94 DISEASES OF THE SWEET PEA 



plied as well as the amount of moisture in 

 the air of the greenhouse may determine 

 the presence or absence of disease. A rela- 

 tively high percentage of moisture in the 

 soil or in the air favors damping off. The 

 humidity of the atmosphere in a green- 

 house is usually much higher than that out 

 of doors. From this it follows that good 

 ventilation should be provided. Over- 

 watering of sweet peas gives rise to un- 

 healthy conditions, especially in cloudy 

 weather, when evaporation and transpira- 

 tion are arrested. Thus the soil becomes 

 flooded with water which interferes with 

 the respiration of the roots and the plants 

 become more susceptible to diseases which 

 may be present in the soil in the green- 

 house. An excess of moisture in the air 

 also favors the development and spread 

 of the powdery mildew. During bright 

 sunny weather there is little danger from 



