The fungicidal value of lime sulfur was discovered in America about 1880 when California peach-growers using it on dormant trees for the San Jose scale found that it controlled the Leaf Curl. As a matter of interest it may be noted that several years earlier a boiled solution of lime and sulfur much diluted was in use by European gardeners as a fungicide on growing plants. Its use was, however, not general and its possibilities were disregarded or overlooked in the general belief that the newly discovered bordeaux mixture was of universal application. The re-discovery of lime sulfur as a summer spray about 1906 by Cordley of Oregon marks the beginning of a new epoch in the history of fungicides.
The fungicidal properties of lime sulfur reside in the sulfur or, more accurately perhaps, in the sulfuric acid which is eventually formed by the oxidation of the sulfur in the presence of water. The special virtue of the lime sulfur solution (poly-sulfides of calcium) lies in the fact that after evaporation of the water they are gradually oxidized, leaving pure sulfur in exceedingly finely divided particles on the sprayed surface. Injury by lime sulfur usually occurs as the solution dries, and especially where the trees have been drenched. Large quantities of the solution are left along the curved edges of the leaves, where as evaporation goes on concentration and burning by the caustic polysulfides results. In the case of grapes, however, and sometimes in the case of burning of peach and apple foliage, the injury is due to the sulfuric acid formed later from the sulfur. If the lime sulfur is properly diluted and properly applied, serious injury seldom results except on grapes and on some varieties of peaches. What has sometimes been taken for lime sulfur injury is doubtless injury from the arsenicals used with it.
If commercial concentrated solutions are to be used, simply dilute as directed for the particular disease or diseases to be combated. Concentrated solutions may be prepared as follows:
Use only fresh, lump-lime free from dust, grit, air-slaked material and magnesium oxid. The lime should not contain less than 90 per cent of pure calcium - oxid. A high grade of sulfur flour, flowers of sulfur, or finely ground brimstone will do equally well. The best formula appears to be:
Lime 90 per cent pure,
Moisten the sulfur thoroughly, working it into a smooth, even fluid-paste. Slake the lime in 10 gallons of hot water, adding the lime slowly to avoid boiling over. Pour in the sulfur paste gradually during the slaking, and stir constantly. When all the lime is slaked and all the sulfur thoroughly mixed in, add water to make fifty gallons and boil vigorously for one hour. The cooking may be done in a kettle over a fire or in a barrel by means of live steam. Allow the solution to settle, decant off the clear dark concentrate, and store in barrels. Keep where it will not freeze. Home - made concentrate will usually show a lower Baume test than the commercial concentrate. In either case the dilution should be made on the basis of the hydrometer reading.