Family Leguminosae 275 



THIELAVIA ROOT ROT 

 Caused by Thielavia basicola Zopf . 



Root rot is a common disease in fields devoted to 

 peas for a period of years. The trouble on the garden 

 and field pea is identical with that on the sweet pea. 



Symptoms. Plants severely infected with Thie- 

 lavia have practically no root system, since this is 

 destroyed by the fungus as rapidly as formed (fig. 

 51 a). All that is left of the root system is a charred 

 blackened stub. The diseased host constantly at- 

 tempts to produce new roots above the injured part 

 but these in turn also become infected. Such plants 

 linger for a long time, but fail to set pods which are 

 of any value. 



The Organism. The mycelium of Thielavia basicola 

 is hyaline, septate, and branched. The mycelium 

 becomes somewhat grayish with age. Three kinds of 

 spore forms are produced endospores (fig. 51 d), 

 chlamydospores (fig. 51 e), and ascospores. Endo- 

 spores are so called because they are formed inside 

 a special thread of the mycelium. This is the spore 

 form that commonly occurs in pure cultures of arti- 

 ficial media and on the host. The endospore case is 

 formed on terminal branches with a somewhat 

 swollen base and a long tapering cell. The endo- 

 spores are formed in the apex of this terminal cell and 

 are pushed out of the ruptured end by the growth of 

 the unfragmented protoplasm of the base. They are 

 hyaline, thin walled, and oblong to linear in shape. 



