38 Walter H. Burkholder 



diseases produced by Phyt. phaseoli and Phyt. flaccum.faciens has caused 

 it to be overlooked as a separate entity. 



The symptoms of the disease caused by Phyt. medicaginis var. phaseoli- 

 cola have been described, but perhaps not fully. Careful observation of 

 this disease in the field has been maintained during the past five years, 

 and many variations of the symptoms have been noted. It has been 

 observed that certain types of lesions, especially on the leaves of the host 

 plant, occur in one season in profusion and in the following season are 

 inconspicuous. These variations in symptoms appear to be due to ecologi- 

 cal factors, and no indications have been observed that various strains 

 of the pathogene are involved. Consequently, after these observations 

 over a considerable period of time, a better picture may be given of the 

 disease as it appears under natural conditions in the field than was given 

 in the first report. 



In the following descriptions of the symptoms of the disease, it must be 

 remembered that observations have been made primarily in the field on 

 the Red Kidney variety of beans. The symptoms do not differ, however, 

 except in intensity, on such common varieties as Pea, Marrow, Yellow Eye, 

 White Kidney, and Refugee. In the majority of cases the symptoms 

 observed in the field under natural conditions can be reproduced in the 

 greenhouse. 



LIFE HISTORY 



The life history of Phytomonas medicaginis var. phaseolicola is very 

 similar in detail to that of Phyt. phaseoli, and for this reason only the salient 

 points are given here. 



The organism overwinters on and in the bean seed, and becomes active 

 in the spring when such seeds are planted and have germinated. Sufficiently 

 high temperature and enough moisture are necessary for the abundant 

 development of the bacteria in the cotyledons. Here they multiply rapidly 

 in the host tissue, and may serve as inoculum to be splashed by rain to 

 the leaves and other parts of the infected individual or to near-by plants. 

 The organism evidently gains entrance to the plant through the stomates, 

 since mechanical injury is not necessary for infection to take place. A 

 second mode of invasion is for the bacteria to enter the plant from the coty- 

 ledons by way of the vascular system. Under such conditions an infection 

 of the vascular system takes place, the xylem being the tissue most 

 generally invaded although the bacteria may break forth into the adjacent 

 cells and even to the surface of the plant. Since Phyt. medicaginis var. 

 phaseolicola exhibits a greater virulence than does Phyt . phaseoli, the disease 

 from this last-named type of invasion is more likely to become acute, 

 rather than remain dormant for some time as it does with Phyt. phaseoli. 

 A further difference between the two organisms is that Phyt. medicaginis 

 var. phaseolicola appears to produce a toxin which precedes the organism 

 in its harmful effect on the plant. This is not so noticeable with Phyt. 



