196 
Journal of Agricultural Research 
Vol. XXIV, No. 3 
On healthy, rapidly growing lemon trees the area of killed and dark¬ 
ened bark, elliptical to irregular in outline, is usually 15 to 30 cm. in 
vertical length and half that in width, when the gum first becomes 
apparent. By that time the fungus has been invading the tissue usually 
for a period of from two to four months. The removal of the bark at 
this time will show that the outer margin of the invaded zone is about 
coextensive with that seen on the surface (PL 3, B). Most often in 
young trees the death of the cambium and inner bark precedes slightly 
the death of the outer bark. The upward extension from the point of 
infection is usually many times its lateral and usually much greater than 
its downward extension. 
In the cambium region surrounding an actively invaded area evidence 
of an influence extending from the margins of the dead bark will be found. 
There is simply a production of clear, watery gum which seems to origi¬ 
nate in the region of the embryonic wood among the live cells without 
any apparent fermentation or decay. This region which is not yet dark¬ 
ened beyond the invaded portion, will be spoken of in this paper as the 
“outer gummous zone.*’ It may in time extend considerable distances 
upward and downward and small distances laterally from the margin of 
the invaded zone. It has been traced for 60 and 90 cm. upward. The 
extent of this outer gummous zone varies especially with the age and 
rapidity of development of the disease lesion and the condition of the 
tree. In some cases it is much larger than the invaded zone and in others 
much smaller. 
The inner surface of the bark in the invaded zone in a lesion of con¬ 
siderable size varies from mineral brown to burnt umber or fawn color • 
and the same discolorations will be found on the surface of the wood just 
at or beneath the cambium. (PI. 3, A, B.) The discoloration does not 
extend far radially (usually only 2 to 5 mm.) into the woody layers. 
The cambium region in the outer gummous zone is chamois to yellow 
ochre in color, fading out gradually at the margins into the normal white 
color of the sound woody surface. 
Frequently when the bark is irregular in contour, gum pockets will be 
formed, 2.5 to 5 cm. in longest axis, due to the rapid and unequal forma¬ 
tion of gum. The gum accumulates near the cambium and by pressure 
separates the bark from the wood at certain places, forming definite 
pockets. The pressure is usually relieved by a break in the bark before 
the pockets become large. A few deeper gum pockets of considerable 
size have also been found, situated in the outer gummous zone, beneath 
layers of wood 3 to 6 mm. in thickness, showing accumulations of gum 
under pressure. The gum, which is watery and clear when first formed, 
hardens as it comes to the surface, apparently by loss of water, and finally 
becomes brittle. On the surface the hardened gum usually ranges from 
mahogany to chestnut in color.® The gum accumulates on the surface 
in long narrow ridges (PI. i. A, B; 4, A; 5, A, D; 7, A) or in oval masses, 
or runs down and collects in masses on the soil, depending upon the 
rapidity of formation and the dryness of the air. During periods of 
heavy dews and rains it gradually dissolves and disappears (PI. i, D; 
5, E). 
Only in rare cases where the surface of the bark is moist during the 
development of the disease is there any sign of fungus development to 
• Ridgway, Robert. cotOR standards and coi.or nombnclaturb. 43 P-. 53 col* pl* Washington, 
D. C. 1912. 
