140 PLANT DISEASES 



(e) foliage spots; (/) perforation of foliage; (g) variegation 

 of foliage; (h) dying of leaves and twigs; (i) dwarfing or 

 atrophy of parts; (j) development of distorted or abnormal 

 growths, such as witches broom, galls, corky out-growths, etc. ; 

 (&) cankers; (I) increase in size or modification of parts; (m) 

 curling of foliage and formation of rosettes; (n) hairy roots; 

 (o) exudation (gums, resins) ; (p) sun burns; (q) rot of fruits, 

 stems or roots. 



The most noticeable diseases of plants are the rots of fruits 

 and stems which are usually caused by fungi and bacteria; 

 blights of the leaves, stems, flowers and fruits, which are also 

 caused by bacteria and fungi ; spots on leaves and fruits, which 

 are caused by both fungi and bacteria ; mildews, which are white 

 powdery fungous growths on leaves, twigs and fruits ; smuts and 

 rusts, which are fungous diseases of grains and other plants: 

 cankers, which may be due to fungi or other causes ; yellowings, 

 mosaics, and other discolorations, which may be due to any one 

 of many causes. 



Wilts. When the disease injures or destroys the root sys- 

 tem, the food and water supply from soil is reduced or cut off 

 and the plant is either weakened or killed. Diseases of the 

 stem may have the same effect. Diseases of the foliage may 

 interfere with the absorption of carbon dioxide, the transpira- 

 tion of water and the photosynthesis of the plant. 



The wilt diseases are usually due to fungi or bacteria which 

 live in the tracheary tissue of the fibro-vascular bundles and thus 

 interfere with the rise of water. These diseases are especially 

 injurious on herbaceous plants. The fact that the organisms live 

 within the plant make it impossible to treat by means of spray- 

 ing. Many of these wilt organisms live in the soil from year to 

 year and cannot be controlled except by crop rotation in which 

 it is necessary to use crops which are not subject to the disease 

 in question. 



