250 DISEASES OF TROPICAL PLANTS CH . 



The organisms which cause these diseases attack the 

 leaves, stems, and roots, but in a state of nature those 

 on the leaf, rarely become destructive, although they 

 draw upon the tree's strength, reduce its vitality, and 

 frequently make it more susceptible to other and more 

 dangerous parasites. 



The diseases of the roots, trunks, and branches may 

 be due to many causes, which may be grouped under 

 the following heads : 



1. Diseases caused by flowering plants. 



2. Diseases caused by algae, bacteria, slime-moulds, lichens, and 



fungi. 



3. Diseases caused by mechanical injuries. 



4. Diseases caused by unsuitable soil. 



5. Diseases caused by unsuitable climate. 



6. Diseases due to unknown causes. 



Wound Parasites. Many of the parasites which 

 cause very destructive diseases of our trees are unable 

 to attack the plants unless they gain entrance through 

 wounds. This is especially true of the organisms which 

 attack the trunks, stems, or branches of trees, and cause 

 both disease of the living trees and decay of the timbers. 

 Many of these organisms are primarily saprophytic and 

 live on old logs, stumps, dead branches, etc., but when 

 they gain entrance to the living plant through wounds 

 they become partially parasitic and frequently cause 

 the death and destruction of the entire plant. 



This class of organisms and the diseases which are 

 caused by them are especially destructive to forests and 

 to fruit trees. 



Under natural conditions these wounds may be 

 caused by wind-storms, hail-storms, insects and other 

 animals. Under cultivation the wounds may also be 

 the result of careless cultivation, pruning, tapping, and 

 of accidents in gathering the fruit. 



The organisms live in the dead logs, stumps, twigs, 

 and debris of the forest or orchard where they reach 

 maturity and produce their spores. In fact, many 

 species produce spores only on the dead and decaying 



