jtiiy 15. 1920 Relative Susceptibility to Citrus-Canker 347 



susceptibility. This confirms the observations of Lee (4) in the Philip- 

 pines, where he has found Chaetospermum generally infected under field 

 conditions. The spots (PI. 58, C) produced on this plant are ruptured, 

 corky, and more or less typical of those found on the plants in the genus 

 Citrus. They occur on the leaves in the absence of wounds. C. gluti- 

 nosum is the most distantly removed relative so far found which is quite 

 susceptible and produces canker spots typical of those found on Citrus spp. 



It is rather peculiar that the other plants tested in this subtribe are 

 immune or nearly so, especially Balsamocitrus and Aeglopsis. A few 

 small, nontypical, unruptured spots have been found on Aegle marmelos, 

 but only at wounds. Thus Aegle can be classed with the first group 

 discussed in its resistance to canker. Lee (4) failed to obtain any in- 

 fection on Aegle. This is the only plant of the whole group tested by 

 us where Lee's results and mine failed to check. 



On both Feronia limonia and Feroniella Ixicida positive infections have 

 been obtained. While the spots are typical of those described for the 

 rutaceous group, they can develop in the absence of wounds on the leaves. 



Of the plants tested in the subtribe Lavanginae, Triphasia irifolia and 

 Severinia buxifolia have remained free from canker. Lee (4) has likewise 

 failed to obtain infection on these plants after repeated trials. No 

 doubt, both species will prove immune to canker. Hesperthusa crenulata, 

 on the other hand, is quite susceptible, in fact, almost as much so as 

 Chaetospermum. While both leaves and twigs are attacked in the absence 

 of wounds, the spots (PI. 59, B) do not resemble those found on any other 

 host. They are flat, and though they rupture, there is no evidence of 

 the corky tissue so typical of the canker spots on Citrus. 



Citropsis Schweinfurthii, in the subtribe Citrinae, ranks with the 

 rutaceous plants in susceptibility, in that infections occur only at wounds 

 and the spots are small, nontypical, and unruptured. Atalaniia citri- 

 oides and A. ceylonica have proved quite susceptible. The spots (PI. 

 59, A) are medium-sized, ruptured, corky, and resemble somewhat those 

 found on Chaetospermum. Lee reports A. ciirioides and A. disticha 

 rather resistant. 



Poncirus trifoliata is without doubt the most susceptible of the wild 

 relatives. Somewhat less susceptible is Eremociirus glauca. Canker 

 spots (PI. 59, D) have appeared on the leaves, thorns, branches, and 

 stems of this plant. The spots are small but ruptured and corky, while 

 on the branches they are of a girdling type, resembling the stem cankers 

 on P. trifoliata, except in size. Equally susceptible and with the same 

 type of canker spots are Microcitrus australis and M. Garrowayi. M. 

 australasica and its variety sanguinea are more resistant, although spots 

 of the same type occur on the leaves, thorns, and twigs. 



Of the kumquats, Fortunella Hindsii is susceptible. Lee (4) reports 

 that canker occurs naturally on the wild plants on the mountains near 

 Hongkong. The canker spots (PI. 60, C) on F. Hindsii are ruptured, 



