174 Diseases of Greenhouse Crops 



Symptoms. Black rot on radish is confined mostly 

 to the tender white-rooted varieties, especially the 

 Icicle. The black-rot germ penetrates the lateral 

 feeding rootlets, from which it works its way into 

 the main root. In cutting across a diseased radish, 

 its interior iibrovascular bundles are found to be 

 blackened. Such radishes are useless for the mar- 

 ket. The disease seldom attacks the red or the 

 black-skinned varieties. For further consideration 

 see Black Rot, p. 124. 



Scab 



Caused by Actinomyces chromogenus Gasp. 



Scab is not a common field disease of radishes. 

 It is, however, found to be troublesome on the crop 

 grown in greenhouses. The French Breakfast va- 

 riety is commonly susceptible to the disease. The 

 trouble may be expected if the crop is planted in a 

 soil which previously produced a potato crop that 

 was badly scabbed or where infected manure was 

 used, or too much lime applied. For methods of 

 control, soil sterilization with steam or formalde- 

 hyde is recommended (see pp. 32-43). 



Damping Off 



Caused by Kheo sporangium asphanidermatum Ed. 



Symptoms. The disease seldom attacks the leaves. 

 The injury is confined to the roots only. Diseased 

 plants are flabby, pale or yellowish, with a tendency 

 to wilt. The roots when pulled out will show that 



