754 FUNGI 



{d) The fungus lives and develops almost entirely upon the 

 outside of the leaf, and on this account it would appear more 

 easy to deal with it by means of washes and external applications 

 of powdered substances than those cases like the potato-disease, 

 where the growth of the fungus goes on chiefly inside the leaf. 



The application by hand or bellows, or specially constructed 

 machinery (sulphurators), of finely powdered sulphur to the 

 affected leaf is a remedy for mildews of various kinds which has 

 been employed for about half a century. 



Mechanically powdered sulphur, i.e. roll-brimstone reduced to 

 a finely pulverized state, by hand or machinery, often acts better 

 than that form known as ' flowers of sulphur ' obtained by con- 

 densation of its vapour or by precipitation processes. In any 

 case the substance acts in two ways : (i) as a fungicide — that is 

 a definite destroyer of the ' mould ' ; and (2) as a protection 

 against further attacks and spreading, as spores will not germinate 

 upon a sulphured leaf. It is chiefly in the second capacity, 

 namely, a protector, that sulphur is so beneficial, and on this 

 account every endeavour should be made to distribute it upon 

 the youngest growth. As a direct fungicide it possesses little 

 effect, and even for this small benefit it must be repeated 

 frequently where ' mould ' is bad. 



The best results with sulphur are observed when the tempera- 

 ture is above 78° F., and it is, therefore, usually applied with 

 success on clear bright hot days, usually in the middle of the day 

 or early morning, when the leaves are partially damp with dew. 

 In cold weather it is nearly useless, and in wet days the sulphur 

 is soon washed off the leaf. 



The general explanation of its action is that the sulphur 

 becomes oxidised with the ultimate formation of sulphurous acid, 

 and this latter substance is credited with the destroying effect 

 upon the fungus. Sulphurous acid, however, in exceedingly 

 minute quantities has a deleterious influence upon the hop-leaf 

 itself. 



