Prevention or Cure of Plant Diseases 17 



to one half mile, or ))ctter still, a greater distance. When 

 it is not practicable to remove the host, the offending por- 

 tions may be cut away, e. g. in the case of the apple rust 

 the cedar-balls may be removed from adjacent trees. 



8. Avoidance of disease-bearing material or material 

 favoring disease. — Infection may roach hay through sick 

 melon vines (melon wilt), or manure may be contaminated 

 by infected plant parts (potato seal), 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 handling 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 underljang 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- 

 bility 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 disease-infested 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 late- 

 blight), and the proximity of certain kinds of plants leads to 

 disease with others (apple rust). All this should be con- 

 sidered in the selection of land for the crop. 



• 12. The practice of crop rotation. — Constant growth 

 of the same crop in any locality tends to increase the enemies 



