The Kinds of Diseases 323 



tribes of plant-lice and scale insects belong here, 

 and for these the kerosene emulsion, lime-sulfur 

 washes and the like, are the specifics. 



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



black-rot and mildew of the grape, fruit-rot of the 

 peach, leaf-blight of the plum and pear, black-knot 

 of the plum. These diseases are characterized by 

 definite spots, discolorations or excrescences 

 more or less scattered over the surface of the 

 leaf, fruit or branch. As a rule, the attacked leaves 

 and fruits have a tendency to drop from the, 

 tree. The general treatment for these diseases 

 is to spray with some fungicidal mixture, as 

 lime-sulfur or bordeaux mixture. The treatment 

 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 on the plant before the fungus is, the parasite 

 may not be able to gain a foothold. Even after 

 it does secure a foothold, however, the spray may 

 check its spread by preventing the development 

 of its external parts and the formation of spores. 



3. The physiological and bacterial diseases, or the 



so-called constitutional troubles. In these cases, 

 there are rarely any definite spots, as in the 

 attacks of parasitic fungi, but the entire leaf, 

 or even the entire plant, or a large part of it, 

 shows a general weakening and wilting, as if 

 there were some cutting-off of the accustomed 

 source of nourishment. Such diseases are a 

 general yellowing and death of the leaf, the 

 dying of the leaf along the main veins and 



