2 o8 
Journal of Agricultural Research 
Vol. XXIV, No. 3* 
On some trees only a small lesion was evident with much gum exud¬ 
ing, on others the bark was killed to a distance of 15 to 30 cm. above 
the soil, with an abundance of gum, and still other trees were dead. 
Some trees showed a strong tendency to form ridges of callous tissue 
along the edges of the dead strips of bark. A thorough search in the 
block of sour-orange trees failed to reveal a single tree affected. 
Differences in resistance are indicated further by an estimate made by 
W. M. Mertz in a nursery of Citrus seedlings about 2 years old grown at 
the Citrus Experiment Station. The following is the percentage of gum' 
disease (probably Pythiacystis gummosis) which was recorded: 
Species. 
Number of 
trees. 
Percentage 
with gummosis. 
Citrus aurantium (sour orange). 
I, 000 
0. 2.' 
Poncirus trifoliata (trifoliate orange). 
I, 000 
I. O' 
Citrus grandis (pomelo). 
I, 000 
2 . 5 
10. o > 
Citrus sinensis (sweet orange). 
2, 000 
The inoculations recorded in Table VIII were made on trees about 2 
years old from seed, by cutting through the bark on the stem not far fromu 
the soil and inserting bits of the mycelium from cultures of Pythiacystis^ 
citrophthora grown on sterilized orange wood. Oiled paper was tied over 
the cuts. 
Table VIII.— Inoculations made May 15, igi 6 \ observations July 2, 1^15, when the^ 
plants were pulled up 
Host. 
Sour orange... 
Rough lemon. 
Sweet orange. 
Pomelo. 
Number of 
plants. 
Results. 
3 
5 
4 
3 
No gum on exterior. One shows interior gum. AII1 
healing normally. 
No gum. Healing normally. 
Two trees gumming copiously; long strip of bark 
killed. Two trees no gum on surface; wounds 
healing. 
Bark killed slightly on edges of cuts without gum; 
on exterior; healing rapidly. 
Of the four forms tested in this experiment, sweet orange showed the 
greatest effect, pomelo less, and sour orange and rough lemon ^ showed 
no appreciable effect from the inoculation. The weather was very hot 
and dry during the experiment and thus not very favorable to the fungus. 
On February 6, 1915, inoculations into the trunks of 21-month-oldi 
seedling sour-orange and rough-lemon trees were made at Santa Paula. 
A branch of a young common-lemon tree close by was inoculated at the* 
same time. The results on May 18, 1915, were as follows: 
The inoculated sour-orange cuts healed rapidly, with slightly more 
gaping of the wound than in the controls, while the rough lemons showed 
a small amount of dead bark next to the inoculated cuts, which were also- 
healing rapidly. A slightly lower resistance of the rough lemon than of 
the sour orange was indicated. On the common lemon, however, the^ 
leaves had withered on the inoculated branch, the bark was killed 
f The rough lemon is thought to be a hybrid. It is used largely for stocks in Florida and should not be- 
confused with the common lemon. 
