Aug. a6 f 1918 Susceptibility and Resistance to Citrus-Canker 
355 
From the results of the experiments it is a reasonable assumption that 
the virulence of the organism can be increased or decreased by a choice 
of hosts, just as the growth of the organism can be influenced on arti¬ 
ficial media by giving it a favorable or unfavorable media on which to 
develop. By this means a very virulent strain can be built up by keep¬ 
ing it on grapefruit, while a virulent strain can be broken down by using 
kumquat as a host. There may be in nature different strains of canker 
organisms, and no doubt these do occur, but apparently these can be 
decreased or increased in virulence by the use of a susceptible or resistant 
plant, so that it all becomes a question of host relation. 
SUMMARY 
(1) Plants representing the more important wild relatives, species, 
varieties, and hybrids of Citrus were obtained from the United States 
Department of Agriculture and inoculated with Pseudomonas citri in 
the greenhouse to test their comparative susceptibility and resistance 
to Citrus-canker. 
(2) The conditions under which the experiments were carried out 
were such that a maximum amount of infection was possible. The 
factors influencing infection were a high temperature, a relatively high 
humidity, and a rapid and vigorous growing plant. 
(3) From the results of the greenhouse inoculations with young plants, 
Citrus-canker is apparently limited to those plants having edible fruits 
with stalked pulp vesicles of the subtribe Citrinae, which includes the 
genera Poncirus, Fortunella, Eremocitrus, Citrus, and Microcitrus. 
(4) Susceptibility and resistance to Citrus-canker follow closely the 
botanical classification of this group as worked out in recent years by 
Mr. W. T. Swingle. 
(5) Of the so-called relatives, the plants belonging to the genera 
Fortunella, Eremocitrus, and Microcitrus show some resistance to Citrus- 
canker, while Poncirus is extremely susceptible, 
(6) All the species and varieties of Citrus tested are susceptible to 
canker. Citrus nobilis with its many varieties and types, the Kansu 
orange, and possibly C. mitis, exhibit enough resistance to warrant 
trials under Citrus-canker conditions in the field. 
(7) Of the hybrids the citrangequat and citranguma have remained 
free from Citrus-canker in these tests, while the citrandarins, limequats, 
and tangelos show some resistance. The citranges, with the possible 
exception of Willits, cicitranges, citrumelos, and limelos are all extremely 
susceptible and can be discarded in the further search for resistant plants. 
(8) The number, size, and character of spots on the leaves are of 
great assistance in judging the relative susceptibility and resistance of 
the plants. 
