12 BULLETIN 120, IT. 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 1 published a method for preparing 

 what has been called iron-sulphid spray. 2 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 together 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 



1 Volck, W. H. The apple powdery mildew in the Pajaro Valley. Office of County 

 Entomologist for Monterey and Santa Cruz Counties, Cal. Special Bui. No. 1. 1909. 



2 The 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. 



