Family Solanaceae 315 



rity always filled with black spore balls. The sori 

 of common scab are shallow and made up of corky 

 compact tissue. After being handled or shipped long 

 distances, potatoes infected with powdery scab can- 

 not be distinguished from those suffering from com- 

 mon scab. However, a microscopical examination 

 of the sori will soon reveal the difference. 



In storage, potatoes infected with powdery scab 

 will dry rot. This is but the final stage of the disease 

 (fig. 59 b) . While it is not uncommon for the dry rot 

 to invade the whole tuber, it generally extends only in 

 spots. The effect of powdery scab on stored potatoes 

 is a more rapid drying of the tubers and the opening 

 of a way for the invasion of secondary infection. 



In the field, the disease is favored only by cool, 

 damp, and rainy weather. Besides the potato, the 

 tomato too is attacked by powdery scab. In this 

 case, infection is confined to the root system, which 

 is much more distorted than is the case with the po- 

 tato. Of the other hosts affected may be mentioned 

 Solanum warscewicii, S. hematoclodum, S. mammosum, 

 S. marginatum, S. ciliatum, and S. commersoni. 



The Organism. The plasmodium within the host 

 cells is irregular in shape. It is composed of proto- 

 plasm within which are evenly distributed nuclei. 

 Within the host cell the protoplasm of the plasmodium 

 is closely applied to the host nucleus. Infection 

 seems to take place by means of a plasmodium rather 

 than by single amoebae. The parasite is confined to 

 the phloem of the host. The invaded cells are not 

 killed, but are stimulated to an abnormal cell division. 



