CHAPTER XXIV 

 Preventives of Fungous Diseases 



In a general way the methods used to prevent injury by 

 fungous diseases come under the head of preventives rather 

 than of remedies. Even in cases where an injured part of 

 a plant is cut off, the aim is to prevent further spread of the 

 disease within the tissues of the plant or to the tissues of 

 other plants. When the fungus has once invaded the cells, 

 there is little hope of remedy so far as the, affected part 

 of the host is concerned. In the case of the external 

 mycelium of the powdery mildews, however, the fungicide 

 may have a direct effect. 



The various fungicides are really preventives rather than 

 remedies. They kill the spores or mycelium threads upon 

 the outer parts of the plants treated, or they lie in wait to 

 kill whatever spores may come to seek entrance to the tis- 

 sues. The chemicals of the fungicides do not enter the tis- 

 sues of the host to destroy the fungus threads among the cells. 



It is evident that the prevention of the ravages of fun- 

 gous diseases is a broad subject. Many ways will readily 

 occur to one who thinks of the myriads of parasitic fungi 

 and their varied host plants. These ways may all be 

 grouped, however, under two general topics : namely, ( i ) 

 the destruction of the parasite ; (2) the prevention of its 

 growth upon or within the host. 



Preventive Measures 



Taking up the latter topic first, we find that for many 

 fungous diseases the most important method of prevention 



264 



