230 
Journal of Agricultural Research 
Vol. XXVIII, No. 3 
At the conclusion of the investigation, no distinct difference in the suscepti¬ 
bility of the two strains of Satsumas was apparent. While the number of Ikeda 
trees infected earlier in the season was greater than the Owari, the reverse occurred 
later in the season, so that more of the Owari trees were infected than Ikeda. 
Further, infection was somewhat lighter on Owari than Ikeda although the per¬ 
centage of Owari trees infected by canker was larger than that of Ikeda. Thus, from 
the results of this experiment conducted with a fair number of plants over a period of 
three years, absolutely no differences in the susceptibility of these two strains was 
apparent. With only a few exceptions, canker was limited to the young growing 
leaves and an occasional twig. On the other hand, the grapefruit plants were 
very badly infected at all times during the season, canker being found on the 
leaves, thorns, and round wood. 
Citrus mitis Blanco proved to be more resistant than the Satsuma under field 
conditions. Canker at all times was limited to a few young leaves and an occa¬ 
sional twig. In the greenhouse experiments, the plants were more susceptible as 
twig infection was common and occasionally the round wood was attacked. 
Likewise, the Kansu orange proved to be decidedly resistant. Very few canker 
spots on the leaves and no twig infection were noted. Of the other Citrus fruits 
tested, all proved to be susceptible. 
fn the continued search among the Citrus fruits for promising canker resistant 
types, the search will have to be confined to a few groups which show some promise 
of being resistant to canker. 
RELATIVE SUSCEPTIBILITY OF THE CITRUS HYBRIDS 
Both faustrime and faustrimon are more susceptible than Microcitrus austra- 
lasica (F. Muell.) Swingle. Faustrimedin is not as susceptible in that infection is 
limited to a few spots on the leaves and an occasional twig. 
All of the Poncirus trifoliata crosses, including citranges, citrumelo, citradia, 
citrandarin, citrunshu, cicitrange, and citraldin, are susceptible, and in no in¬ 
stance can any one number be selected as showing enough resistance to citrus 
canker to warrant its use as a canker resistant stock or orchard tree. While none 
of these hybrids are resistant to canker, several of them stand out above the com¬ 
mon level and may be of some use for further breeding and crossing purposes. 
Throughout this investigation the Willets citrange has consistently shown less 
infection than any of the other citranges. The fact that it is one of the parents 
of the citrangequat, the most resistant hybrid so far found, substantiates to some 
extent these results. 
On the whole, it can be stated that when Poncirus trifoliata is crossed with any 
of the more resistant Citrus species the resistance of the progeny is lessened. 
Of the four citrange crosses, citrangequat, citranguma, citrangarin, and cit- 
rangedin, it happens that the second parent in each case is resistant to canker. Un¬ 
fortunately, no hybrids were tested with a susceptible parent, so that we do not 
know what is the direct influence of the citrange parent on canker susceptibility. 
The citrangequat has consistently proved very resistant, in fact, as much so as 
its second parent. It appears to be the most promising canker resistant hybrid 
so far found. One disadvantage in the use of citrangequat as a stock has been the 
small number of seeds produced. However, Swingle and Robinson 6 have recently 
developed a method of propagation by the use of root cuttings, so that stock of 
this hybrid can now be rapidly developed. If it is found that it will-make as de¬ 
sirable a stock as Poncirus trifoliata , it can replace the trifoliate orange, which at 
the present time is a menace to the final eradication of citrus canker in the Gulf 
Coast States. The other three citrange crosses, while they do not show as much 
8 Swingle, W. T., and Robinson, T. R. two important new types of citrous hybrids for the 
home garden—citrangequats and LiMEQUATS. Jour. Agr. Research, 23: 229-238, illus. 1923. 
