Dec. 15, 1920 Effect of Temperature and Humidity on Citrus-Canker 497 



CONCLUSIONS ON THE INFLUENCE OF HUMIDITY ON GROWTH OF 

 THE ORGANISM AND HOST PLANTS AND ON INFECTION AND 

 DEVELOPMENT OF THE DISEASE 



(1) The results of the silk thread experiment prove very conclusively 

 that there is a distinct relation between temperature and humidity on 

 the viability of Pseudomonas citri. 



(2) The citrus-canker organism is very susceptible to a combination 

 of high temperature and humidity. Medium humidities at all tempera- 

 tures are not injurious to the organism. With all humidities at low 

 temperatures none of the organisms are killed. 



(3) Apparently, at the humidities and temperatures used there is an 

 inhibiting action on the growth of the Citrus plants, with the exception 

 of Citrus mitis. 



(4) Little or no infection occurred at the humidities and temperatures 



used. 



DISCUSSION 



The writer realizes keenly the imperfections and incompleteness of 

 the experimental data presented, because of the complexity of the pro- 

 blem with its numerous and diverse factors. However, he feels that 

 enough qualitative data have been accumulated to indicate that a most 

 excellent field of endeavor lies in investigations of this nature. Several 

 fundamental principles have been uncovered, which, with further study, 

 should lead to promising results. Thus, with the incompletness of the 

 experimental work in mind, the writer will attempt to discuss his results 

 as a whole and correlate them with actual field conditions as he has 

 observed them during the past four years. 



A superficial study of the temperature relations, in culture, of the 

 bacteria causing plant diseases shows that, in the main, the temperature 

 relations of Pseudomonas citri agree very well with those which have 

 been studied from time to time. One point which pathologists have not 

 considered in their studies of temperature relations of bacteria in culture 

 has been the time element. However, when this factor is considered, 

 the plant-disease bacteria belonging to the Pseudomonas group have a 

 minimum temperature of approximately 5 C. or slightly higher. By 

 the use of former methods, lower minimums have been obtained in some 

 cases. They have an optimum between 20 and 30 , a maximum varying 

 with the time factor, but between 35 and 38 for a period of 24 hours, 

 and a thermal death point between 49 and 51 . The plant-disease 

 organisms of the bacillus group, with but few exceptions, have a maxi- 

 mum temperature and thermal death point several degrees lower than 

 the Pseudomonas group. , 



Temperatures below the minimum simply inhibit the growth of the 

 bacteria, so that low temperatures within reasonable limits and with 

 the length of exposure considered do not cause their death. It should 

 16917—20 7 



