634 
Journal of Agricultural Research 
Vol. XXX, No. 7 
The cases recorded above as doubtful 
showed decided death of tissue, but 
did not develop typical Gloeosporium 
limetticolum sporulation. It is very 
likely that such lesions were produced 
by saprophytic organisms that were 
able to invade and break down tissues 
under the rather extreme conditions of 
inoculation. In the writer’s opinion 
these doubtful cases are in all proba¬ 
bility negative. 
In the case of the citrons, 10 strains 
or varieties were furnished by the 
Office of Crop Physiology and Breeding 
Investigations. Six of these lots did 
not show any positive evidence of 
infection by G. limetticolum, and they 
are averaged together in the tabulation. 
Four other lots did show low percent¬ 
ages of what seemed to be positive, 
although not perfectly typical infection. 
This was verified by culturing G. 
limetticolum from the lesions on 
punctured shoots, and by securing 
typical infection on limes from such 
cultures; and on this basis a positive 
record was made in such cases. How¬ 
ever, the absence of any such cases on 
the unpunctured shoots, and the occur¬ 
rence on citrus of a large proportion 
of doubtful lesions in which the presence 
of G. limetticolum could not be demon¬ 
strated, would lend support to the view 
that tender shoots of these citrons are 
rather subject, under the conditions of 
the puncture experiments, to invasion 
by saprophytic fungi among which G . 
limetticolum may sometimes be found. 
Certainly these citrons, classed as 
susceptible, are affected with far less 
frequency than the susceptible varieties 
of limes, and the lesions recorded as 
positive did not have all the typical 
features of lime wither tip. 
Besides these somewhat questionable 
results from certain citrons, there is 
no positive susceptibility except in the 
group of limes, and among the varieties 
tested in this group the susceptibility 
is sharply confined to the West Indian 
lime and to Dominican Thornless lime. 
GENERAL DISCUSSION 
Clausen, 7 in his description of G. 
limetticolum, gives three host plants, 
Citrus medica var. acida Hook. ( = C. 
aurantifolia Sw., and includes the 
West Indian lime which is referred to 
by Clausen as sour lime), C. limetta 
Risso, and C. limonis L. Clausen 
explains that the last two were arti¬ 
ficially infected in greenhouses and that 
“the report of pathogenic properties on 
the sweet lime, C. media var. limetta, is 
according to C. O. Smith who is work¬ 
ing at Whittier, Calif., with a strain of 
fungus supplied by the writer.” The 
strain of Sweet lime used in the present 
investigation did not give positive 
results in 7 tests including 45 shoots, 
punctured and unpunctured as well 
as in 2 special tests of 3 plants each. 
Clausen reports only 2 tests of 
lemons {C. limonis L. -~C. limonia 
Osb.) with G. limetticolum. Two trees 
were used in each; 1 test was negative 
for both trees, in the other test one tree 
developed “an infection of three young 
leaves, the other lemon, as well as the 
control, was not affected in any way.” 
He further states, “the extent of 
parasitism on the lemon was not fully 
demonstrated in the experiments, but 
the parasitism in the one positive case 
was unmistakable.” Clausen’s inocu¬ 
lation tests included: 19 on sour (West 
Indian) lime of which 13 were positive, 
4 on lemon of which 3 were negative, 8 
negative on sweet orange, 8 negative 
on sour orange, 7 negative on grape¬ 
fruit, and 4 negative on tangerine 
orange. Adopting the view that the 
sour lime, the sweet lime, and the lemon 
are all varieties of C. medica L., he 
states in his summary that the wither- 
tip fungus in his artificial infection 
attacked only forms and varieties of 
this one species. 
The present investigation gives much 
more extensive evidence that most 
citrus types are immune to attack 
by G. limetticolum. It casts doubt on 
the reported susceptibility of sweet 
lime and lemon, and raises the question 
of whether or not certain citrons 
may have slight susceptibility when 
wounded. 
It is interesting to note that com¬ 
mercial plantings of the West Indian 
lime are commonly made from seed¬ 
lings. These have remarkable simi¬ 
larity in their general characteristics, 
and all seem to be highly susceptible 
to the wither-tip disease. A constant 
lookout has been kept in affected 
groves, and special tests of seedlings 
have been made for individuals im¬ 
mune to the disease. As yet no such 
individual has been found. It is 
possible that such individuals do 
exist, and the prompt discovery of 
one plant with high resistance or 
complete immunity would mean a 
quicker and more satisfactory solution 
of the wither-tip-control problem than 
the substitution of some other citrus 
variety that is immune, or the de¬ 
velopment of some new type by hy¬ 
bridizing. 
7 Clausen, R. E. Op. cit. p. 232. 
