Apr. 19,1924 Canker Susceptibility of Citrus Species and Hybrids 229 
above are sufficiently susceptible to warrant further attention. However, it is 
of scientific interest to know that Pseudomonas citri is not limited in its attack 
to the genus Citrus but can produce, under certain conditions, infections on a 
wide range of plants in the family Rutaceae to which Citrus belongs. In this 
connection, the plant breeder is furnished facts which may assist him in develop- 
*ng hybrids of a commercial nature which are resistant to citrus canker. 
SUSCEPTIBILITY OF CITRUS FRUITS 
The difference in the susceptibility of the two forms of Citrus hystrix DC 
has been consistent throughout the investigation. The round leaf form equals 
grapefruit in its susceptibility to canker, while the pointed leaf form shows 
some resistance to canker. Various forms of this obscure but large group should 
be tested further in the Philippine Islands where it is native, and undoubtedly, 
forms resistant to citrus canker will be found. 
No changes were noted in the reaction of the citrons, lemons, and limes tested 
in the greenhouse. None of these plants were used in the field, so that their 
susceptibility to canker must be based on greenhouse results. While plants of 
the Ichang lemon were tested both in the greenhouse and field, no changes have 
occurred to modify previous reports. 
All grapefruit plants tested are susceptible to citrus canker. Of -the 
pummelos, the Hirado Buntan and Siamese have shown some resistance in that 
infection was almost wholly confined to the leaves with only an occasional spot 
on a young, growing twig. 
Of the sweet orange group, no variety or species stands out as resistant, al¬ 
though they are not as susceptible as some of the other citrus fruits. 
All of the plants belonging to the Citrus nobilis group show resistance to 
canker. In the majority of instances, canker was limited to scattering spots on 
the young leaves and on an occasional twig. The Cleopatra tangerine, which 
appears to be an excellent stock for some types of Citrus might after further trials 
be found suitable to replace the trifoliate orange as a stock. 
As has been pointed out in a previous report, Tanaka 4 and Scott 5 have 
shown that there are several distinct strains of Satsuma grown in southern 
Alabama. The two most commonly found are the Owari and Ikeda. Scott 
describes them as follows: 
The fruits of the Owari are flat, thin skinned, depressed at both the blossom and stem ends, and practi¬ 
cally seedless, maturing in Alabama the latter part of October or first of November. The leaves are very 
broad, particularly at the base, and the tree has an upright habit of growth. 
The fruits of the Ikeda variety are not as flat as those of the Owari, are not depressed at either the stem 
or blossom end, have a very coarse texture and a thick skin, and generally contain a few seeds. The season 
of the Ikeda is three or four weeks later than that of the Owari variety. The leaves are narrow, and the 
tree has a very spreading habit of growth. 
From various sources reports of a difference in susceptibility between these 
strains were received, and in order to determine if they varied in susceptibility the 
following experiment was conducted. 
During the early spring of 1918, thirty 3-year-old trees budded on Poncirus 
trifoliata of the Owari strain and a like number of the Ikeda strain were planted 
in the isolation field. As a control, thirty 3-year-old Duncan grapefruits budded 
on Poncirus trifoliata were planted between the two rows of Satsumas. Obser¬ 
vations were made as to the number of trees infected and the parts of the plants 
attacked at intervals during the growing season over a period of three years. 
During this time the number of trees did not remain the same in that a number 
died or were cut out to make room for the remainder to grow. At the conclusion 
of the test, only 13 of each kind of tree remained. Since these trees were always 
surrounded by susceptible trees, natural infection was depended on entirely. 
4 Tanaka, T. varieties of the satsuma orange group in japan. U. S. Dept. Agr. Bur. Plant 
Indus. Crop Physiol, and Breeding Inves. C. P. & B. I. Circ. 5, 10 p., illus. 1918. 
5 Scott, L. B. varieties of the satsuma orange group in the united states. U. S. Dept. Agr. 
Bur. Plant Indus. Hort. and Pomol. Inves. H. & P. Circ. 1, 7 p., 1918 
