Dec. 23. 1918 Susceptibility of Rutaceous Plants to Citrus-Canker 66,5 



The evidence is that Xanthoxylum rhetsa and Triphasia trifoha are 

 immune, while Chalcas (Murraya) exotica, Atalantia disticha, and Fort- 

 unella {Citrus) japonica show positive results only under the most fav- 

 orable circumstances for infection. 



It is noteworthy that some of the immune and highly resistant species 

 possess thick, coriaceous brittle leaves — for example, Severinia buxifolia, 

 A talantia disticha, Fortunella japonica, and Chalcas exotica. This suggests 

 the possibility that resistance to Citrus-canker may be influenced in some 

 measure by histological or morphological differences. 



SUMMARY 



(i) Inoculation tests with Pseudontinas citri upon 24 species repre- 

 senting 20 genera of the family Rutaceae show that 19 of the species are 

 susceptible in greater or less degree. It thus appears that Citrus-canker 

 is not closely limited to the genus Citrus, but has a wide range of hosts 

 among the Rutaceae. 



(2) Severinia buxifolia, Aegle marmelos, and Balsamocitrus gabonensis, 

 all close relatives of Citrus, may safely be called immune to Citrus-canker. 

 Xanthoxylu-m rhetsa and Triphasia trifolia seem to be immune. 



(3) Chalcas {Murraya) exotica, Atalantia disticha, and Fortunella 

 {Citrus) japonica, also closely related to the genus Citrus, are strongly 

 resistant to Citrus-canker. 



(4) Claucena lansium, Feronia limonia, Feroniella lu^ida, ChcLetos- 

 permum glutinosa, Hesperethusa cremdata, Paramignya longipedunculata, 

 Citropsis schwemfurthii, A tlantia citrioides, Eremocitrus glauca, Fortunella 

 hindsii, Microcitrus australis, M. australasica, Toddalia asiatica, Evodia 

 ridleyei, E. latifolia, and Melicope triphylla, of different relationships to 

 the genus Citrus, all produce positive results when inoculated with 

 Pseudomonas citri, at needle punctures. Of these Claucena lansium and 

 Feronia limonia develop infection very slowly, the others fairly quickly. 



(5) Chaetospermuni glutinosa shows naturally occurring infections of 

 Citrus-canker and in the Philippines its susceptibility is easily greater 

 than that of the sweet orange {Citrus sinensis). Fortunella hindsii o/s- 

 curs naturally in South China, very much isolated from sources of Citrus- 

 canker infection. The abundance of cankers found on such trees gives 

 rise to the theory that this species may have been an original \vild host 

 from which Citrus-canker spread to cultivated species. 



