232 



AGRONOMY 



light and may float in the air for long distances before coming 

 to rest, thus spreading the species very widely. Like other 

 plants, they need warmth and moisture to grow, and increase 

 most rapidly in warm, cloudy weather. The harmful bacteria 

 in the soil may be carried from one field to another in the dirt 



FIG. 169. Bacterial wilt of melons 

 From Duggar's "Fungous Diseases of Plants" 



that adheres to the feet of animals, on the implements used in 

 stirring the soil, and even by currents of water during rains. 



Number of plant diseases. An immense number of organ- 

 isms produce disease in plants, and if these could all live on 

 the first species encountered, it is likely that few plants of 

 any kind would come to maturity. Fortunately most plant 

 diseases are restricted to a single species or a related group 



