Dec. i 5> 1920 Effect of Temperature and Humidity on Citrus-Canker 479 



good growing condition when first inoculated, while the others had com- 

 pleted their growth or were dormant. Even though some of these 

 plants developed new growth when transferred to the higher tempera- 

 ture, they remained free from canker. 



As on the citranges, just the new foilage was attacked in the majority 

 of instances. The spots were present at the base of the new growth or 

 petioles, and when present on the leaves most of them were on the 

 midrib or near the tip of the leaves. The majority of the spots were 

 elonagted rather than round and became visible in from five to eight 

 days after the plants were placed in the 30 C. case. 



The grapefruit plants were all in excellent condition for infection 

 when inoculated and placed in the 5 , io°, and 15 C. cases. However, 

 in no instance did the disease appear at these temperatures. Immediately 

 after the plants were transferred to the 30 case, growth proceeded at 

 the normal rate for that temperature, and all plants showed visible spots 

 within five days of the transfer. Canker was much more severe than on 

 the citranges and calamondin However, the spots were limited 

 to the young growth and were usually grouped at the tips of the young 

 leaves. Very few spots were found scattered over the leaves in general. 



Thus, while no canker occurred on any of the plants held at 5 , io°, 

 and 1 5 C. for a 15-day period, it did develop on those plants irrespective 

 of species which were in good growing condition when inoculated, after 

 they were all transferred to a temperature of 30 . Even though the 

 plants did start growing after they were transferred, no canker occurred 

 at this temperature on any which had completed their growth or were 

 dormant when inoculated, except that one elongated spot developed at 

 the base of the new growth on one citrange plant. Apparently, the 

 organisms were able to enter the stomata of the very young growth 

 and remain inactive at the lower temperatures, but when the plants 

 were placed at a higher temperature the organisms became active and 

 produced canker. From the location and type of the spots there is no 

 doubt that the organism entered the tissues and remained quiescent 

 until a higher temperature was available. 



In Table XIII are given the results obtained between temperatures of 

 20 and 30 C. for a period of approximately four weeks. At 20 all the 

 citrange plants became diseased. However, the spots were limited to 

 the new growth and did not become visible until 15 days after inocula- 

 tion. Only a few spots occurred on the twigs, and no mature or old 

 leaves were attacked. 



Canker was much more severe at 25 C, causing some defoliation and 

 producing numerous spots on all plants. The spots were first visible 

 eight days after inoculation, which is one week earlier than at 20 . 

 The majority of the spots occurred on the young foliage. Twig canker 

 was much more general than at 20 , and some spots were formed on 

 the old leaves. 



