10 DISEASES OF TREES 



to canker, whereas larches mixed with other trees may remain 

 unaffected. The climatic conditions peculiar to a given district 

 may render it specially liable to outbreaks of certain diseases. 

 Thus in Alpine districts proximity to lakes and narrow valleys 

 specially predisposes to certain fungoid diseases, because the 

 moist air of such places favours the fructification of fungi in 

 a high degree. In the forest one meets with certain localities, 

 so-called " frost-beds," which favour the injurious action of 

 frost. The character of the soil may predispose to definite 

 diseases, in that, for instance, it specially favours the growth of 

 underground parasitic fungi, or the conditions may induce the 

 appearance of "root-rot." In very many cases one can say 

 forthwith of certain localities that they predispose to definite 

 diseases, and the latter must occur when some factor or other 

 of the environment is present, although in other localities the 

 same factor may be harmless to the vegetable world. Of course 

 this predisposition which is linked to the locality only forms 

 a part of the multifarious circumstances favourable to the 

 occurrence and spread of diseases that are to be ascribed to 

 the environment of the plant, and it must not be confounded 

 with the idea of a predisposition to disease in the narrower 

 sense. 



In the first place, the normal predisposition of plants may 

 consist in phases of development which naturally exist for a time 

 in every plant. To this class belongs tlie period of youth of the 

 plant, and the young condition of its new shoots, leaves, 

 and roots. These are at first covered only by a delicate 

 epidermis, which is but slightly if at all cuticularized, and 

 which can offer no resistance to the attacks of parasitic 

 fungi ; whereas later on in life, when a cuticle has been formed 

 on the outer cell-walls, and when periderm and bark have been 

 formed on the axial organs, the predisposition for many forms 

 of disease disappears. 



On the other hand, later periods of life may also induce 

 a predisposition to certain diseases. Young conifers which 

 possess resin-canals are almost perfectly protected from 

 infection by wood-fungi, at least in so far as these find an 

 entrance only through wounds caused by the removal of branches, 

 because each fresh wound is at once covered by a protecting 



