THE spores of rust are so small that when freed from the diseased plant they rise into the air by means of currents produced by alterations in atmospheric temperature, and can float through the air for a distance of some thirty miles before coming to rest after an initial rise of only a few feet. It has been shown by means of traps exposed from aeroplanes that spores can float through the air a mile above the earth. The aeroplane has also come into use in other ways. It has been found that various substances when sprayed in a solution on to the plant can kill the disease, and recently it has been demonstrated that fine dusts, chiefly of sulphur compounds,
are particularly valuable in this respect. To-day in Canada low-flying aeroplanes spray clouds of antiseptic dusts over growing corn.
The farmer to-day has to sterilise his machines and tools in the same way as a surgeon has to treat his instruments, before operating on a patient. If a threshing machine is used on wheat that is even partially rusted, it may simply become a means of spreading infection broadcast wherever it is used unless carefully sterilised with some antiseptic. Chemical sprays have been in use for controlling fungus pests almost from the beginnings of plant pathology as a science. The well-known Bordeaux and Burgundy mixtures were originally used for spraying grapes in the French vineyards to prevent people from eating them. It was found subsequently that where crops had been sprayed grapewas kept within bounds. Since that time spraying has become a method of control for nearly all plant diseases. Immense quantities of spray are used to prevent the appearance of potato blight, a dangerous fungus disease that can so far affect man’s affairs that it caused the Irish potato famine, the repeal of the Corn Laws, and the consequent fall of Peel’s government in England.
It is not merely diseases of standing crops that can be caused by fungi. Many crops develop rots in storage, and in recent years with the increase of the fruit trade throughout the world, storage rots have come to be increasingly studied. Commonly, as with apples, a large number of these rots are shown to be due to common infections of orchards, so that here again aseptic and antiseptic methods will prevent trouble.