Family Convolvulaceae 155 



White Rust 

 Caused by Cystopus ipomoece-pand Manes Farl. 



White rust is a disease of the foliage only. It is 

 present in nearly every field where sweet potatoes are 

 grown. Although prevalent, the disease is almost 

 unrecognized as such by the growers. The losses 

 from it are indirect. While it is true that the sweet 

 potato is grown only for its roots, nevertheless, a good 

 crop depends upon a healthy and abundant stand of 

 leaves. The sugar and the starch in the potato are 

 not manufactured from the soil, but are made by the 

 leaves from the air and sunlight, and are then stored 

 in the roots. The effect of white rust is to kill much 

 of the foliage, thereby curtailing the amount of sugar 

 and starch manufactured, and this in turn results in a 

 shorter crop and poorer keeping roots. It is gener- 

 ally agreed that the more starch a root has, the better 

 it keeps. The White Southern Queen variety is one 

 of the best keepers, being richest in starch content. 



Symptoms. White rust appears late when the 

 plants have usually made most of the vine growth and 

 when the hills are beginning to set, i.e., to form new 

 sweet potatoes in the soil. A typical symptom of 

 white rust then is the yellowing of leaves in the center 

 of the hill, which later turn brown, shrivel, and die. 



In carefully examining the center leaves as they 

 begin to yellow, we see that on the under side of such 

 leaves there are many minute white raised pimples, 

 (fig. 25 e-f) each of which when touched with the 



