The Bacterial Diseases of the Bean 61 



plant above a lesion will die. The collapse in most cases is sudden and 

 complete. This death of the upper part of the plant is in no respect 

 similar to the wilt due to the xylem invasion by many of the other bacterial 

 bean pathogenes. 



Below the lesion the tissue of the plant remains healthy, and if a toxin 

 is formed by the bacteria it is not one which rapidly spreads ahead of the 

 invaded cells, as is found in the case of Phytomonas medicaginis var. 

 phaseolicola. There is no gradation between the dead tissue and the 

 healthy, as may be found in many necrotic diseases; instead, a sharp 

 line of demarcation exists between the two tissues, and this remains 

 stationary since the disease does not move downward. The healthy tissue, 

 however, begins to form a gall, which breaks out from underneath the 

 epidermis and often becomes twice the diameter of the original stem 

 (figure 13). 



It is reasonable to expect that the host plant would react by the forma- 

 tion of such a gall merely because of the accumulation of food material 

 at this point. In other words, the reaction may be due to the mechanical 

 injury of the disease. Phyt. vignae var. leguminophila may cause a similar 

 type of necrotic lesion on the bean plant, but in no case has gall formation 

 been observed. This would appear to indicate that the pathogene under 

 consideration played some part in the formation of the gall. This indica- 

 tion was strengthened further when it was observed that hypertrophy 

 occurred about relatively small lesions caused by the Swiss organism on 

 the stems of sieva beans. It must be admitted, however, that no careful 

 histological studies or attempts have been made to ascertain whether 

 bacteria are present in the galls or whether the bacteria produce a substance 

 which would stimulate gall formation. 



The action of the pathogene on the bean pod is somewhat different from 

 its action on the stem. On the pod no hypertrophy has been observed, 

 and, while the bacteria move rapidly in the tissue, there is not that flash 

 of invasion followed by sudden ceasing of growth which is found in the stem 

 tissue. If inoculations are made by injuring the surface of the pod, the 

 pathogene progresses rapidly from the point of inoculation and produces a 

 spot which is slightly sunken and from pecan brown to walnut brown in 

 color. There is no green, water-soaked appearance, as is found in the 

 early stages of pod lesions produced by Phyt. phaseoli, its variety fuscans, 

 and Phyt. medicaginis var. phaseolicola, or by the suture lesions produced 

 by Phyt. flaccumfaciens. The spot has more the appearance of a late 

 stage of a spot produced by Phyt. vignae var. leguminophila. In certain 

 instances when the injury to the pod tissue has been deep, the bacteria 

 develop rapidly and ooze out with the sap in small, reddish brown drops. 



In a few cases, inoculation experiments were conducted on seedlings and 

 on pods without injuring the plants. The seedlings were sprayed with a 

 suspension of the bacteria in water, and a similar suspension was painted 



