The Bacterial Diseases of the Bean 11 



less severely infected seed, or when weather conditions are less favorable 

 for the bacteria, they may be confined at first entirely within the cotyle- 

 dons. Here they may multiply in the fleshy tissue, and be washed or 

 splashed by rain to various parts of the young growing plant. An injury 

 is not necessary for the bacteria to cause infection, and almost any tissue 

 above ground may be invaded. In some cases, during the early stages 

 of development the pathogene in the cotyledons may enter the stem of the 

 plant through the vascular system and establish itself in the xylem vessels. 

 The walls of these vessels apparently are very resistant to the organism, 

 since the bacteria may exist there for weeks without breaking through 

 into other tissue. In the xylem vessels, however, the bacteria may travel 

 throughout the plant. Their progress usually is upward, probably owing 

 to the water stream; but they may also travel downward, and in rare 

 cases have been recovered from the roots of diseased plants. When they 

 are confined to the stem alone, little effect may be produced on the plant 

 aside from a slight stunting; but when they pass into the leaves, the 

 young growing tips, and other plant parts, wilts and necrotic lesions may 

 result. In their passing through the xylem vessels of the pedicel into 

 fairly mature pods and infecting the seed, no external evidence of the 

 presence of the bacteria may be noticed. Zaumeyer (1929) has recently 

 described in detail the tissues involved in this latter process. 



The tendency for Phyt. phaseoli to systemically invade a plant was 

 first noted by Barlow (1904) and was later emphasized by the writer 

 (Burkholder, 1921). Plants so infected are not the rule in the fields of 

 New York; nevertheless they do serve a purpose in the life history of the 

 organism in this climate. The first month-and-a-half of the growing 

 period of the bean plant is unfavorable for the development of the blight. 

 June and early July are inclined to be cool, and Phyt. phaseoli prefers a 

 warm temperature. In late July and during August, when the pathogene 

 has come to the surface in systemically infected plants through various 

 lesions, these plants act as a source of inoculum for the spread of the 

 disease. At times, in a bean field under close observation, one may watch 

 the spread of the blight in an ever-growing radius about such a plant. 

 The overwhelming majority of the leaf and pod lesions which one observes 

 in a field, are local and of exogenous origin. Even in these lesions, however, 

 and especially on the leaves, the pathogene usually localizes or develops 

 more rapidly in the xylem vessels. 



The rapid development of the blight in a field depends considerably upon 

 warm weather. It may be observed that the disease is more prevalent 

 in hot seasons than in cool ones. In the greenhouse, infection is more 

 easily obtained at 80° F. than at 65° F., and at the latter temperature the 

 leaf spots develop very slowly. One observation in the field on the relation 

 of temperature to the blight is possibly worthy of recording here. In 

 1922 the writer had a small field of beans at Ithaca on a northern slope. 



