162 
The Philippine Journal of Science 
1921 
DIFFERENCES IN SUSCEPTIBILITY OF CITRUS SPECIES AND VARIETIES 
IN THE EXPERIMENTS 
It became apparent at Lamao that there are wide differences 
in the susceptibility to canker(9) of the different species of 
Citrus which necessitate a separate consideration for each 
species when working with, or discussing, such subjects as the 
control of canker, the injuries resulting from canker, and the 
eradication of canker. Many of the contradictory statements 
with regard to various phases of citrus canker would seem to 
be due to a confusion of the species or, in some cases, to the 
failure to connect such statements with the host under observa- 
tion. A consideration of the susceptibility of the different 
species at Lamao in connection with the feasibility of control 
methods has been possible and therefore is briefly presented 
here. 
At the beginning of these experiments the trees of the 
mandarin orange varieties had little or no canker. Very few 
infections were found, and it was a carefully recorded observa- 
tion that more than 33 per cent of the few infections that were 
found upon mandarin orange varieties occurred at very evident 
wounds, either spine wounds or insect injuries. Under such 
conditions, as shown in the foregoing tables, complete elimina- 
tion of citrus canker upon the mandarin orange varieties was 
very prompt and there were no recurrences of infection; also, 
the operations were simple and inexpensive. Many of the 
citrons (C. medico) were also slightly susceptible and very easily 
freed from canker. The mature calamondin trees were never 
observed in the writer’s experience to be cankered, and control 
measures for citrus canker on this host were entirely unneces- 
sary. These species, the mandarin oranges ( C . nobilis var. 
deliciosa) , the citron ( C . medico) together with the calamondins 
( C . mitis) and, of course, the round kumquats ( Fortunella 
japonica) , constitute a class of commercially grown citrus fruits 
upon which control is simple and very often unnecessary, at least 
when trees are mature. These varieties might be classed as a 
group, class 1, the members of which are so slightly susceptible 
that they can mature and produce fruits with no injury from 
citrus canker, even in the absence of control treatments. 
At Lamao almost complete elimination of canker was obtained 
very quickly on the American-grown lemon varieties (C. limonia) 
and the so-called Mediterranean varieties of the sweet oranges 
(C. sinensis), such as the Valencia, Jaffa, Mediterranean Sweet, 
Ruby, St. Michael, and Du Roi. In this class should also be 
