Dec. i S , 1920 Effect of Temperature and Humidity on Citrus-Canker 475 



the temperature was varied, but a saturated atmosphere was main- 

 tained. 



Prior to placing the plants under bell jars at the various temperatures 

 in the experiments reported on under the heading "Influence of temper- 

 ature on growth of the host plants" they were thoroughly sprayed with 

 a 48-hour-old culture of Pseudomonas citri in beef bouillon. All in- 

 oculations were made at 10 a. m., about which time the stomata have 

 reached their maximum opening. As readings and observations were 

 made on the growth of the plants notes were taken on the development 

 of canker. Thus, a correlation could be obtained on the condition of 

 the plant and its relation to infection and development of the disease. 

 In Tables XI to XVII, the total number of spots and the part attacked 

 are given. On consulting Tables VI to X it will be noted that all new 

 spots are starred. Thus, a double check was obtained between the con- 

 dition of the plant, infection, and development of the disease. 



EXPERIMENT IA 



On consulting Table VI it will be seen that no spots developed on any 

 of the dormant plants of Poncirus trifoliata, nor on any plants subjected 

 to temperatures below 20 C. Thus, in Table XI, only the positive re- 

 sults with Citrus mitis and C. grandis are included. 



No spots occurred on the calamondin plants at 20 C. Canker first 

 appeared on these plants held at 25 . At 30 the spots were more 

 numerous, while at greenhouse temperature the number fell off. Canker 

 was not general on these plants because they are somewhat resistant. 

 The spots in all cases were small, unruptured, and occurred for the most 

 part on the mature or old leaves. 



Even though an extremely slow growth of grapefruit occurred at 15 C. 

 no canker was produced. On the grapefruit plants canker first devel- 

 oped at 20 , the spots increasing in numbers at 25 . At 30 the number 

 of spots dropped off considerably, while under greenhouse conditions 

 the disease was more severe. It should be noted, however, that the 

 grapefruit plants at 25 and those kept at the greenhouse temperature 

 were in much better condition for infection. 



