Sweet Pea Diseases 341 



vessels, thus clogging the upward flow of the water 

 from the roots to the stem. Freshly collapsed 

 plants usually have a water-soaked appearance, and 

 are later overrun by a white weft, which is merely 

 the mycelium of the fungus; this is followed by 

 sclerotia (fig. 69, d) (resting bodies), which are 

 found scattered here and there on or within the af- 

 fected stems. The fungus is a soil organism which 

 occasionally causes trouble in clover fields. It is in- 

 troduced with animal manure. For a description of 

 the causal organism and methods of control, see p. 

 151. 



Thielavia Root Rot 



Caused by Thielavia hasicola Zopf. 



Symptoms. Plants severely infected with Thie- 

 lavia develop practically little or no root system, 

 since the new roots are destroyed as soon as they are 

 formed. Generally all that is left is a stub, which 

 is charred in appearance (fig. 70, b.). The dis- 

 ease often w^orks up from the affected roots to the 

 stems, some 2 or 3 inches above ground. Affected 

 plants neither die nor wilt, but remain dwarfed, 

 stunted, and sickly pale in color, and produce few 

 or no blossoms. For a description of the causal or- 

 ganism and methods of control, see p. 355. 



Powdery Mildew 



Caused by Microsphera alni (Waller) Salm. 

 Symptoms. Powdery mildew is a very common 

 trouble of greenhouse sweet peas. The causal fun- 



