3i8 Diseases of Truck Crops 



tion depends on the variety of tuber and the cailtural 

 conditions. Common scab may often be confused 

 with powdery scab; but a careful examination will 

 reveal striking differences. 



Symptoms. The disease attacks the tubers only. 

 It begins as small surface spots or stains, which soon 

 spread and increase in depth, penetrating to a depth 

 of a half centimeter. The spots consist of accumu- 

 lated corky tissue which may be readily removed 

 (fig- 59 g)- The diseased cells lose their starch and 

 are filled instead with what appears as fat globules. 

 The scab spot is merely the resvdt of the corky cam- 

 bium cells which are formed to protect the inner 

 starch-bearing parenchyma tissue from the irrita- 

 tion of the parasite. Scab does not impair the germi- 

 nation of the seed, but it reduces the jrield as well as 

 prejudicing the keeping qualities of the tubers. It 

 does not in any way impair their edible quality. 



The Organism. The scab-causing organism was 

 formerly believed to belong to the class of fungi and 

 was originally named Oospora scabies Thaxter. But 

 two American workers, Lutman and Cunningham,' 

 found that the scab organism is not a fungus, ^sut 

 belongs to the thread bacteria (fig. 59 h, i). A. 

 chromogenus consists of long irregular filaments; the 

 cross walls of the branches are scarcely visible. On 

 agar, under lack of moisture conditions or concentra- 

 tion of medium, the filaments grow out, and become 

 closely segmented into short rods known as gonidia 



' Lutman, B. F., and Ctinningham, G. C, Vermont Agr. Expt. Sta. 

 Bui. 184 : 3-64, J914. 



