Dec. is. I9 2o Effect of Temperature and Humidity on Citrus-Canker 48 7 



Because of the limited amount of infection the results are not tabulated. 

 No sign of canker developed on any of the plants in set 4, which had been 

 inoculated with a culture of the organism grown at 35 ° C. As was to be 

 expected, only three spots (two on grapefruit and one on sweet lemon) 

 occurred on this type of plant in the other three sets. This extremely 

 light infection was due to the distinctly inhibitive influence of the high 

 temperature on the growth of these plants. 



Many spots occurred on both the limequat and trifoliate orange plants 

 in the remaining sets. Incubation required from 5 to 1 1 days on the 

 trifoliate orange and 11 or more on the limequat plants. The spots 

 were medium-sized, ruptured, and very corky. In no case did any of the 

 trifoliate orange plants, which were dormant when inoculated, become 

 infected when new growth appeared later. Furthermore, where a new 

 shoot had started prior to inoculation, many spots developed on this 

 shoot, but no canker appeared on any shoots which developed after 

 inoculation. Evidently, at this temperature, the organism is unable to 

 survive for any length of time and is only able to infect the actively 

 growing tissue of the plant. 



CONCLUSIONS ON THE INFLUENCES OF TEMPERATURE ON INFEC- 

 TION AND THE DEVELOPMENT OF THE DISEASE 



(1) No canker whatsoever has been produced on dormant plants. 



(2) The minimum temperature for the successful inoculation of 

 Poncirus trifoliata, Rusk citrange, and Citrus grandis plants is 20 C. 

 Apparently, it is a little higher for plants of C. mitis. 



(3) The optimum temperature for infection of the Citrus plants used, 

 which were in an active growing condition, lies between 20 and 30 C, 

 with the possible exception of C. mitis. 



(4) A low night temperature has a decidedly inhibiting effect on 

 infection and development of the disease on citrange and Citrus mitis 

 plants. This does not hold true for C. grandis. 



(5) At 20 C. only the new growth was attacked with few or no twig 

 cankers; not only the new growth but twigs developed cankers at 25 , 

 and there were few spots on old leaves; while at 30 all of these parts 

 were readily attacked. 



(6) The period of incubation varied not only with the host plant but 

 also with the temperature. With citrange and Citrus mitis, the period of 

 incubation was shortest at 30 C. With grapefruit, the period^of incu- 

 bation was very short at all temperatures between 20 and 30 . 



(7) At 20 C. the spots produced on the plants are more typical of 

 those found under natural conditions, while at 25 ° and 30 they are 

 extremely loose, soft, and spongy. 



(8) Judging from the location, parts of the plant attacked, and type of 

 spots produced on growing plants when transferred to a temperature of 

 30 C. after being held from two weeks to one month at 5 , io°, and 15 C, 



