Dec. 15, 1920 Effect of Temperature and Humidity on Citrus-Canker 48 1 



At 30 C, canker on old leaves and twigs was general and was much 

 more severe on the new growth than at the lower temperatures. On one 

 plant, the spots were visible four days after inoculation, on the others at 

 eight days. 



No consistent results were obtained at the greenhouse temperature. 

 Very little canker occurred on the new foilage or twigs, while spots on the 

 old leaves were common. Canker did not develop until 15 and 20 days 

 after inoculation. 



The results with the calamondin plants were rather variable. No 

 canker occurred on these plants at 20 C. Only one rapidly growing 

 plant inoculated at 25 ° became diseased, even though the other two 

 plants made some growth later on. At 30 , canker was general on the 

 mature and old leaves of all three plants, only two spots occurring on 

 the new growth. One plant kept at the greenhouse temperature devel- 

 oped canker, and the spots here were limited to the new growth. Canker 

 was visible 12 days after inoculation. At the other temperatures, the 

 spots were visible in eight days. The spots produced on the calamondin 

 plants were small and unruptured. 



With the exception of the grapefruit plants kept at the greenhouse 

 temperature, all developed canker within four days. Only two leaves 

 were attacked at 20 C, and in both cases the spots were localized at the 

 tip of the leaves or along the midrib. The plants held at 25 did not 

 grow nvell, so that only a few spots developed on some of the mature 

 leaves. At 30 , canker was fairly well distributed over the new foliage 

 and twigs. Several leaves were defoliated by the severe attack, but no 

 spots occurred on the old leaves. This is in contrast to the general dis- 

 tribution of canker on the plant held at the greenhouse temperature. 

 The spots produced on the grapefruit varied with the temperature. At 

 20 , the spots were more typical of those found under natural conditions, 

 while at 26 and 30 they were extremely spongy and corky. The same 

 was true for the spots on the citranges and calamondin. 



Experiment 6 



In this experiment, another attempt was made to obtain infection at 

 1 5 C. There were two plants each of the trifoliate orange, Rusk cit- 

 range, calamondin, and one of grapefruit. All plants chosen were in 

 *good condition for infection. As a control a similar set was included at 

 20 . The plants were inoculated with a 6-day-old culture of Pseudo- 

 monas citri in beef bouillon, grown at 15 and 20 , respectively, set 

 under bell jars, and kept in a saturated atmosphere for 1 month. Obser- 

 vations on the condition of the plants were made from time to time. It 

 was noticed that at 15 , the young growth matured rapidly, especially 

 that of the grapefruit plant. No spots were found at the end of the 

 month. At 20 , on the other hand, spots were visible on the grapefruit 

 16917°— 20 6 



