack of sufficient air is two-fold. Not only do such conditions favour the growth of Pythium, but these very conditions tend to produce weak drawn seedlings which of course are more susceptible to attack. Very careful attention to cultural conditions will therefore make it possible to ward off and, in most cases, entirely prevent the disease. The seedlings should never be overcrowded, and they should be given as much air and as little water as possible. I f through inattention to these conditions a few of the seedlings in a bed should show signs of disease they should be immediately removed and burned, and more air and less water given to those that remain. In any treatment it is also necessary to prevent the infection of the seed-bed at the beginning of the season. This infection invariably arises from the presence of resting spores in the soil. Naturally, then, soil known to have previously produced the disease should not be used, but since the resting spores are present in most garden soils it is wisest to endeavour to get rid of them. They can be killed by partially sterilising the soil by heating with steam or with boiling; water. It has been shown in an earlier lecture that the partial sterilisation of soil is valuable for other reasons. By adopting the practice it is possible to eliminate the resting cells note only of Pythium but also of other harmful organisms.