IT has been seen that fungi and their allies play a most useful part in the world but the adverse side of the balance-sheet has still to be considered. This is chiefly a disclosure of diseases that can affect both plants and animals. Disease may be said to indicate a condition when two organisms are constantly associated beyond mutual tolerance. The lack of tolerance means that some kind of reaction will occur in the host. There may be unhealthy enlargement at the site of attack. A rot may develop in a vital part, or in some bacterial diseases of plants (such as ‘crown-gall ‘) a tumour may develop at some distance from the seat of attack, in the same way as in cancer of the human subject. It is found that a number of disease-producing fungi are ones which usually live in dead organisms but which occasionally become virulent. Little purpose would be served here in giving a catalogue of various diseases of plants and animals caused by fungi, bacteria or mycetozoa. It will probably be of greater interest to indicate the kinds of diseases that occur and the methods
used to control them. We can hardly do better than continue to discuss the ‘rust’ disease of wheat as it affords almost a typical example. It has been mentioned that efforts have been made to break the life-cycle of the ‘rust ‘disease of wheat by eradicating the intermediate host. This is a general method of attack on parasites living on two hosts, but it is not, unfortunately, successful. At the same time it had been noted for years by fanners that ‘rust ‘of wheat developed especially strongly when corn-fields were associated with Barberry. At this time the double life-history was unknown and botanists generally regarded the practical experience of the farmer as a piece of unscientific stupidity. Finally the farmers had their way and in various countries, particularly in France and the United States, laws were passed prohibiting a farmer from having Barberry on his land. This undoubtedly had a great effect in controlling the spread of the disease as it demonstrably led to its decrease in certain areas.
To-day the problem is dealt with in two ways. In the first place an effort is made to produce strains of wheat that will resist attacks of ‘rust ‘, and in the second, by good cultivation, strong healthy plants are produced; it has been shown, for instance, that excessive manuring with nitrogenous manures tends to increase the susceptibility to disease. Apart from these methods, attempts are made to control the disease by the use of antisepsis in the same manner as disinfectants are used in hospitals to check the spread of human disease. To control rust of wheat is a large problem rather like trying to check a world-wide attack of influenza.