686 PROTECTION AGAINST DISEASES. 



2. Modifying Factors, 

 a. Species 



The Scots jDine suffers most from needle-cast, also the 

 black, cluster and mountain pines. The disease does not 

 appear to have been as yet observed on Weymouth pine, nor 

 on Piiius rigida,lSU\\., that is now extensively used for planting 

 waste land. 



b. Aijc of Plaiils. 



Two-year-old plants suffer most from this disease, but 3- 

 to 4- year-old plants may also be attacked, though the 

 danger becomes less every year. In years when the disease 

 is very prevalent, weakly plants may suffer up to the age of 

 15 years, but only up to about G feet from the ground. 



c. LocaHtij. 



Pines growing in all kinds of localities are subject to this 

 disease, but it is chiefly prevalent in damp or wet places 

 exposed to frequent fogs. Thus valleys and plains suffer more 

 than hills and mountains, where the snow protects the plants 

 during winter. In depressions and in cold valleys, the disease 

 is often very destructive. As regards aspect, southern and 

 western slopes are most endangered ; eastern slopes also suffer, 

 but northern slopes either not at all or only exceptionally. 



The soil appears to have some influence on the disease, but 

 its effects have not yet been clearly explained. Stein* states 

 that Scots pine suffers most on pure sandy soils, but after all 

 it is on such soils that most indigenous Scots pine- woods are 

 found. Von Loffelholz t has observed that plants suffer less 

 on thoroughly cultivated soil than when the land has not been 

 previously broken up, and this may be due to the better root- 

 systems and superior hardiness of the plants in such cases. 

 It also appears that on peaty soil needle-cast is little to be 

 feared, which fact may be due rather to the treatment of the 

 pine-forests on such localities than to the nature of the soil. 



* "Ueberdie Schiitte," Dr. F. Stein, " Tharaiidter Jalubucii," vol. viii,, 1852, 

 pp. 208—225. 



t The same, p. 41. 



