22 PLANT DISEASES 



It is an indisputable fact that the presence of numerous 

 plants of the same kind, and of the same age, massed 

 together, not to say crowded, favours the spread of a given 

 disease ; in fact, all epidemics on a large scale naturally 

 require such conditions, as the potato disease (Phytophthora 

 infestans), rust of wheat {Puccinia graminis), vine mildew 

 {JPlasmopara viticola), larch canker (Dasyscypha calycina), 

 and others. 



It is fully realised that in many instances myriads of 

 plants of the same kind, and of the same age, must exist, 

 as in the case of cereals, and in fact practically all field 

 crops ; nevertheless it is well that the disadvantage of this 

 method should be known, even if it cannot at present 

 be obviated. 



On the other hand, is it absolutely necessary, or even 

 advisable, to crowd a house with tomatoes alone, or with 

 chrysanthemums, or to plant pure forests or plantations? 

 Experience proves that such a proceeding is not con- 

 ducive to the well-being of plants so treated, and it rests 

 with the practical man to avoid the danger as far as 

 possible. 



Avoid massing together plants of the same kind whenever 

 circumstances permit. 



Mention has already been made of the group of fungi 

 called wound-parasites on account of their gaining admis- 

 sion to living tissues through some wound or broken 

 surface. Many such wounds are beyond the control of 

 human agency, either to avert or to protect from serving 

 as a starting-point for the attacks of fungi. Among these 

 are wounds caused by hailstones, cracks and wrenches 



