The effect of the fungus is to produce a lesion on its host, which becomes in the course of time filled with spores of the fungus that, because of their colour, show like patches of iron-rust on the plant. Frequently, as may be seen on juniper in this country, a large gall-growth is produced, and sometimes, as with wheat and other cereals, the result of an attack is that the plant is useless for normal purposes. It will readily be appreciated that when large areas of ground are cultivated with one crop, very serious loss may ensue if a disease appears. The very concentration of host material will lead to widespread infection once a disease has broken out, and the ‘rusts ‘in the world’s wheat-growing belts causes enormous financial loss.

‘Rust ‘is always present in the great wheat zones, but in some years it is more troublesome than in others. In 1916 which was a bad ‘rust’ year, the wheat crop lost in Western Australia was valued at two million pounds; in western Canada twenty million pounds of damage was done and the story was repeated round the world. Thousands of acres of wheat were never even garnered as the yield could not possibly have paid for the labour and materials that would be spent.

Wheat and cereals are not the only crops adversely affected by rust. Apples, small fruit, forage crops and garden flowers all have their particular rust disease that, as in the famous instance of the rust of coffee in Ceylon, can become so menacing as practically to kill an industry. The rust of coffee that started in Ceylon and spread through the Old World reduced the value of the coffee exports of Ceylon from about three million pounds to twenty-five thousand pounds sterling—a drop to jig part of its original value in about thirty years. Even to-day the life-history of this dangerous disease is not fully known in spite of the fact that on the completeness of that knowledge depends the success of methods of controlling rust diseases.

‘LIVE AND LET LIVE ‘: THE PARASITE’S BEST POLICY TO appreciate the peculiarities of the life-histories of rusts it is necessary to consider the properties of parasites in general. First of all it is as well to avoid the moral threat this word implies when levelled at people whom we happen to dislike as a group, and to remember that biologically a parasite is a highly specialised organism that has come to conform fairly closely to the environment that is provided by its host. If we could endow a parasite with the power of thought so that it could be said to have conscious action, we should be justified in saying that its policy was very short-sighted if it so depleted its host that the host died. For, once the host was dead the parasite would be without a food supply. Again, the parasite endowed with thought might realise that the elaborate organisation it had built up to comply with the conditions inside a host might not only be useless, but even a source of weakness and therefore of danger in the great world outside. It would be wise, therefore, for a parasite to stop short of killing its host and to conduct itself on a ‘live and let live ‘principle merely with an eye to its own future.

However careful a parasite might be about this, hosts sometimes die, and this fact has to be reckoned with. Should a host incontinently die the parasite must either die with its host, or get out of its host’s body—and get out quickly. A double problem then faces it. It might very probably stand a greater chance of survival if it could adopt quickly some other form, but it needs mobility to reach a new host. So long as a parasite is concerned merely with its own affairs

these problems stand, but should it, may we say, elect to have some pride of race, to wish to produce ‘more and better parasites,’ these problems would become intensified. To produce more of its own kind to compete with it for food in its own particular environment would be foolish, so that a desire for reproduction implies a need for distribution. It must not be assumed that a parasite is capable of considering possibilities and acting accordingly. The fact is that it cannot appear otherwise than if it were so subtly governed, because death is the reward of error. If a parasite made a mistake, it would commit suicide.

One method of solving the problem of protection and distribution at the same time is to adopt an ‘alternative ‘or ‘intermediate ‘host endowed with the power of mobility. A notable example of this is to be found in malaria, which is a disease caused by the growth of a parasite in the blood of man. It is transferred from one person to another by mosquitoes which act as intermediate hosts.

THE FUNGUS THAT TAKES A ‘HOUSE FOR THE SEASON ‘A SIMILAR state of affairs is found in the rust fungi. Some are confined to one host while others require two hosts, one of which is known as the principal host, and the other is called the alternate host. If the principal host happens to be an annual plant that will die at the end of summer the rust fungus is compelled to have some method of living during the winter and frequently finds an alternative host that is evergreen or at least perennial. Dispersal is made by the wind which carries off a large number of tiny spores. Whether the spores ever reach a suitable host must therefore be largely a matter of chance. It has been found that there are different spore forms, which according to their function have been distinguished as winter-spores and summer-spores. Summer-spores form pustules in the host that show red through cracks in the epidermis or skin. Sometimes a special form of ‘spring-spores ‘may occur and produce a rusted appearance. The winter-spore is a special two-celled spore that produces four spores, called basidiospores, on a special stalk or basidium.

All, some, or one form only may exist in any life-cycle. We may take the ‘rust ‘of wheat, which is typical of the life-history of a ‘rust ‘, as an example of what happens. When the wheat is about to produce flower spikes the rust appears as cracks on the leaves and stalks. That is the summer-spore

stage, and several crops of rust may succeed each other on the wheat plant. Then in time the winter-spore stage, which is black, follows, and in this stage the rust endures the winter. Sometimes during late winter or early spring the winter-spore germinates and gives rise to an evanescent spore stage which is blown by the wind on to the alternative host, in this particular instance, the common Barberry. There, two forms of cups are produced on the leaves, one on the upper surface containing the spring spores, and one on the lower surface holding another spore stage, the function of which is still obscure, though it is connected with sex. The fusion of two nuclei from sister cells during the formation of spring-spores is probably not a sexual process as was at one time thought but a pseudo-sexual fusion that can replace sex. Recent evidence tends to suggest that the cells of the cup on the lower surface may produce male sexual cells.

It was at one time thought that this parasite had become so highly specialised that it would die if its life-cycle was broken by the disappearance of one host, but the disease occurs on wheat in countries where Barberry is unknown, and re-infection is continued by the summer-spores.

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