272 GENERAL PLANT PATHOLOGY 



irritability of the living plant organisms. A knowledge of insect life 

 is essential, as also the chemistry of the plant, of the soils, of the ferti- 

 lizers, of the insecticides and fungicides. The physics of sap ascent, 

 of osmosis, of turgescence, and of the soil must be studied.^ 



The investigation of malformed organs and cells may be classified 

 under the head of Pathologic Morphology, and if the microscope is 

 used, it may include Pathologic Histology and Pathologic Cytology. 

 Disturbed conditions of the reproductive cells and organs bring about 

 anomaUes in the offspring, so that genetically speaking freaks, bizarre 

 forms, or chimeras arise. Diseased conditions may be traceable to 

 disturbed nutrition, to excessive or retarded growth and to abnormal 

 irritability. Therefore to be a successful pathologist, one must be a 

 good morphologist, histologist, geneticist and physiologist. 



Phytopathology is that phase of botanic inquiry which treats of the 

 diseases of plants. Itshistory dates from about 1850. Disease may be 

 looked upon as an unwholesome condition, derangement of, perversion 

 of, or departure from the normal in structure, in function, or in both com- 

 bined. It is a morbid state. One who studies phytopathology is con- 

 cerned with the characteristic symptoms of disease (Symptomatology), 

 the interpretation of symptoms (Diagnosis), with the causes of diseases 

 (Etiology) and with the remedies (Therapeutics) and prevention of dis- 

 ease (Prophylaxis). Recently considerable attention has been given 

 prophylaxis, following out the old adage that an ounce of prevention is 

 worth a pound of cure. Curative agents are therapeutic agents. 



ETIOLOGY. — At the outset it is important to consider the causes of 

 disease. These may be considered under two heads, predisposing and 

 determining. 



Predisposing Causes of Disease. — The normal plant can to a cer- 

 tain extent ward off the attack of disease, but the power to do so varies 

 within wide limits, which may be conditioned upon racial, or individual 

 characteristics of resistance. The degree of this resistance determines 

 the degree of the immunity of the plant organism. It is well known 

 that the normal constitution of plants varies considerably in individ- 

 uals of the same variety and among different races and varieties of the 

 same species. Some individuals and varieties are constitutionally weak, 

 others are strong and resistant to external influences of every descrip- 



'Appel, O.: The Relations between Scientific Botany and Phytopathology. 

 Annals Mo. Bot. Gard., 2: 275-285, February, April, i9r5. 



