This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Owing to the attention that has been given to the various fungoid diseases of fruits, flowers, and vegetables of late years, a large industry has developed amongst chemists to supply remedies for checking or killing the various diseases. What has already been said about the efficacy or otherwise of insecticides at p. 167 applies with almost equal force to fungicides. The use of these has increased enormously of late years, but the various fungoid diseases seem to enjoy themselves as much as formerly on our crops. Indeed the writer has seen some of the worst cases of fungoid diseases in market gardens upon which large sums of money have been spent annually in insecticides, while other gardens, upon which not a farthing has been spent on either insecticides or fungicides, are practically immune from fungoid diseases and insect pests. Apple trees and pear trees that have been carefully sprayed for Fusicladium have had their fruits attacked quite early in the season; while in other cases, where the usual methods of cultivation were practised, but where no fungicides were used, the fruits were perfectly free from fungoid attack.
The only fungoid diseases that seem to defy all fungicides and all efforts to check or eradicate them appear to be the "Silver-leaf" of Plums and the "Die-back" of Gooseberries. Some day, perhaps, when more is known about these two terrible diseases, it may be possible to find a means of destroying them. In the meantime Victoria and Gisborne Plums are rapidly falling a prey to the "Silver-leaf" fungus (Stereum purpurewm), while Gooseberries are being mowed down wholesale in places by the "Die-back" (Botrytis cinerea).
Just as many insect pests are undoubtedly due to inferior methods of cultivation, it is possible that the prevalence of many fungoid diseases is due to the same cause. The cultivator, therefore, who pays more attention to the arts of cultivation and manuring his soil, and not so much perhaps to the sciences of chemistry and entomology, will probably find his crops more free from disease of every kind. At the same time, as it is beyond the bounds of human possibility to keep fungoid diseases like that of the Potato absolutely in check, he should be prepared to suppress any sudden outbreaks by means of fungicides that have been found more or less effectual.
Fungicides are applied either in the form of sprays or washes, as dry powders, or as vapour. So far as the sprays and washes are concerned the grower will find fungicides applied in a hot or warm state much better than in a cold one. The writer has carried out many experiments with fungicides and insecticides dissolved in boiling water, and has applied them in the form of a fine misty spray to outdoor crops without causing the least injury to the plants, but often destroying the pests and diseases with one good application. The reader must distinguish between applying an insecticide or fungicide boiling hot through a fine-spray nozzle, and dipping the leaves and shoots of a plant in the same solution. When the hot liquid is sent with force through a fine-spray nozzle, it impinges on the leaf or stem surface in the form of tiny globules in a mist-like spray. A good deal of heat is lost in transit, and the tiny globules are still further cooled to air temperature almost as soon as they touch the leaf surface. They have, however, retained a higher temperature than is healthy for the fungus or insect pest; hence these are usually killed outright. In spraying large areas, the only difficulty is to maintain the liquid up to a temperature of 212° F. or boiling-point, but this may be overcome by having a small portable boiler and fire attached.
The following is a list of the most effective insecticides and fungicides on the market at present: -