474 Journal of Agricultural Research vol. xx, no. 6 



twigs. Satsuma does not produce as much summer growth as others, which is another 

 reason for escaping from the severe summer infection. 



All writers agree that the young and tender growth of trees in a good 

 growing condition favors the development of the disease. Some few 

 go so far as to give the age of the parts most susceptible. Thus, Jehle 

 (4) found that — 



medium sized, thrifty leaves seem to be most susceptible, and canker is seldom found 

 on those which are yellowish, unhealthy, very young or very old.. . . The young 

 tender twigs and thorns are more subject to citrus canker than are the older more 

 corky ones. . . . As the fruit matures, it seems to become less and less susceptible 

 to citrus canker, and mature picked fruits seem to be immune. 



Other investigators have also noted the absence of infection on the mature 

 fruits. 



The writer (7) has stated that — 



even though ideal conditions of temperature and humidity were supplied for infec- 

 tion, few or no canker spots developed if the plant was not in good growing condition. 

 The largest number of spots naturally occurred on mature leaves which were still 

 tender and of a light-green color. Few spots appeared on the young leaves, while 

 spots developed on the old foliage of the more susceptible plants only. 



The writer (7) has gone one step further in discussing the relations of 

 the condition of the plant to infection when he stated that — 



apparently resistance is in part mechanical — for example, the texture of the leaf 

 determines to a large extent the size and character of the spot. Leaf texture plays an 

 important role in the resistance of the host plant to Citrus-canker and seems closely 

 related to the rapidity with which the leaves mature. There is a considerable varia- 

 tion in the time required for the maturation of the leaves of the various Citrus plants. 

 Thus, the leaves of the kumquat, which are rather thick and highly resistant, reach 

 maturity much sooner than the thin, extremely susceptible leaves of the grapefruit. 



Weather conditions which influence not only the growth of the organ- 

 ism but the trees themselves, are also responsible for retarding growth, 

 both of the organism and the host. Thus, Jehle (5) finds that — 



the disease has a peculiar faculty for lying dormant for long periods without producing 

 any visible symptoms, but sooner or later making its appearance in a typical form. 

 There are numerous instances on record in which it has remained dormant in this 

 way for many months on trees which have been shipped from an infected nursery. 



Examples of dormancy of the organism have been encountered in the 

 field, especially with nursery stock. The writer with Neal (8) proved 

 experimentally under field conditions that the canker organisms could 

 remain dormant through the winter in the outer bark tissue of some of the 

 hardy hybrids for a period of 6>£ months. 



It is clearly evident from the facts brought out that it is extremely 

 difficult to separate the influence of weather conditions on the develop- 

 ment of the disease from its relations to the growth and development of 

 the host. Even experimentally it is impossible to separate the influ- 

 ence of temperature and humidity. Thus, in the following experiments 



