256 DISEASES OF TROPICAL PLANTS 



CH. 



Lichens. These plants are very common on trees 

 of all kinds. They are epiphytic or semi-saprophytic 

 in nature, but they interfere to a greater or less 

 extent with the normal physiological activities of the 

 plants. When they prove a pest or give an unsightly 

 appearance to fruit, shade, or ornamental trees, they 

 should be treated in the manner described on page 123. 



Soil and Climate. The diseases due to soil and 

 climatic conditions are many. Sometimes they can be 

 recognised and remedied, while in other cases they are 

 difficult to recognise and often more difficult to remedy. 

 In a state of nature these troubles are not so notice- 

 able, since if the conditions are unfavourable for a 

 certain species of plants, they naturally give way to 

 those plants which find the conditions favourable. 



In the case of plantings, the unfavourable conditions 

 of either soil or climate usually so weaken the vitality 

 of the tree as to make it especially susceptible to organic 

 troubles which frequently result in its death. Many of 

 the diseases of fruit and ornamental trees are due to 

 attempts to grow these trees under unfavourable con- 

 ditions, thus causing them to be weak and unable to 

 resist the attacks of root fungi and other organisms. 



Diseases of Foliage. This class of diseases has been 

 treated under other heads, so that it is not necessary to 

 take it up at this time. The native foliage diseases to 

 which the species of trees of any part of the world may 

 be subject are usually of comparatively little importance, 

 but diseases may be introduced which may prove very 

 destructive. Foliage diseases may be treated upon 

 many classes of cultivated trees, but it is not practicable 

 to give treatment to forest trees for such troubles. 



Diseases of Roots. The diseases of roots of trees 

 are very poorly understood, but they doubtless do 

 considerable damage to forest trees as well as of 

 cultivated trees. They have been referred to in con- 

 nection with other subjects. 



Mycorrhizas. The roots of many plants are fre- 

 quently, and with some species usually, found to be 



