14 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 



an opportunity to act on heavy sulphur deposits, the skin and outer 

 layers of the flesh became brown and leathery. Mild cases of such 

 injury have very much the appearance of ordinary sun scald, but 

 when the damage becomes more serious the growth of the affected 

 area is checked and the fruit cracks open. This type of injury has 

 developed only occasionally, and then only when a strong dosage of 

 sulphur or iron-sulphid mixture has been used. No such damage 

 has ever been produced by the weak dosages of sulphur that are 

 recommended in this bulletin for mildew control. 



When it became evident that the tendency to cause fruit and foliage 

 dropping was one of the general properties of these finely divided 

 forms of sulphur, attention was turned to the investigation of a large 

 number of other materials, in the hope that some substance might be 

 found which would prove as effective as sulphur against the mildew 

 and yet be free from the injurious property of causing fruit and 

 foliage to drop. Among such substances tested, the most satisfac- 

 tory results in mildew control were obtained from a number of dye 

 materials. Fifty or more commercial dyes and laboratory stains 

 were tested, and it was found that a number of them, when applied 

 in water solutions, were capable of staining the mycelium and killing 

 the mildew. Such sprays, however, are curative rather than pre- 

 ventive in their action, and while, with the exception of eosin, no 

 injurious physiologic effects were encountered, their fungicidal prop- 

 erties were not entirely satisfactory. 



Meantime, investigations with the iron-sulphid and other precipi- 

 tated sulphur sprays were continued, and it has been found that by 

 using weak mixtures, starting the spraying early, and repeating it 

 frequently a very satisfactory mildew control can be obtained without 

 danger of causing fruit to drop. Thus, after six years of investi- 

 gations, in which 250 or 300 spraying experiments were conducted 

 and over 100 different materials tested, sulphur in some very finely 

 divided form still remains the most satisfactory fungicide for use 

 against apple powdery mildew in the Pajaro Valley. 



It may be repeated here that those forms of sulphur known com- 

 mercially as sublimed sulphur, or flowers of sulphur, sulphur flour, 

 and ground sulphur are far too coarse to be effective. Sulphur in 

 its colloidal form gives excellent mildew control and possesses some 

 distinct advantages of its own, especially in the matter of covering 

 power. Certain difficulties involved in its preparation, however, 

 prevent the grower from making his own supply. Precipitated sul- 

 phur may be made in a number of different ways, but what is here 

 called the iron-sulphid mixture is the simplest and safest form in 

 which the grower can prepare it. 



