498 Journal of Agricultural Research voi.xx, no.6 



be noted that all the active plant-disease bacteria can develop in cultures 

 at temperatures lower than that of their host plants. The writer wishes 

 to point out here again the pronounced lag in the growth of Pseudomonas 

 citri on media between temperatures of 15 and 20 C. To him, this 

 difference is of marked significance. No explanation of this phenomenon 

 can be offered at this time. 



In most cases, the optimum temperature for the growth of these 

 organisms is approximately the same as that of the host plant. Thus, 

 the temperatures at which the best development of the host plant occurs 

 are the same as those which yield the best growth of the bacteria in culture. 



For extended periods of time, the host plant develops at temperatures 

 slightly higher than the bacteria in culture, although the plant's develop- 

 ment is likewise retarded at the high temperatures. The extent to 

 which the growth of the bacteria at or near the maximum is retarded or 

 inhibited depends on the length of exposure. 



While studies of the temperature relations of the bacteria in cultures 

 are necessary, the results can not be strictly interpreted in the light of 

 field conditions. They serve only in indicating an approximation, 

 especially where minimum and maximum temperatures are concerned. 



Our present methods of determining the resistance of bacteria to drying 

 have been exceedingly crude, and with but few exceptions no attention 

 has been paid to conditions which might influence the results. At best, 

 the usual methods do not even have an empirical value, in that the 

 results are not comparable. A glance at the literature on the subject 

 will reveal this fact. 



Different investigators have obtained widely divergent results with 

 the same orgainsm. To illustrate, Stevens (12) states that — 

 bacteria (P. citri) from young and old cultures exposed for two weeks on glass slips 

 to dry in the air of the laboratory failed to germinate, 



while Wolf (17) comes to the conclusion that — 



the organisms seems to exhibit a very considerable resistance to drying 



and further that — 



the organism can retain its viability for about two months. 



Smith (10) — 



found this organism (P. campestris) much more resistant to dry air than Harding's 

 first report would indicate, to wit; in Harding's experiments, invariably destroyed 

 in 45 hours, and 7 out of 8 cover-slips sterile at the end of 2 1 hours. In my own tests, 

 the organism on 8 out of 24 cover-slips was alive after 34 days, when inoculated from 

 a potato culture 2 days old and on 2 out of 23 cover-slips when inoculated from bouillon . 



Later Harding, Stewart, and Prucha (j) found that Pseudomonas cam- 

 pestris could live on cabbage seed for a year under certain conditions. 



In the experiments carried out by the writer, strict attention has been 

 paid to the amount of the inoculum on the threads, as well as to tempera- 

 ture and humidity. The most striking results obtained indicate that 

 at low temperatures humidity has little influence on the viability of the 



