Dec. 15, 1920 Effect of Temperature and Humidity on Citrus-Canker 47 1 



It can be concluded from this experiment that the growth of grape- 

 fruit and plants of a similar type is decidedly inhibited at a temperature 

 of 35 C, while the trifoliate orange and limequat can make a normal 

 growth, at least for the period of time covered by the experiment. 



CONCLUSIONS ON THE TEMPERATURE RELATION OF THE HOST PLANTS 



(1) With actively growing Citrus grandis plants in a saturated atmos- 

 phere the optimum temperature lies between 20 and 30 C. The lower 

 limit of the optimum temperature is a little higher for C. mitis, while for 

 Poncirus irifoliata and allied plants the upper range of the optimum 

 temperature is above 30 . 



(2) No temperature used was able to force the dormant Poncirus iri- 

 foliata plants into active growth. 



(3) The minimum temperature for the growth of Citrus grandis is 1 5 C, 

 and for the others tested it was 20 . 



(4) Citrus grandis plants kept at a temperature of 15 C. matured their 

 foliage very rapidly and in most instances within a week's time. At 

 temperatures of 20 and above, growth was more rapid and extensive. 

 The period of maturation of the leaves was extended so that 16 to 20 days 

 or more were required, which is twice as long as at 15 . 



(5) Leaves that have once reached their maturity at low temperatures 

 can not be forced to increase their size by a change of environment. 



(6) A low night temperature checks the growth of plants held at a 

 high temperature during the day and also hastens maturation of the 

 leaves. This is especially noticeable with the Poncirus trifoliata, cit- 

 range, and Citrus mitis plants. C. grandis, on the other hand, is not so 

 easily influenced. 



(7) Plants grown at a high temperature are inhibited in their growth 

 when transferred to low temperatures. Citrus grandis is only slightly 

 inhibited, while Poncirus trifoliata, Rusk citrange, and Citrus mitis plants 

 are completely checked. 



(8) Growth of Citrus grandis and plants of a similar type is decidedly 

 inhibited at a temperature of 35 C, while Poncirus trifoliata and limequat 

 make a normal growth, at least for the period of experiment. 



INFLUENCE OF TEMPERATURE ON INFECTION AND DEVELOPMENT 



OF THE DISEASE 



In discussing infection and development of citrus-canker, two factors 

 have been stressed by the workers in this field. Both have been given 

 equal prominence and can not very well be dissociated. These factors 

 are weather conditions and the condition of the host plant. In discussions 

 of weather conditions, most of the emphasis has been placed on humidity 

 as favoring the more rapid development of the disease, while little has 

 been said regarding the influence of temperature. However, it has 

 usually been inferred that a favorable temperature for infection existed. 



