CHAPTER XXVI 



INJURIOUS INSECTS AND DISEASES AFFECTING FRUIT TREES 



By J. J. WILLIS 



In the culture of fruit, whether in the orchard, open garden, or under glass, the 

 question of injurious insects and diseases is apt to be looked upon with some 

 amount of dismay, but after all these are rarely to be counted upon as per- 

 manent factors, and considered rightly they are the direct and perhaps the most 

 efficient means of keeping the fruit grower in a state of mental alertness. 



There are three general types of difficulties in fruit growing, which have 

 been classified by Professor Bailey as follows : — 



1. Attacks by insects. 



(a) The damage by those insects which eat or chew a part of the 

 plant, and which, therefore, are killed by the application of some poison, 

 such as Paris green. These insects include the whole tribe of cater- 

 pillars, worms, and beetles. 



{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 may be placed under this head- 

 ing, and for these kerosene emulsion, resin washes, and the like are the 

 specifics. 



2. Parasitic fungoid diseases, such as the apple scab, mildew of the grape, 

 leaf blight of the plum and pear, currant gall, canker, and the like are 

 characterised by definite spots, discolorations, or excrescences, which are more 

 or less distributed over the surface of root, branch, leaf, or fruit. As a rule, 

 the leaves and fruits which are attacked have a tendency to drop from the 

 tree. The general treatment for these diseases is to spray with some fungicidal 

 mixture. The treatment is useful in proportion as it is applied early and 

 thoroughly. After the disease once gets into the tissues of the host plant, 

 it is difficult, if not impossible, to kill it. If, however, the fungicide is applied 

 before the parasite gets permanently established there will be less danger of its 

 obtaining a foothold. Even if the fungus has made headway, a thorough spray- 

 ing will check its progress. 



3. The physiological and bacterial diseases or those which are termed con- 

 stitutional 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 by disease, as if there were some cutting 

 off of the accustomed source of nourishment. Such diseases are very likely to 

 be seen in the general yellowing and dearth of the foliage ; in the dying of the 



