136 DISEASES OF THE SWEET PEA 



soils, will naturally cause loss and disease. 

 Thielavia root rot commonly found in the 

 greenhouse produces a similar disease on 

 sweet peas in the^ field. In the latter case 

 it assumes greater economic importance, 

 especially on trial grounds. Root knot 

 has so far been encountered only on green- 

 house sweet peas. It will doubtless be 

 found in the field, however, before long. 

 The same is also true for the Rhizoctonia 

 root rot, the Fusarium root rot, and the 

 Sclerotinia collar rot. Growers of sweet 

 peas on a large scale for seed or for orna- 

 mental purposes should be on their guard 

 to prevent the introduction of these trou- 

 bles into their soils. It is much easier to 

 prevent infection than to eradicate a dis- 

 ease that has once gained a strong foot- 

 hold. Powdery mildew, a common leaf 

 trouble in the greenhouse, is found also 

 outdoors to some extent. On irrigated 



