196 SHADE-TREES IN TOWNS AND CITIES 



deaux mixture will not be too conspicuous, it may be applied 

 at intervals of from two to four weeks during July and 

 August. If the trees are in a very conspicuous place, the 

 dilute copper sulfate, one pound to two hundred and fifty 

 or three hundred gallons, may be used, as often as every 

 two weeks and after heavy rains. The great advantage of 

 Bordeaux mixture is that it holds the copper firmly to the 

 leaves and with every rain or heavy dew, minute quantities 

 are dissolved and the spores are prevented from germi- 

 nating. 



Mildews. These are parasites which produce white 

 spores and more or less white patches on the leaf or other 

 parts. The powdery mildew of the willow {Uncinula salicis 

 (DeC.) Wint.) and the mildew occurring on some species 

 of maple {Uncinula aceris (DeC.) Wint.) may be taken as 

 examples. Coming on later in the season than the brown 

 rusts, two or three applications of either the Bordeaux mix- 

 ture or dilute copper sulfate will be effectual, but no applica- 

 tion need be made generally until the middle or last of July. 



Blights. The leaf blights are unlike the rusts in that 

 they attack and destroy small portions of the leaves, and 

 when these spots or places become numerous the whole leaf 

 is destroyed and it soon falls off. 



The leaf spot of the horse-chestnut and the sycamore leaf 

 blight are two of the most destructive leaf diseases of this 

 class. 



The destructive work of the leaf spot of the horse-chest- 

 nut {Phyllosticta pavice Desm.) becomes conspicuous early in 

 the season by the appearance of small brown spots on the 

 leaflets ; these spread to a large size and produce dead areas 

 of curled and broken tissue until the foliage becomes ragged. 

 This is followed by early defoliation of the trees. 



