76 Walter H. Burkholder 



Differences in loss of virulence of the pathogene in pure culture 



One point of difference among these bacteria which should be touched 

 upon here, although it can scarcely come under the head of cultural reac- 

 tions, is the variability in the loss of virulence, and even loss of pathogeni- 

 city, which exist among them. This fact has been mentioned in the 

 individual descriptions, but it should be emphasized here where the pecu- 

 liarities of each form are under discussion. The ability of the organisms 

 to retain their virulence in pure culture varies with the species. Phyt. 

 flaccumfaciens is an outstanding example of a pathogene that does not 

 lose its virulence. Strains of this organism four and five years old are 

 at hand which appear to have lost nothing of their ability to infect their 

 host, or at least such a loss is not evident. Phyt. medicaginis var. 

 phaseolicola is a second organism which may be placed in this class. One 

 strain has been under observation for four years and still has the ability 

 to kill young Red Kidney bean plants when these are inoculated. Certain 

 symptoms, however, such as the stem lesions, are not so striking in appear- 

 ance when produced by these old strains as when produced by a recently 

 isolated strain, but the final result appears to be similar. 



Strains of Phyt. phaseoli five and six years old will still cause definite 

 pod spots. They do not, however, infect the vascular system and pro- 

 duce a wilt as rapidly as do recently isolated strains. Some old strains have 

 apparently lost this ability, and a bean plant inoculated at a stem node 

 with such an organism will show no generalized signs of the disease. Phyt. 

 viridiflava and Phyt. vignae var. leguminophila lose their virulence very 

 rapidly in pure culture. Continued inoculation of the bean plant with 

 each, and reisolation of the organism, were necessary for inoculation 

 experiments in order to have at hand a culture of the pathogene which 

 showed any virulence. With Phyt. viridiflava the pathogenicity was 

 at times even entirely lost. 



In considering the loss of virulence in some of these species, one is forced 

 to look upon this ability of organisms to produce disease as a rather delicate 

 character, a variable one in many cases and one about which very little 

 is known. The plant pathologist has been inclined, in naming species, 

 to consider pathogenicity as a primary character. In this respect his 

 position as plant pathologist has made him a little biased. The fermen- 

 tation of a sugar is more stable, in many cases, than is the pathogenicity, 

 and therefore is a more reliable character. We pay little attention to 

 the sugar, however, and place importance on pathogenicity. It is there 

 that our interest lies. It is possibly for this reason that many of these 

 bacterial pathogenes have been overlooked in the past. All bacteria 

 causing a blight in beans have been considered as one and the same. The 

 problem, however, is more complicated than it appears on the surface, 

 and probably the six forms discussed in this memoir are not all that attack 

 beans. The cultural characteristics of the bacteria should be considered 

 more carefully, and, when possible, differential media should be devised. 



