The Bacterial Diseases of the Bean 71 



ecological factors. Temperature and moisture, in some instances, appear 

 to play an important part. In other cases the origin of the lesion, whether 

 exogenous or endogenous, may be of significance. The possibility of 

 there being races of the pathogene with different symptom-producing 

 abilities should not be ruled out of consideration, although such cases 

 have not been proved. 



Certain general aspects of the diseases which might prove to be of some 

 diagnostic value should be considered first. As already stated, the diseases 

 caused by Phytomonas phaseoli and Phyt. flaccumfaciens appear to be 

 favored by hot weather, while the amount of rainfall apparently is not 

 an important factor. On the other hand, Phyt. medicaginis var. phaseo- 

 licola occurs with greater severity in cool seasons. This disease at such 

 times may spread rapidly through a field and work with great speed in 

 the individual plant. In New York no other bacterial disease has caused 

 so much damage, and by the very severity of its action one may almost 

 determine the causal organism. 



In considering the individual symptoms of the diseases on the plant 

 as of diagnostic value, they are merely pointed out in this discussion, 

 since they have been described in some detail earlier in the memoir. The 

 most noticeable symptoms of these diseases occur on the leaf, and it is 

 here that most of the peculiarities and characteristic lesions are to be 

 found. The halo-blight leaf spot (figure 6) caused by Phyt. medicaginis 

 var. phaseolicola is a symptom which can scarcely be mistaken or con- 

 fused with other leaf spots. This organism, however, does not always 

 produce this type of lesion, and in New York, during the hot months, 

 the common symptom is the small angular spot which occurs in such 

 great numbers over the leaf (figure 7). Phyt. phaseoli, however, has 

 been known to produce a spot similar to this latter one, and so this organ- 

 ism should always be kept in mind. In plants systemically infected, 

 certain leaf symptoms indicate when Phyt. medicaginis var. phaseolicola 

 is the causative agent. These are the mosaic appearance of the upper 

 leaves, and, in severe cases, the dwarfed and yellowish crinkled condition 

 of these leaves. However, in the absence of these symptoms, even though 

 the vascular tissue is affected it is not safe to assume that the disease is 

 caused by some other vascular-invading organism. It should be mentioned 

 here, too, that the mosaic symptoms on the leaves have been produced 

 in the greenhouse on Red Kidney beans artificially inoculated with Phyt. 

 flaccumfaciens. While this experiment has been repeated a number of 

 times in the greenhouse, the symptom has never been observed in the 

 field due to this latter organism. 



Phyt. phaseoli produces, on the leaf, brown necrotic lesions of varying 

 sizes, with yellow borders (figure 1). These spots may enlarge and cover 

 most of the leaf tissue. The lesion illustrated in figure 8, however, caused 

 by Phyt. medicaginis var. phaseolicola, is not unlike this type of leaf spot. 



