Beet Diseases 115 



Crown gall is a very important disease because of 

 its cosmopolitan nature, for it is widely prevalent 

 and attacks a large number of hosts. 



Symptoms. The disease does not usually mani- 

 fest itself until the roots are nearly half grown. 

 Abnormal outgrowths or galls (fig. 16, b.) appear 

 which vary in size from that of a garden pea to 

 nearly two inches in diameter, according to the se- 

 verity of the attack. The galls are usually attached 

 to the beet by a narrow string. In light cases of 

 infection there may be but one gall on the root; in 

 severe cases, however, the roots may be covered with 

 numerous galls. 



The Organism. The cause of crown gall is a bac- 

 terial organism, Fseudomonas tumefaciens Sm. and 

 Town. It is a short rod, multiplying by fission, and 

 moves about by means of polar flagella. On agar 

 or gelatin it forms small round white colonies. Un- 

 der unfavorable conditions it readily develops in- 

 volution forms; the organism is short lived in pure 

 culture. P. tumefaciens lives over in the soil from 

 year to year. 



Control. The disease may be introduced with in- 

 fected soil. Sterilizing the soil with steam or for- 

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



Scab 



Caused by Actinomyces chromogenus Gasp. 

 Scab on beets is the same as the scab of the Irish 

 potato, the radish, and the carrot. 



