2 DISEASES OF CROP-PLANTS 



4. Parasitic flowering plants. 



5. Nematode worms. 



6. Mites. 



7. Insects. 



(b) Non-parasitic Diseases. 



Non-transferable, with specific S5niiptoms and fre- 

 quently with characteristic lesions. 



As no strict definition can be given of disease there is no 

 fixed limit to such a list as this. Injuries, mostly direct, may be 

 produced by grazing animals, by hail, or by lightning, while 

 deficiencies in soil, unfavourable weather, an unsuitable climate, 

 and last, but not least, the unskilful or negligent cultivator, 

 may all he set down as disease-producing agencies, and as such 

 will be referred to upon occasion in subsequent discussions. 



Naturally there exist at any given time recognised diseases 

 of which the cause is unknown or disputed. The safest rule in 

 such cases is to regard them tentatively as being of parasitic 

 origin if their mode of occurrence suggests that they are trans- 

 ferable. 



The use of the term physiological disease is by common 

 consent undesirable, though there must be few pathologists 

 who have not found it convenient on occasion. It has repre- 

 sented a sort of Cave of Adullam to which diseases not known 

 to be of parasitic origin could be relegated, but is .objectionable 

 in form since all diseases properly considered involve disturb- 

 ances of physiological processes. 



In relation to agricultural practice the most important 

 distinction among plant diseases is to be made between those 

 which are able to develop upon plants in normal condition and 

 those which occur to a serious extent only upon plants reduced 

 in vigour by unfavourable circumstances, as of soil, chmate, 

 insect infestation, or methods of cultivation. For those which 

 have this character in a marked degree the term debility diseases 

 is used in the following pages. 



