138 MASS. EXPERIMENT STATION BULLETIN 170. 



Considering the amount of deterioration in trees during recent years 

 many tree wardens and city foresters have been in a quaHdary as to what 

 species to plant. But there is reason to beUeve that these severe condi- 

 tions are past, and it may be a century before they occur again. One of 

 our most valuable and beautiful species, i.e., the elm, has been practically 

 abandoned as a shade tree in some places owing to its rapid and general 

 deterioration. There are many other species that have been affected in a 

 similar manner to the elm, although perhaps not so seriously. Notwith- 

 standing the fact that some trees have suffered particularly from various 

 causes, we believe that these should still- be utilized for planting, their 

 aesthetic and other qualifications being such that they cannot be dis- 

 pensed with. Moreover, affection by insects and fungous diseases must 

 not always be considered too seriously in judging the value of a species, 

 since control of many of them is possible with the use of modern methods. 

 It should be borne in mind that many of the pests are secondary or are 

 subservient to other causes. 



The European cut-leaf birch, wliich has been dying off in wholesale 

 fashion of late, is always associated with borers, which are considered 

 a specific cause of the dying of this tree. Quite the reverse is true, how- 

 ever, as the borers are secondary to drought injury. In fact, every serious 

 drought period affects the cut-leaf birch in tliis manner; the roots become 

 dried out and the tree falls a prey to borers. Borers in trees may not 

 always occur secondarily to some other cause, but it is extremely rare to 

 find healthy trees affected with borers. As soon as a tree becomes slightly 

 abnormal from any cause, infection follows. Even the slightest poisoning 

 from gas or injury to the roots by drought or winterkilling is sufficient 

 cause for weakening the trees, and borers and other insects follow as a 

 secondary cause. There is no reason, however, why the European cut- 

 leaf birch or other trees should fall a prey to borers if properly planted in 

 a suitable soil and well suppHed with water during drought periods, prefer- 

 ably by subirrigation methods. 



The elm has suffered from elm-leaf beetle to some extent, although 

 rarely is one found dying from this cause. Many elms have been prac- 

 tically ruined by the leopard moth. However, it can be stated as a general 

 principle that weak trees, or those that are under more or less abnormal 

 conditions, are more likely to be affected by insects and fungi than strong, 

 healthy, vigorous ones. In our opinion this holds true for the elm-leaf 

 beetle infestation, and some of our most careful observers regard the 

 leopard moth as secondary to other causes. The so-called "chestnut 

 blight" is held by some competent pathologists to be secondary to some 

 other cause or deteriorating factors common to the chestnut. In sup- 

 port of tills idea it is known that numerous chestnut trees have been dying 

 the last few years, from New England to Tennessee, which are not and 

 never have been affected with the blight fungus. 



The most important lesson to be learned from the behavior of shade 

 trees during the past years of trying experience is that we must give more 



