INTRODUCTION. 



INJURIES BY INSECTS DISTINGUISHED FROM ORGANIC DISEASE 



DISEASES AFFECTING THE CONDITION OF THE PLANT. 



Ill plants, as in animals, the outward symptoms of disease do not al- 

 ways indicate i)lainly the cause of the physical disturbance. There is 

 often need of skilful diafjnosis before it is possible to intelligently apply 

 a remedy. 



Naturally, in exogenous plants, whose vital growing parts are near 

 the surface, we look for the most part to external enemies for the cause 

 of disease, and especially upon insects, the natural foes of the vegeta- 

 ble world, suspicion falls most readily and with greatest reason. Do the 

 leaves of an orange tree turn yellow and fall to the ground, some 

 worm, we suspect, is gnawing at the root, or Scale-insects are sapping 

 the vital fluids from the bark. Are the blossoms blasted, the fruit drop- 

 ping or splitting, we are inclined to lay the blame upon some sucking 

 bug or upon some, it may be, harmless insect that we chance to see 

 upon the plant. 



But these phenomena are not always attributable to insect agencies, 

 although they are often correctly so assigned. They are frequently the 

 result of pathological disturbances, as obscure in their origin as are 

 many diseases of animals. 



As the object of the present treatise is solely to make known to or- 

 ange growers the insect enemies and friends with which they have to 

 deal, we cannot here enter upon a discussion of the principles of vege- 

 table hygiene as applied to orange trees. 



There are, however, several organic diseases which in their etiectupon 

 the tree closely copy the work of insects, and it is desirable that they 

 should be clearly distinguished. "We may, therefore, at the outset 

 briefly examine the forms of fungus and other attections most commonly 

 met with on plants of the citrus family, and give their distinguishing 

 characters, with so much as is certainly known or can bo plainly conjec- 

 tured concerning their nature or origin. 



DiE-BACK.— (Plate 11, Fig. 1.) This is a disease of the bark and young 

 wood, affecting chiefly the tendei- shoots. These grow to a length of 1 

 foot or 18 inches, and then become stunted, and finally die. Trees af- 

 fected with this disease continually push ottt new growth which soon 



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