July 15, 1920 Relative Susceptibility to Citrus-Canker 353 



When no twig or stem infection occurs and only small, scattering, 

 unruptured spots appear on the leaves, the plants show enough resistance 

 to be classed as resistant. An intermediate group can be formed where 

 the spots are larger, ruptured, and more general on the leaves, with 

 occasional twig and stem infections. Plants placed in this group might 

 be found promising under certain conditions. Plants which show large, 

 ruptured spots on the leaves, severe enough to cause defoliation, and 

 large girdling cankers on the twigs and stem should be classed as extremely 

 susceptible. With these remarks in mind, the writers will attempt to rank 

 the Citrus fruits, provisionally, in groups according to their suscepti- 

 bility to citrus-canker. 



The plants of the grapefruit and pummelo group are extremely sus- 

 ceptible. However, the Siamese and possibly the Hirado Buntan pum- 

 melos give promise of showing some resistance to canker. 



Of the numbers tested belonging to Citrus hysirix, those with rounded 

 leaves are as susceptible as grapefruit. The plants with pointed leaves are 

 apparently more resistant to canker. More study is needed to determine 

 whether this will hold for all forms of Citrus hysirix in the Philippines. 



All numbers of the lemons tested, including the Ichang lemon, show 

 about equal susceptibility, which is slightly less than that of grapefruit. 



The plants of the sweet-orange group vary somewhat in susceptibility. 

 Leaf infections are severe, and twig and stem cankers are common. As 

 a whole, they are not as susceptible as the lemons. 



Citrus excelsa and the Natsu-mikan are equally as susceptible as the 

 plants of the sweet orange group. 



Since only one number of the limes was tested, the position which the 

 limes should take in the scale of susceptibility is doubtful. The sour 

 lime tested proved to be somewhat resistant. However, Lee finds that 

 with one exception the limes are susceptible. 



While the citrons tested are easily infected, the spots are small, in- 

 creasing very slowly in size. Twig infection occurs only occasionally 

 and is the exception rather than the rule. 



Citrus mitis, at least seedling plants such as were used in the inoculation 

 experiments in the greenhouse, while showing some resistance are more 

 susceptible than in the field. Leaf infections are scattering, and twig 

 cankers are rarely produced. 



The Kansu (Yuzu) orange so far has proved decidedly resistant. No 

 twig cankers have occurred, and only small, scattering spots have devel- 

 oped on the foliage. 



All numbers of the Citrus nobilis group tested have proved to be decid- 

 edly resistant, and, no doubt, all of these plants, if not mixed with sus- 

 ceptible varieties, could be grown under canker conditions. That does 

 not mean that it would be economical or at all advisable to allow canker 

 to persist even in unmixed plantings of Satsuma or other varieties of 

 C. nobilis. 



