July IS. 1920 Relative Susceptibility to Citrus-Canker 359 



The influence of pummelo on the second parent varies. As is to be 

 expected, pummelo crossed with lime and orange produced hybrids which 

 are as susceptible as grapefruit. Hybrids between pummelo and Clem- 

 entine orange, King of Siam orange, tangerine, and Satsuma are suscep- 

 tible to some extent, variation in susceptibility depending more or less 

 on whether the leaf is of the grapefruit or the mandarin type. 



The calarin, hybrid between calamondin and tangerine, and the calas- 

 shu, hybrid between calamondin and Satsuma, so far give promise of 

 being as resistant as their parents. 



FALSE HYBRIDS 



A rather large number of "false hybrids" or nucellar bud sports, the 

 the results of Mr. Swingle's crosses with varieties of Chinese pummelo 

 and American grapefruit were tested in both the field and the greenhouse. 

 The plants were vigorous growers and produced abundance of new 

 growth. Most of these false hybrids resemble the Chinese pummelo. 



All proved extremely susceptible, in fact, even more so than grapefruit 

 (PI. 68, A). Leaf infections were so severe as to cause defoliation. The 

 spots on the twigs and stems were large and of a girdling nature. A 

 number of twigs and several plants were killed outright by complete 

 girdling. The spots in all cases resembled those found on grapefruit. 

 Thus, all the false hybrids tested are extremely susceptible in both the 

 field and the greenhouse, and no one of them gives any promise of canker 

 resistance. 



SUMMARY 



(i) The investigations on the susceptibility and resistance to Pseu- 

 domonas citri Hasse of the wild relatives. Citrus fruits, and hybrids of 

 the genus Citrus reported on in a preliminary paper have been contin- 

 ued in both the field and greenhouse. Many more numbers have been 

 successfully inoculated, others have proved to be extremely susceptible, 

 while some still show considerable resistance. 



(2) The successful inoculation of a large number of wild relatives m 

 the greenhouse shows that Pseudomonas citri has a wide range of hosts 

 and is not limited to the genus Citrus. 



(a) Of the rutaceous plants not closely related to Citrus, positive in- 

 fections have been obtained on Casimiroa edulis, Chalcas exotica, and 

 Claucena lansium. Xanthoxylum sp., and Glycosmis pentaphylla have re- 

 mained immune. In all cases a few nontypical, unruptured spots have 

 been produced, but only at wounds or scratches on the leaves. Chalcas^ 

 exotica responded the least, and the period of incubation was long. 



ih) Of the tribe Citreae, subtribe Aeglinae, Chaetospermum glutinosutn 

 is the most distantly removed relative, so far found, that is quite suscep- 

 tible and on which canker spots are more like those found on Citrus. 

 Aegle martnelos has been only slightly infected, while Balsamocitrus 

 Dawei and Aeglopsis Chevalieri have remained immune. 



