Dec. i S . 1920 Effect of Temperature and Humidity on Citrus-Canker 485 



EXPERIMENT 4A 



In experiment 4, it was pointed out that where plants were held for a 

 short time at 30 C. and then placed at 15 a marked inhibition of growth 

 occurred, although the grapefruit leaves made an extremely slow growth 

 and the younger leaves matured to some extent. However, when 

 transferred back to the 30 case, growth of all the plants except one 

 proceeded at a regular rate for that temperature. 



When the two sets of plants were placed in the 30 C. case, both were 

 inoculated in the usual way. At the end of 24 hours set 2 was transferred 

 to the 1 5 case to determine whether canker would develop at this tem- 

 perature. No doubt the organisms were able to enter the host plants 

 during the 24-hour interval, for canker was observed on the grapefruit 

 plants of the control 48 hours after they were inoculated. 



At 1 5 C. all the plants remained free from canker, with the exception 

 of the larger grapefruit plant. Nine days after the transfer of the plants 

 a few small, unruptured spots occurred on one grapefruit leaf (Table XVI) . 

 However, after the plants were transferred back to the 30 case, the 

 severity of canker was as great as on the control plants, except on the 

 one citrange plant which did not produce new growth. These results 

 indicate quite clearly that the organisms were able to enter the plants 

 during the interval they were held at 30 in as great a number as in the 

 control plants, but when the plants were transferred to the 15 case, 

 growth of the plants and likewise the development of the organism were 

 inhibited, although in culture at this temperature a fairly good growth 

 is made by the organism. When the. plants were again placed in the 30 

 case and normal growth for that temperature was resumed, as much canker 

 subsequently appeared on these as on the control plants. All experi- 

 ments so far presented along this line indicate quite clearly that the 

 development of the disease is primarily dependent upon the activity of 

 the plant. 



Table XVI. — Percentage of infection on plants at an alternating high and low temperature 



EXPERIMENT 4A 



Tempera- 

 ture. 



Rusk citranse. 



Cilrus milis. 



Citrus grandis. 



Plant No. 1. 



Plant No. 2. 



100 per cent leaf in- 

 fection; spots few, 

 small, and corky; 

 1 spot on twig 

 at base of new 

 growth. 



30 C, 

 transferred 

 to 15 . 



is'C, 



transferred 



to 30°. 



Clean. 



Few small, scatter- 

 ing, compact spots 

 on lower leaves. 



Clean . 



Clean; no 

 growth. 



Spots plentiful at 

 old leaf scars. 



100 per cent leaf in- 

 fect i on ; spots 

 many, small to 

 medium, corky; 2 

 spots at tip of 2 

 twigs, large and 

 corky. 



Few small, scatter- 

 ing, unruptured 

 spots on one leaf. 



100 per cent leaf in- 

 f e c t i o n ; spots 

 many, small to 

 large, corky; 2 

 twig spots at tip. 



100 per cent leaf in- 

 fection ; spots small 

 to medium, few 

 corky; 1 twig 

 spot, large and 

 corky. 



Clean. 



100 per cent leaf in- 

 fection; spots few 

 small to medium, 

 corky. 



