8 DISEASES OF TROPICAL PLANTS CH . 



of fruits ; (n) witches' brooms ; (o) leaf curls ; (p) leaf 

 rosettes ; (q) hairy root ; (r) exudations (gums, resins) ; 

 (5) sun burns ; (t) rots. 



A symptom may appear as the result of any one 

 of a number of causes, for example, wilting may be due 

 to attacks from insects, fungi, nematodes, or unsuitable 

 soil conditions ; hypertrophies may be due to the 

 attacks of insects, nematodes, fungi, or bacteria ; 

 mechanical injuries and witches' brooms to insects or 

 fungi. 



However, it must be remembered that the part in 

 which the disease becomes manifest to the eye is not 

 necessarily the seat of the disease ; the cause of the 

 disease may be in a part of the plant very remote from 

 the part in which some one or more of the preceding 

 symptoms appear, e.g. a disease of the roots may cause 

 a discoloration, wilting, or dropping of the leaves. It 

 is also true that some diseases may be present in the 

 plant for a long time before the plant shows any 

 external evidence of the disease. 



This last fact increases the difficulties of detection 

 of certain diseases by inspection, and thereby increases 

 the dangers in the introduction of foreign plants. 



If the disease is due to a specific organism, the 

 organism may bring about the unhealthy condition in 

 its host : (a) by feeding upon its tissues or sucking its 

 juices or otherwise drawing upon it for food ; (b) by 

 covering the surface of the leaves or stem and thus 

 interfering with the action of the light and proper 

 transpiration ; (c) by clogging up the tubes of the 

 fibro- vascular bundles and interfering with the circula- 

 tion of water and other fluids; (d) by attacking the 

 growing regions, especially the root tips, and interfering 

 with or preventing their development ; (e) by attacking 

 the cambium, the growing part of the stems, as in the 

 case of the canker fungi ; (/) by living inside the host 

 and feeding upon it and secreting poisons which kill, 

 as in the case of some of the "damping off" fungi; 

 (g) by attacking the fruit but doing little or no damage 



