SHADE TREES. 



199 



this trouble with Bordeaux mixture many years ago. This rust affects 

 the lower foliage, usually when the dew is most abundant. Infection is 

 sometimes so severe that it destroys the twigs and branches. However, 

 the use of Bordeaux mixture as a spray for ornamental trees is objection- 

 able on account of the discoloration of the foliage, and some prefer the 

 fungus to the unsightly foliage. If possible, some less objectionable 

 sprajdng material should be employed for ornamental 

 trees and shrubs. Although Bordeaux mixture has 

 proved after manj'' years' trial to be the best all-round 

 summer spray for leaf spots, of late the diluted lime 

 and sulfur solution is being substituted for it with more 

 or less good results. Lime and sulfur applied to dormant 

 trees for the San Jose scale has proved invaluable as a 

 means of controlling leaf spots, and in some cases it 

 can undoubtedly be used to advantage for certain of the 

 such, for example, as the Glaosporium infection fungi, 

 oak and sycamore. It should be applied in late winter 

 before the leaves have begun to appear. 



A valuable preventive treatment for fungous infec- 

 tions of trees, in some cases at least, consists in burn- 

 ing the leaves each fall. This is especially valuable 

 with Rhytisma, common to maples, for this fungus 

 does not mature its spores wliile the leaves are on the 

 tree, and burning the contaminated leaves would lessen 

 the chance of infection. 



Finally, attention should be given to keeping trees 

 in a healthy condition. Countless examples could be 

 given of the lessened chances of infection possessed 

 by a healthy tree. 



Winter Injuries. 



Fig. 70. —Elm tree 

 showing pitted 

 trunk associated 

 with borers. Of- 

 ten observed on 

 trees under un- 

 congenial con- 

 ditions. 



Injuries resulting from low temperature are common 

 and often cause considerable damage to vegetation. 

 Whether a species is native or introduced it is likely 

 to suffer from winter injury if the proper condition 

 prevails, but plants introduced from regions where the climate is mild 

 are more likely to suffer from the effects of severe cold, although this 

 does not always follow. Moreover, plants grown out of their customary 

 habitats, or under uncongenial conditions, become more susceptible to 

 winter injury. The red maple, for instance, which usually grows in wet 

 places, becomes more susceptible to winter injury when grown in a dry 

 situation, and the same holds true for other swamp species. 



Winter injury is often restricted geographically, although during some 

 seasons it may be quite universal. The same type of injury may also 

 be more common, as well as more serious, in one locality than another. 

 The effects of winter injury to trees may also be local, i.e., only the 



