PLANT DISEASES 433 



(host) brought about by the presence of the parasite, a con- 

 dition which is more or less unfavorable to the work of the 

 plant. This means that while we investigate the parasite 

 to discover its habits, the patient that has the disease is the 

 host plant. The study of plant diseases, therefore, so far as 

 plant parasites are concerned, is the study of the effect of 

 the parasite on the host plant. 



The practical application of our knowledge of parasites 

 and diseases has not resulted so much in curing diseases as 

 in preventing them, which means preventing the attack of 

 parasites. This involves enough knowledge of the parasite 

 to know the form in which it makes its attack, as well as the 

 part attacked and the time of attack. 



139. The groups of parasites. Almost all of the groups 

 of fungi contain parasites that are dangerous to cultivated 

 plants. These parasites are more or less selective, that is, 

 they do not all attack all plants indiscriminately. Each 

 parasite is more or less restricted to certain hosts, and often 

 to a single host. This explains why one kind of plant is 

 subject to a certain disease (" susceptible "), and another 

 is not (" immune "). 



The groups of parasites are very numerous, and it would 

 be impossible for an elementary student to learn to recog- 

 nize them ; but this is not important for our purpose. All 

 that is necessary in this first contact is to learn to recognize 

 certain " symptoms " of disease which attacked plants 

 show. Any symptom suggests troubles which may be 

 brought about by a great variety of parasites, and it is not 

 always necessary to distinguish the -parasite exactly before 

 using the appropriate preventive measures. 



140. The groups of diseases. All plant diseases can be 

 referred to three groups, which differ as to the relation of 

 parasite and host. 



In one group, the parasite kills living cells, and its de- 

 structiveness depends upon the number and kind of cells 



