Classification of Diseases. 349 



(b) Attacks of insects which suck their food, 

 and which are, therefore, destroyed by caustic 

 applications which injure the bodies of the 

 pests. All the tribes of plant -lice and scale 

 insects belong here, and for these the kerosene 

 emulsion, resin washes, and the like, are the 

 specifics. 



2. Parasitic- fungous diseases, such as the apple- 

 scab, black -rot and mildew of the grape, leaf -blight 

 of the plum and pear, black -knot, and the like. 

 These diseases are characterized by definite spots, 

 diseolorations or excrescences, which are more or less 

 scattered over the surface of the leaf, fruit or branch. 

 As a rule, the leaves and fruits which are attacked 

 have a tendency to drop from the tree. The gen- 

 eral treatment for these diseases is to spray with 

 some fimgicidal mixture, like the Bordeaux mixture 

 or the ammoniaeal carbonate of copper. The treat- 

 ment is useful in proportion as it is applied early 

 and thoroughly. After the fungus once gets into 

 the tissues of the host- plant, it is difficult, if not 

 impossible, to kill it. If, however, the fungicide is 

 upon the plant before the fungus is, the parasite may 

 not be able to obtain a foothold. Even after it does 

 obtain a foothold, it is probable, however, that the 

 spray will check its spread by preventing the devel- 

 opment of its external parts. 



3. The physiological and bacterial diseases, or 

 those which are termed constitutional troubles. In 

 these cases, there are rarely any definite spots, as 

 in the attacks of parasitic- fungi, but the entire 



