Apr. 19,1924 
Canker Susceptibility of Citrus Species and Hybrids 231 
resistance as the citrangequat, are sufficiently resistant to warrant further trials 
both as a stock and as orchard trees. 
Limelo, orangelo, satsumelo, clemelo, siamelo, and tangelo, hybrids with grape¬ 
fruit as one parent, vary in their susceptibility to canker. In no instance are 
any of these hybrids as resistant as Satsuma to citrus canker, although a few 
numbers are only slightly more susceptible. Because of the fact that one of 
these hybrids, the tangelo, is being grown commercially and has merits which 
are demanding attention from the orchardists and the public, the reaction of 
these hybrids to canker is of importance. Until further information is at hand 
it is safe to state that the tangelo and satsumelo, with some numbers of siamelo, 
show enough canker resistance to place them beside the mandarin oranges in 
their resistance to canker. 
Likewise, the limequat and orangequat can be regarded as canker resistant 
as Satsuma. The orangequat has been observed very closely in the field because 
of its apparent similarity to the Satsuma, both in the type of growth and in its 
reaction to citrus canker and scab. Its relation to the Satsuma, if there is any, 
will not be known until fruit of the orangequat is produced and compared with 
Satsuma. Oranguma and siamar are slightly less resistant than the above 
hybrids. 
Judging from the results obtained with the various hybrids of Citrus mitis, it 
appears while somewhat resistant itself, it does not carry this resistance in the 
hybrid. Thus, in the order of their susceptibility, we have sopomaldin, citraldin, 
calarin, and calashu, which are equally as susceptible as the other parent, grape¬ 
fruit, trifoliate orange, tangerine, and Satsuma, respectively. 
In the search for promising canker-resistant plants, the results of over four 
years’ investigations seems to point to the fact that our best plants will come 
from the hybrids. Already one or two of the hybrids showing almost as much 
resistance as Satsuma have been set out in a small way. No doubt, as the 
result of further tests by the Office of Crop Physiology and Breeding Investi¬ 
gations, Bureau of Plant Industry, United States Department of Agriculture, 
other canker-resistant hybrids will be found. 
DISCUSSION 
In Tables I, II, and III we have listed each number tested, giving the type 
and number of plants used, its index of susceptibility, and the part or parts of the 
plant attacked. The index of susceptibility is the maximum that the plant has 
shown while it was under investigation. The same is true of the parts of plants 
attacked. Thus, the list as it stands records the maximum susceptibility shown 
by the plants of each individual number. 
It must also be remembered that for the most part these plants consisted 
of small seedlings of budded plants. In only a few cases were the plants large 
enough to bloom and set fruit. Further, they were fertilized to stimulate new 
growth, for it was only on the new growth that canker developed in the majority 
of cases. Also during the greater part of the work many of the more susceptible 
plants were badly infected with canker, so that an epidemic condition existed 
both in the greenhouse and field, and consequently natural infections could be 
counted on at all times. Overwintering of canker in the field occurred each 
season, so that during the last three seasons very few of the plants were inoculated. 
In all of our publications on citrus canker one point has been continually 
stressed, namely, that the plants must be in good growing condition for canker 
to infect. In other words, the plant must be making a good, vigorous growth 
to produce maximum infection. Therefore every means was used to keep the 
plants in a good, vigorous growing condition during the investigation. 
Under grove conditions with older trees no doubt many of the numbers tested 
would show greater resistance to canker. However, the fact that under the 
conditions existing maximum susceptibility was obtained renders the results^more 
valuable in that it gives us a truer index of susceptibility. 
88287—24f-3 
