70 PLANT DISEASES 



frequently impregnated with sulphate of copper to prevent 

 decay, and it was observed during the autumn of 1884 

 that vines supported on poles treated in this manner were 

 comparatively free from mildew. It is interesting to note, 

 in connection with the above discovery, the extension of a 

 rule-of-thumb practice into an important and indispensable 

 branch of modern horticulture, when fully explained and 

 utilised in a scientific or methodical and exact manner. 

 The value of sulphate of copper in preventing the decay 

 of the poles depended entirely on the property it possessed 

 of preventing the attacks of fungi, which would otherwise 

 soon have destroyed the wood. Although this explanation 

 was not probably known at the time, or even suspected, 

 until its action in arresting the growth of mildew on living 

 vines was observed, it must be considered as suggesting 

 the idea of fungicides and spraying, which at the present 

 day is practised with very beneficial results by all intelli- 

 gent plant cultivators in every country, probably least so 

 in Great Britain. 



Mildew attacks the foliage, young shoots, tendrils, 

 flowers, and young fruit of the vine, but usually appears 

 first, and is most abundant, on the leaves, where its 

 presence is indicated by the appearance of pale green 

 or yellowish spots on the upper surface. These spots 

 usually continue to increase in number and size, and 

 gradually change to a brown colour as the tissue of the 

 leaf dies; finally the leaf becomes brittle and falls, often 

 within ten days or a fortnight of the first indication of 

 disease. Very frequently nearly all the leaves of a vine 

 are attacked, and their premature fall arrests the develop- 

 ment of the fruit, even if the latter is not directly attacked 

 by the fungus ; the tree is also so weakened that the pro- 



