352 
Journal of Agricultural Research 
Vol. XIV, No. 9 
field conditions the more resistant plants would show almost absolute 
immunity, while the less susceptible plants would show more resistance 
to Citrus-canker. For example, a larger number of canker spots were 
produced on the oval kumquat in the greenhouse than has been reported 
on all the kumquats in the field since Citrus-canker was introduced into 
this country. Where the plants were in good condition for infection and 
remained resistant we may rest assured that they will be resistant under 
ordinary field conditions. 
The fact that the plants were thoroughly mixed in the cases also helped 
to produce the greatest amount of infection. The writer found that the 
organisms isolated from susceptible plants grow very rapidly on potato 
plugs, while those from the more resistant plants develop slowly. 
It is also very difficult to isolate the organism from the spots on the 
more resistant plants. No doubt, if all the resistant plants had been 
placed together in a case and inoculated, little or no canker would have 
developed and no reinfections would have been possible. Thus, by mix¬ 
ing the plants a higher percentage of hosts were infected and reinfected, 
owing to the spread of the more virulent organisms from the extremely 
susceptible plants. 
The results may be influenced somewhat by the fact that small, rather 
young plants, mostly derived from seedlings, were used. Just how much 
the individual variations of the seedlings enters into the work will not 
be known until a number of inoculation experiments are carried out. 
These plants are natives of various parts of the world and grow under 
diverse climatic and soil conditions. When brought together in an ex¬ 
periment of this nature, undoubtedly many of them are weakened, a 
condition that influences their behavior toward Citrus-canker. 
It is indeed surprising to find from the inoculation experiments in 
the greenhouse that, so far Citrus-canker is apparently limited to those 
plants having edible fruits with stalked pulp vesicles, of the subtribe 
Citrinae, which includes only five genera. Of these, the genera Poncirus, 
Fortunella, and Microcitrus were formerly included in the genus Citrus, 
while Eremocitrus was first described as a Triphasia and then as Ata- 
lantia before being placed in a new genus by Mr. Swingle. It is inter¬ 
esting to note how closely susceptibility to Citrus-canker under green¬ 
house conditions ties up with the botanical classification of this group 
of plants as worked out by Mr. Swingle. 
Jehle (ro) (u) has reported Xanthoxylum fagara (L.) Sarg., X. Clava 
Hercules, and Chalcas exotica Millsp. (Murraea exotica, L.) as being sus¬ 
ceptible to Citrus-canker. His method of inoculation (needle pricks) and 
the fact that the plants were kept in a tightly screened cage (8) into 
which no direct sunlight could penetrate, 1 might make his results possible. 
In the Philippines, Wester (29) has noted “what is apparently Citrus- 
canker’ J on Chaetospermum glutinosa (Aegle glutinosa Merrill). No 
1 Sides and top were double-screened on the outside with galvanized netting and on the inside with 
bronze screen oi fine mesh, with a mote at the base. 
