502 Journal of Agricultural Research voi.xx. no. 6 



On the other hand, conditions are met with where a suitable temper- 

 ature for growth and infection is present, but there is a decided deficiency 

 in rainfall. The conditions existing in the Philippines can be cited as a 

 typical example. Thus, Mackie (6) states that — 



during the dry season which occurs from January until the monsoon changes in June, 

 the disease is apparently quiescent. * * * However, after the rains begin, the 

 trees send out new growth and it is on this new growth that the canker appears, 

 coming into evidence in about a week. * * * Throughout the rainy season, the 

 disease thrives. 



Initial infection can take place under conditions which do not favor 

 the development of the disease. Furthermore, it may occur and the 

 organisms may remain quiescent in the tissues for long periods of time 

 without any signs of the disease being manifested. In fact, we may 

 assume that there are occasions when initial infection takes place without 

 the subsequent developent of the disease because of unfavorable con- 

 ditions for its development after the organism enters the tissues of the 

 host plant. 



The writer has shown that initial infection did occur at low temper- 

 atures, although no canker developed until the plants were transferred 

 to a higher temperature. These experiments were repeated under green- 

 house conditions several times with the same results. In the field, 

 plants were inoculated in September, 191 7, and no canker became visible 

 until the following April, when it developed very rapidly and was ex- 

 tremely severe on the twigs and stems of some of the hardy hybrids. 

 No doubt, in the case of kumquat, the organism is able to enter the 

 stomata but is unable to develop because of the resistance offered by 

 the tissues. Where the tissues are broken, kumquat leaves can be 

 readily infected. Thus, initial infection requires a definite set of con- 

 ditions entirely different from those required for the development of the 

 organism after it enters the host plant. 



No canker whatsoever has been obtained under any conditions at 

 15 C. or lower on any of the plants experimented with in the greenhouse. 

 At 20 the disease has been produced on all plants, although the amount 

 of canker and the period of incubation varied greatly with the different 

 plants. Thus, only one calamondin plant was successfully inoculated at 

 20 in all the greenhouse experiments ; all growing plants became diseased 

 at 25 ° ; while at 30 the number of spots increased very rapidly in number 

 over those produced at 25 °. 



On the trifoliate orange only a few leaf spots occurred at 20 C. after 

 15 days. At 25 spots were more numerous on the young leaves; a few 

 old leaves became diseased and a few twig spots were formed. The 

 period of incubation at this temperature was only 8 days. Canker was 

 general on all the plants at 30 , with the period of incubation shortened 

 to 4 days. It is interesting to note in this connection that in the eradi- 

 cation of canker in Alabama practically all canker on trifoliate orange 



