PREVENTION OR CURE OF PLANT DISEASES 23 



parts (potato scab, cabbage rot, melon wilt, etc.). Soil 

 may wash from one field to another and bear disease with 

 it (tobacco wilt, melon wilt, etc.). 



9. Prevention of wound infection. — Examples : heart 

 rots, sap rots, cankers, fruit rots. Wound infection may 

 often be avoided by care in handhng the plant so as not 

 to wound it. In trees place ladders carefully, not roughly, 

 among the limbs; avoid walking on limbs with nailed or 

 hard boots, etc. Gather all fruit, root crops, etc., carefully 

 to avoid breaking the skin or bruising the underlying tissue. 

 In case of removal of large limbs coat the exposed wood 

 with paint or tar to prevent invasion by parasites. 



10. Avoidance of susceptible varieties ; selection of resist- 

 ant varieties. — There is much difference in the suscepti- 

 bihty of different varieties of plants. Study the different 

 varieties of the crop to be raised, particularly in this regard, 

 and use those that are most resistant. In case resistant 

 individuals are observed among sick plants, save their seed 

 carefully and test the resistance in succeeding years. In 

 this way new and resistant varieties may be developed. 



11. Avoidance of diseased localities. — This is in many 

 cases imperative where the soil is invaded by a germ 

 inimical to the growth of crops of certain kinds, as in the 

 case of cabbage black rot, cotton and melon wilt, etc. 

 Similarly, low damp locations favor diseases of some crops 

 (potato blight), and the proximity of certain kinds of plants 

 leads to disease with others (apple rust). All this should 

 be considered in the selection of land for the crop. 



12. The practice of crop rotation. — Constant growth 

 of the same crop in any locaUty tends to increase the 

 enemies of that crop. Rotation breaks the continuity 

 and hinders the multiplication of the parasite. 



