44 Walter H. Burkholder 



When the infection is local and arises from external infection, the xylem 

 may or may not be involved. 



With plants in which the bacteria have multiplied rapidly in the xylem 

 vessels and have become very numerous, certain symptoms appear on the 

 leaves before the pathogene itself has extended to them. There is first a 

 definite mosaic appearance (figure 9), with lighter and darker areas dis- 

 tributed over the leaf, the darker areas being nearer the larger veins. 

 This symptom has been mistaken for the true mosaic, but necrotic lesions 

 appearing on the plant soon make evident the difference. The mosaic-like 

 symptom is not difficult to produce in the greenhouse in artificially inocu- 

 lated plants. 



A more severe manifestation of this type of chlorotic symptom has been 

 noticed frequently in the field on Red Kidney beans. In this case the 

 young leaves of an infected plant are stunted in size, very pale green in 

 color, and extremely crinkled, and the veins are very prominent. Such 

 plants usually succumb very quickly to the disease. 



On the stem 



Longitudinal streaks due to this pathogene may appear on the stalk, 

 the stem, and the leaf petioles of the bean. On seedlings, the tissue of 

 which is very tender and succulent, such lesions at first are water-soaked 

 and dark green. Later they may dry down and become somewhat reddish, 

 though the color is not always present. On older plants the water-soaked 

 condition is not so evident. Stem lesions, especially those on the 

 seedlings, are local and usually of exogenous origin. When the stem lesions 

 are of endogenous origin a somewhat different picture is produced. The 

 bacteria breaking through the xylem vessels cause longer lesions, often 

 with longitudinal cracks. One very characteristic condition may occur on 

 such lesions which is not found in any of the other bacterial diseases of 

 the bean. The pathogene may ooze out in relatively large droplets on the 

 surface of the stem (figure 10). These droplets are colorless to milky, 

 and in appearance are not unlike those due to Erwinia amylovora in fire 

 blight of apples and pears. As a rule they have been observed in the field 

 in the early part of the growing season and during cool, wet weather. 

 They have also been produced in the greenhouse on Red Kidney plants. 



On the pods 



The individual spots produced by the disease on bean pods are more or 

 less circular in outline, but they may anastomose and give rise to rather 

 irregular lesions (figure 11). These lesions are at first green and water- 

 soaked, but later they may dry out and become sunken and reddish brown 

 in color. In this respect they are similar to spots produced by Phyt. 

 phaseoli. In some cases the water-soaked appearance may still be present 

 even after the pods have ripened and dried. Over these pod lesions one 



