6 DISEASES OF TROPICAL PLANTS CH . 



The diseases of plants may be attributed to some 

 one or more of the following causes : 



Fungi. Soils. 



Bacteria. Moisture. 



Slime moulds. Temperature. 



Flowering plants. Gas and smoke. 



Insects. Physiological. 

 Mites. 

 Nematodes. 



The first group may be considered organic, and the 

 second group non-organic ; or the first group may be 

 considered parasitic (although some of them are not 

 strictly parasitic), and the second non-parasitic. The 

 first group includes that great host of living organisms 

 which live within or are attached to the host plants, 

 affecting their health and frequently causing death. 

 The extent of this parasitism varies from that of 

 organisms which depend upon the hosts for all of their 

 food supply, to that of those which receive very little 

 sustenance. Some parasites destroy their hosts in very 

 short periods of time, while others live upon their hosts 

 for long seasons, even for many years, before death 

 results. Still other parasites never cause the death of 

 the host plant, but do reduce its vitality, growth, or 

 fruit production. Some parasites do not require a 

 great amount of food from the host plant but grow in 

 such a manner as to interfere with the performance of 

 its ordinary functions. Some which are of little 

 importance in themselves open the way for the attacks 

 of organisms which cause other and more destructive 

 diseases. A disease may also produce conditions suit- 

 able for the growth of organisms which are in no way 

 the cause of the disease in question. The presence of 

 two or more organisms in or on a diseased plant 

 frequently prevents a satisfactory diagnosis and 

 treatment. 



Diseases resulting from the second or non-organic 

 group of causes are frequently very difficult to diagnose. 

 Often they are of little importance in themselves, but 



