230 The Spraying of Plants. 



Yet all parasitic fungi do not attack the host-plant in the 

 same manner. Some immediately penetrate into the interior 

 tissues, and there they flourish, being well protected from outer 

 influences by the exterior covering of the plant. The fungi 

 causing all the more serious diseases develop in this manner, 

 and in fact the vast majority of plant diseases are caused by 

 such organisms. There are others, however, in which the 

 body of the fungus is almost entirely upon the surface of the 

 host-plant, only a comparatively small number of threads pene- 

 trating the tissues in order to obtain nourishment. These 

 parasites can be rubbed off, and unless the attack has been 

 very severe, the green, healthy tissue will be seen underneath. 

 This class may for convenience be termed " surf ace fungi," to 

 distinguish them from those which grow within the host-plant ; 

 it is represented by the common powdery mildew of the grape, 

 one mildew of the gooseberry, one of the strawberry, and a few 

 others. 



The life histories of the various fungi must form the basis 

 for any methods of treatment which may be adopted. During 

 certain stages of their existence, parasitic fungi may be checked 

 quite easily, and at such times the remedies should be applied. 



It is evident that when a fungus has once become established 

 inside the host-plant, it cannot be reached without destroying the 

 tissues of the host in the affected places, which is by no means 

 desirable. The fungus must be destroyed before it enters the 

 host ; in other words, the spores must be killed as soon as they 

 germinate, or better, they must not be allowed to germinate. 

 All applications must be preventive, not curative, since a cure 

 is practically impossible when the fungus is once established, 

 unless it grows upon the surface of the host. 



The line of treatment indicated is this : to cover the stems 

 and foliage of the cultivated plant with some substance that 

 will destroy the spores which may be present, as soon as they 

 germinate, or with one that will have the power of prevent- 

 ing this germination. If that is done, the plant will remain 

 healthy, so far as fungi are concerned ; otherwise it will not, 

 unless, indeed, no fungus attacks it. Several substances which 

 destroy these spores, as well as the surface fungi, have already 

 been found. They are easily applied, safe, and effective, and 

 any grower who suffers his fruit to be ruined by these para- 



