Most fungous diseases of fruits and fruit-trees are more or less profitably controlled by the application of external protective substances known collectively as fungicides. The essentials of a good fungicide are: it must have ability to protect the host from the attacks of the fungi; it must be non - injurious to the host; it must be adhesive, i.e. not readily washed off by rains; and must be relatively inexpensive and easy of application.

A fungicide usually acts to protect a host by destroying or inhibiting, through some toxic property, the growth of the fungous spores which fall on the exposed surfaces after the fungicide has been applied. Fungicides do not penetrate the host and kill the fungus after it has entered and established itself, hence the futility of applications after the host is infected. Fungicides prevent, they do not cure. In the case of a few fruit diseases, the fungicide acts as a disinfectant rather than as a protector, i.e. it destroys the spores of the pathogene which are already present when applied. The control of peach Leaf Curl or of the surface-growing Powdery Mildews are cases of disinfection rather than protection. Pathogenic fungi vary considerably in their sensitiveness to fungicidal poisons, both as to kinds and strengths, hence the necessity for using different kinds and strengths of fungicides for different fungous diseases. Sulfur fungicides, for example, so effective against apple scab are of no value for the control of Bitter Rot, against which copper fungicides must be used.

That a fungicide must be non-injurious to the host is self-evident. Hosts, like pathogenes, vary in their sensitiveness to fungicides. Some, like the peach, are notably susceptible to injury from almost any toxic substance, while others, like the apple, will tolerate a wide range in kind and concentration of fungicides. Varieties and even individual trees exhibit great variation in their susceptibility to injury, so that only after extensive testing may one say with safety what kinds and strengths are to be applied to the different fruits.Moreover , the liability to injury depends on a number of variable conditions, such as the stage of seasonal development of the host, or its vitality, climatic conditions, especially temperature, moisture and the like. It is common knowledge among growers that in rainy seasons bordeaux mixture is apt to be very injurious to apples, while in dry seasons little or no injury follows its application. Foliage on apple-trees of low or impaired vitality is much more liable to lime sulfur injury than that of healthy vigorous trees. Grapes of American origin are almost uniformly injured by the application of sulfur in any form while showing no evidence of injury from the application of copper fungicides. European grapes, on the other hand, are not injured by sulfur. Young, expanding leaves of the apple will tolerate a much more concentrated solution of lime sulfur than will the fully expanded and mature leaves later in the season, strange as this may seem.

Adhesiveness, especially where the fungicide is to act in a protective way, is a very necessary requisite of a good fungicide. The spores of most pathogenic fungi are disseminated and infect the host during rains. Not only must the fungicide be present on the host in advance of the rains, but at least enough of it must remain, in spite of the washing of the rain, to destroy or inhibit the growth of the spores. Adhesiveness may be due to the gelatinous nature of the fungicide as in the case of bordeaux mixture, or to the finely divided character of the resulting particles as in lime sulfur. Other substances having adhesive qualities are often added to assist in holding the fungicide in place on the host surface. Iron-sulfate and arsenate of lead act in this way when added to lime sulfur.

The cost of the fungicide is often a limiting factor. A substance ideal as a fungicide in every other respect may cost more than is warranted by the profits realized from its application. Under average conditions, however, this is rarely the determining factor.

The ease with which a fungicide may be applied or the simplicity of the operations involved are of much importance in determining its general and effective use. The ease and rapidity with which dust fungicides may be applied in contrast to the slow and laborious application of liquid sprays will have much to do with the substitution of spraying by dusting.