Aug. 26,1918 Susceptibility and Resistance to Citrus-Canker 
3 53 
isolations were made of this material, so that this observation has not 
yet been definitely substantiated. 
These are the only Citrus relatives so far reported in the literature as 
being susceptable to Citrus-canker. It should be noted that the relatives 
mentioned in this connection are widely removed from Citrus, and if 
these are as susceptible as reported, we should surely anticipate results 
with the closely related plants as Citropsis and Atalantia, which belong 
to the same subtribe. Further inoculation in the greenhouse and field 
this season should go a long way in clearing up this situation. 
Of the relatives Poncirus trifoliaia is undoubtedly the most susceptible, 
followed by Microcitrus australis , while Eremocitrus glauca , Fortunella 
Hindsii , and F. margarita show some resistance to Citrus-canker. No 
doubt some of the plants which did not become infected will probably 
prove susceptible to some extent when good vigorous plants are used. 
All of the species and varieties of true Citrus proved susceptible. 
The citrons, lemons, lime, grapefruits, pummelos, and similar plailts 
are so susceptible that in the search for resistant forms they can be 
discarded, unless other pummelo strains possessing resistant qualities 
may be found. The plants belonging to the species C. nobilis and the 
Kansu orange are the only forms which exhibited any resistance to 
Citrus-canker. It is from this group and type of plants that resistant 
forms will be obtained in the future. 
The results obtained with the hybrids have by far been the most 
interesting and instructive. Two of the hybrids, the citrangequat and 
citranguma, have thus far remained completely resistant, while the 
other hybrids having the mandarin type of orange for one parent have 
shown considerable resistance. 
When susceptible plants like grapefruit and lime have been used in 
crossing, the resulting hybrid generally retains this susceptibility, 
while the resistant parents pass their resistance on to the hybrid. The 
most far-reaching results in the search for commercial resistant varieties 
will be obtained in the development and hybridizing of the forms which 
show some resistance to Citrus-canker. 
The type and number of spots varies directly with the resistance 
exhibited by a plant. This offers a means of judging and comparing 
the relative susceptibility and resistance of the whole group. Appar¬ 
ently resistance is in part mechanical—for example, the texture of the 
leaf determines to a large extent the size and character of the spot. 
Leaf texture plays an important role in the resistance of the host plant 
to Citrus-canker and seems closely related to the rapidity with which the 
leaves mature. There is a considerable variation in the time required 
for the maturation of the leaves of the various Citrus plants. Thus, 
the leaves of the kumquat, which are rather thick and highly resistant, 
reaches maturity much sooner than the thin, extremely susceptible 
leaves of the grapefruit. 
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