1913.] PUBLIC DOCUMENT — No. 31. 79 



result. lu cities where the streets are constantly being regradeJ, 

 the surfaces of the larger roots may often be found exposed, 

 but in perfectly normal condition. 



Injupjes from Various Treatments. 



We have at different times had occasion to observe injuries to 

 shade trees brought about by various causes, in many cases 

 resulting from treatment applied by careless or ignorant per- 

 sons. The present widespread treatment of various pests, as 

 might be expected, has made necessary the testing of different 

 remedies, some of which have proved to be more injurious than 

 the pests themselves. These materials have not only been used 

 by careful people trained in the use of different methods for 

 controlling insects and fungi, but by others who are reckless 

 to the point of seeming to delight in taking chances. 



Oil. — Many different oils have been used for spraying insect 

 pests, some of which have proved reliable and others injurious. 

 Kerosene oil can be used on some plants under certain condi- 

 tions without causing injury, while in other cases it will kill 

 them. A few years ago there was placed on the market a 

 spraj'ing device for the mechanical mixing of kerosene and 

 water in different proportions, but when these materials are 

 mixed mechanically they usually separate, and they have been 

 responsible for the death of many trees. The oil soaks into the 

 bark and often reaches the cambium and sapwood, destroying 

 the tissue ; and we have seen quite a few shade trees killed by 

 spraying with kerosene and water to exterminate wooley aphis. 

 In some cases every part of the tree touched by the kerosene 

 was injured, while in others the injury was only local, a more 

 commonly noticed condition on thick-bark trees, while the for- 

 mer case was invariabl}'' restricted to trees with thin bark. The 

 bark of trees killed by the use of kerosene presents a different 

 appearance and develops usually a different type of fungous 

 flora from the bark of trees dying from other causes ; besides, 

 traces of the oil, which remain on the tree for a long time, can 

 be detected by the sense of smell. A fair diagnosis of this 

 type of injury may be made from specimens of the bark, but 

 when there are comparatively slight local injuries it is best to 

 examine the tree in situ. Even slight traces of oil may be 



