PART I 



INTRODUCTION 



THE NATURE AND CLASSIFICATION OF PLANT 

 DISEASES 



Health is a state in which each organ performs its own function 

 and acts in harmony with every other, and disease in the broadest 

 sense of the word consists of any departure from this state. 

 A condition of disease proceeds from a derangement of the 

 function of any organ, and in plants this most frequently follows 

 upon derangement in structure. 



Two classes of diseases are generally recognised as coming 

 within the scope of the mycologist or plant pathologist : parasitic 

 diseases caused by fungi or bacteria, and non-parasitic diseases, 

 in the sense of affections characterized by specific symptoms 

 and believed to result from some disturbance of function caused 

 otherwise than by a parasitic organism. 



This limitation is one of convenience, not of logic. Insect 

 injuries to the living tissues react on the health or well-being 

 of the plant, and often produce serious derangements of function 

 resulting in general symptoms which in some cases have no 

 obvious relation to the wounds inflicted. These are as definitely 

 diseases as the affections produced by fungi, but are in practice 

 mostly left to the entomologist. It is a defect of this system that 

 the botanical aspects of insect injury often remain unstudied. 



The term mycologist, it may be noted here, as applied to a 

 student of plant diseases, dates' from the time when the idea of 

 their causation was limited to fungus parasitism. It properly 

 applies to the student of fungi ; the general practitioner in 

 plant pathology is more largely concerned with the host, and 

 the range of his vision must extend at least to bacterial diseases, 

 virus diseases and those of non-parasitic origin. 



A convenient approximation to a classification of plant 

 diseases, in the broader sense, which is not complete, but includes 

 the chief groups of diseases having more or less obvious specific 

 characters, may be made as follows : — 



{a) Parasitic Diseases. — Caused by : — 



1. Fungi. 



2. Bacteria. 



3. Infectious viruses. (Ultra-microscopic organisms ? ) 



I B 



