The fungicides used on fruits are of two general types, depending on the active principle in each: the copper fungicides and the sulfur fungicides. Although there are a number of kinds of each, bordeaux mixture, ammoniacal copper carbonate and water solutions of copper-sulfate are the copper forms usually used against fruit diseases; while lime sulfur, self-boiled lime sulfur, soda sulfur compounds and finely divided sulfur - dust are the usual forms of sulfur fungicides employed.
So far as fruits are concerned, it may be said that, in general, copper fungicides are always to be used on American grapes and are safe on sour cherries, gooseberries, currants, pears, quinces, strawberries and, in dry seasons, on apples. Sulfur fungicides alone may be used on peaches, plums and sweet cherries, and are also safe and effective on European grapes, apples, quinces, pears, cherries, currants and gooseberries.
In the dormant condition, where disinfection is necessary, either copper or sulfur fungicides are safe and effective.
As regards the physical form, fungicides are of two general types, liquid and dry, or dust. Liquid fungicides have been used almost entirely since the introduction of bordeaux mixture about 1883. Several attempts to introduce copper fungicides in dust form have failed. Recently, with the development of sulfur fungicides, very successful and promising results have been obtained in the application of dry, finely divided sulfur.
Insecticides may usually be safely mixed with fungicides. Arsenate of lead is the insecticide now commonly employed. It may be used with bordeaux, lime sulfur, and in powdered form with finely divided sulfur for dusting purposes. It may not be safely combined with soda sulfur compounds.
Fungicides of all types are now generally manufactured, although they may all be home-prepared. The standard commercial brands are all about equally efficient and safe. They are in general more expensive than home - made preparations, but at the same time they are usually more convenient, especially where relatively small quantities are needed.
Bordeaux mixture was from 1885 to about 1910 the chief fungicide for fruit diseases.. With the introduction of lime sulfur as a summer spray, bordeaux has taken second place. It is by no means, however, entirely replaced by lime sulfur, and remains for American variety of grapes at least the only safe and effective fungicide, while for all other fruits except peaches, plums and apples (Fig. 126) it is just as safe and effective as any sulfur spray. The active principle in bordeaux is of course the copper. This is combined with the calcium in the form of gelatinous colloid membranes suspended in the water. The smaller the membranes, the better the mixture stands up, and the more effectively it covers sprayed surfaces. As the water evaporates from a sprayed surface, these minute 2f membranes dry down and attach themselves like glue to the surface. When wetted by dew or rain, they swell but do not dissolve slowly. The action of the moisture and the air is to free a portion of the copper in the membrane which acts on the germinating spore to inhibit its growth or to kill it. Injury by bordeaux (Fig. 124) results when weather conditions favor excessive solution of the copper from the membranes.
Fig. 124. - Bordeaux injury on apples.