46 A. L. PlERSTORFF 



GENERAL DISCUSSION AND CONCLUSIONS 



From a review of the literature and from the writer's work, it is 

 evident that many species of the Rosaceae are attacked by Bacillus 

 amylovorus. It is apparent also that species in the genus Pyrus as 

 a whole, are the most congenial hosts for B. amylovorus. 



Cultural reactions of the fire-blight organism have been determined 

 by various workers. The w 7 riter rechecked Stewart \s work on acid toler- 

 ance and found that B. amylovorus can live in an acid medium as low 

 as pH 4.60. Very low temperatures do not kill all of the organisms ; 

 some were able to withstand -183° C. for ten minutes in a liquid 

 medium. How long the bacteria would be able to withstand this 

 temperature,, or how many repeated freezings would be necessary to 

 give sterile cultures, was not determined. 



The toxic principle contained in sap from blighted pear fruit has 

 many of the characteristics of a true bacterial toxin as found in human 

 and animal pathogenes. It will pass through a collodion membrane, 

 is precipitated by alcohol, and is specific for B. amylovorus ; in contrast 

 with properties of bacterial toxins, it appears to be stable when exposed 

 to the air, and also thermostable. These characteristics suggest that 

 more than one substance may be involved. 



It is the opinion of the writer that, when the enzymes are better 

 understood, microchemical tests will reveal some of the reasons for sus- 

 ceptibility or immunity of certain host plants, or of a single host 

 plant at different seasons of the year. At least a knowledge of the 

 usable food of B. amylovorus will aid materially in such studies. A 

 •parallel exists between phloridzin content of apple shoots, and fire- 

 blight susceptibility. Whether or not the bacteria can utilize phloridzin 

 has not been determined. Sucrase. maltase, inulase, and arbutase were 

 shown to be formed. Amygdalin is found in very small quantities in 

 apple and pear shoots; B. amylovorus did not split amygdalin. Arbutin, 

 however, is found in greater quantities in these fruits, and the fire- 

 blight bacteria were found to utilize it. This suggests that at least 

 a part of the food used by the organism may be arbutin. Chemically, , 

 arbutin and amygdalin are closely allied. Also, inulin was found to 

 be utilized as a source of carbon, but starch was not. 



From the evidence given earlier in this paper, it is apparent that 

 the fire-blight pathogene and some other bacterial plant pathogenes 

 are able to survive adverse conditions for longer periods than was 

 formerly supposed. Just how these organisms can retain their vitality 

 for several years on dead plant parts is not known. 



As early as 1885, Arthur pointed out that blossoms are a common 

 avenue of entrance for fire-blight bacteria. From the writer's experi- 

 ence it appears that, in a large proportion of cases, entrance of the 



