CHAPTER II. 



NATURE OF PEACH LEAF CURL. 



The study of the nature of plant diseases is intimately linked with 

 the study of plant physiology, and the true science of vegetable 

 pathology is largely, as Ward has defined it, the stud}^ of abnormal 

 physiology. (Introduction to Hartig's Text-book of the Diseases of- 

 Trees.) These facts become evident when studying the etioloo-y of 

 peach leaf curl and the conditions attendant upon its widespread 

 development. The direct cause of peach leaf curl has long been 

 known as a parasitic fungus, Exoascus deformcms (Berk.) Fuckel, 

 but it is evident from a careful stud}^ of the disease that the injurious 

 development of the fungus is distinctly correlated with special physi- 

 ological phenomena of the peach tree itself. These conditions of the 

 tree are in tiu-n dependent upon such external influences as tem- 

 perature, the humidity of the soil and atmosphere, and others. Such 

 facts were foreshadowed by the theories advanced by peach growers 

 as to the cause of the disease. Many growers have considered peach 

 leaf curl as the direct result of excessive moisture and low tem- 

 perature or sudden changes, and as these physical conditions cer- 

 tainly have an important bearing upon the injurious development of 

 the disease, they are considered together with the direct relations of 

 the parasite to its host. However, too much stress can not be placed 

 upon the fact that the fungus alone is responsible for the injur}^ to the 

 tree. Without the parasite not a leaf would curl nor a peach fall on 

 account of this malady — in fact, no such disease would exist. This is 

 shown by the work hereafter detailed. It is fortunate that the direct 

 cause of peach leaf curl is a parasitic fungus rather than unfavorable 

 atmospheric conditions, for the latter could not be controlled, while 

 the control of the fungus has been found practicable, simple, and inex- 

 pensive. 



PHYSICAL CONDITIONS INFLUENCING THE DISEASE. 



The influences of temperature, humidity, situation , soil, etc. , upon leaf 

 curl are often so well marked that the}' have f requentl}^ and in fact quite 

 generally been mistaken for the active cause of the disease. Indeed a 

 veiy large percentage of peach growers have maintained, to within the 

 past ten or fifteen years, that sudden changes of temperature occurring 

 in conjunction with wet weather are, the sole cause of the curling and 

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