Apr. 21,1923 
Gummosis of Citrus 
199 
tl^ough the bark of healthy tree trunkvS after first washing the surface 
with water and with alcohol. Paraffined paper was then tied over the 
cuts, control cuts without inoculum being used for comparison. In 
these preliminary experiments, the inocula used were as follows: (i) Bits 
of diseased bark tissue from near the margin of an actively enlarging 
gummosis lesion; (2) a bit of gum-filled woody tissue taken just outside 
the area of killed bark, in the outer gummous zone; (3) a piece of exuded 
gum from the same lesion; (4) bits of bark from a sound, healthy tree 
(used as controls). A cut was also made without inserting inoculum 
of any kind. 
Of these inoculations, the first (consisting of two trees) brought about 
a development of the disease beginning at the point of inoculation. The 
other cuts all healed without any exudation of gum or visible injury to 
the bark. It will be noticed that the inoculations resulting in disease 
were made from tissue cut from the advancing edges or margins of re¬ 
cently killed tissue, while the others were from tissue or gum beyond this 
killed area (outer gummous zone) or from healthy bark. The develop¬ 
ment of the disease in one tree will be described, sinc3 it proved to be 
typical of the severe form of Pythiacystis gummosis as it occurs on lemon 
trees in the coastal sections of California. 
On April 24, 1912, about two months after inoculation, gum was 
exuding rapidly and flowing downward to the surface of the soil, 15 cm. 
below, and hardening in a 15-mm. ridge. On August 2, over five months 
after inoculation, the exterior area of discolored bark was 15 by 25 cm. 
with a copious exudation of gum, forming three ridges on the bark surface 
and with a large mass of gum on the surface of the soil (PI. i. A). The 
foliage on the inoculated side of the tree was beginning to turn yellow. 
On September 19, 1912, the area of killed bark had extended upward and 
laterally, until it occupied a space 30 by 30 cm. and covered one-third the 
circumference of the trunk (PI. i, B). On November 15, 1912, the bark 
was dead on half the circumference of the trunk and the gum had formed 
ridges on the surface. The leaves on the affected side of the tree were 
v^ yellow and were dropping. The foliage on the side not affected was 
still healthy and normal. This tree now presented the appearance 
typical of many trees in the same locality which were affected naturally 
with Pythiacystis gummosis. On March ii, 1913, a little over a year 
from the time of inoculation, only 10 cm. of the 90-cm. circumference of 
the tree contained live bark at the level of the greatest lateral extension 
of the disease. At the place of greatest extension upward (46 cm. from 
the surface of the soil), the disease was arrested and callus tissue had 
formed. On May 24, 1913, the invaded area had extended only slightly 
beyond that of the previous observation. Pieces cut from the margins 
of the drying bark (PI. i, D) yielded cultures of Pythiacystis citroph- 
thora. Most of the gum first formed had been dissolved and carried away 
by the rains. The bark on the side first affected had dried and contracted, 
showing longitudinal fissures just like those seen in old naturally occur¬ 
ring cases. Gum was exuding on the margins of the killed bark. In 
every respect this was a typical case of the form of gum disease due to 
P. citrophthora. On September 3, 1913 (18 months after inoculation) 
only about 5 cm. of live bark remained on the circumference of the trunk 
(PI. I, C), and the tree was considered useless from a commercial stand¬ 
point. 
