12 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 
be used, on account of the severe foliage injury resulting when they 
are employed in sufficient concentration to be effective against the 
mildew. ; 
At present sulphur in some very finely divided form is the most 
efficient fungicide against apple powdery mildew. However, ordi- 
nary ground sulphur, sulphur flour, and flowers of sulphur, or sub- 
limed sulphur, are all far too coarse to give appreciable results. The 
extremely finely divided form known to the chemist as precipitated 
sulphur and the still finer form known as colloidal sulphur are both 
particularly effective against the mildew. 
In 1909 one of the writers: published a method for preparing 
what has been called iron-sulphid spray. It was made by precipi- 
tating a solution of iron sulphate (copperas) with the required 
amount of lime-sulphur solution. Twenty pounds of iron sulphate 
were used in preparing 200 gallons of the spray mixture. By stir- 
ring togetber the lime-sulphur solution and a solution of iron sul- 
phate, a black, muddy precipitate is formed, which contains precipi- 
tated sulphur, iron sulphid, and calcium sulphate. This spray was 
found particularly effective against the apple powdery mildew. It 
also possessed the very desirable property of stimulating a vigorous 
foliage growth, but it was found that serious injury in the form of 
leaf shedding and fruit dropping might result unless great care 
was exercised in applying it. Subsequent investigations showed that 
of the three compounds contained in this iron-sulphid mixture the 
precipitated sulphur is the ingredient which gives the preparation 
its properties as a fungicide, and it is at the same time responsible 
for the physiologic effects seen in the fruit and foliage shedding 
and in the stimulation of a more vigorous foliage growth. Pure 
precipitated sulphur used in amounts equivalent to that contained in 
the iron-sulphid mixture produced effects similar in all respects to 
those obtained from the iron-sulphid spray, and the same was true of 
colloidal sulphur, which is sulphur in a much more finely divided 
form than precipitated sulphur. Similar results were obtained from 
still other mixtures containing very finely divided sulphur. 
From these investigations, which extended over several seasons, 
it became evident that aside from their power to control mildew, 
precipitated and other very finely divided forms of sulphur were 
capable of producing what may be termed sulphur effects. These 
sulphur effects are of a physiologic nature, and their intensity is 
1Volek, W. H. The apple powdery mildew in the Pajaro Valley. Office of County 
Entomologist for Monterey and Santa Cruz Counties, Cal. Special Bul. No. 1. 1909. 
2The expression ‘“ iron-sulphid spray” or ‘‘ iron-sulphid mixture ”’ will be used in this 
publication to refer to the mixture prepared from iron sulphate and lime-sulphur solution. 
The compound sold by chemical dealers under the name of iron sulphid is an entirely 
different substance and is of no value as a spray material. 
