92 
FLORIDA STATE HORTICULTURAL SOCIETY. 
his book “Citrus Fruits and Their Cul¬ 
ture/’ 
EFFECTS UPON THE PLANT. 
Healthy leaves are the digestive or¬ 
gans of the plant. They manufacture the 
plant-foods, starch and sugar, from wat¬ 
er brought up from the soil, and from 
carbon dioxide obtained directly from the 
air. When the working surface of the 
leaf is greatly reduced by any cause, 
there is a reduction in the quantity of 
food manufactured. By the swelling of 
the cells in the spongy tissue, the air¬ 
spaces between the cells become closed 
up. This interferes with the entrance of 
air into the interior of the leaf. The de¬ 
velopment of corky tissue also excludes 
the air from the leaves at these points. 
This reduces the supply of carbon diox¬ 
ide from which the starch is manufac¬ 
tured. 
The leaves are also the breathing or¬ 
gans of the plant. The energy with 
which the plant works is supplied by this 
respiration. The reduction of the air- 
supply thus weakens the plant by cutting 
down its supply of energy. 
If only a few leaves on a tree are affect¬ 
ed, the normal processes of the plant are 
not interfered with. But when the ma¬ 
jority of the leaves are diseased, the life- 
processes will be enfeebled, and a per¬ 
ceptible weakening of the tree will result. 
Herein lies the chief harm the disease 
does to the tree. This reduction of the 
vitality of the tree allows the withertip 
fungus to gain entrance. 
The withertip fungus is peculiar in 
that when the tree is of its normal 
strength, this fungus is unable to gain en¬ 
trance into the wood. But immediately 
the vitality of the tree is reduced at any 
point, particularly in the region of the 
young tissue, this fungus bores its way 
into the plant. Once within it secretes a 
poison that further weakens the tissue, 
and allows the fungus to proceed farther 
inwards. When the fungus reaches a 
point in the tree where its vitality is 
equalled by that of the tree, it is unable 
to proceed farther. At this point, the 
branch will form a callus which may be 
seen as a ring. 
Though in many places the yellow spot¬ 
ting has been seen associated with die- 
back, yet there is no proof that there is 
any relation between the two. 
Very often the disease is accompanied 
or followed by melanose. This is partic¬ 
ularly so if the trees have not been 
strengthened by proper care and fertiliz¬ 
ers. 
The leaves on trees that are badly 
spotted are usually undersized. This 
stunted growth is probably due to the 
starting of the disease before the leaves 
were fully developed. 
CAUSE. 
In searching for a cause for a disease 
one must distinguish between those fac¬ 
tors that influence it, and those that cause 
it. For example, certain forms of fer¬ 
tilizers or certain methods of treatment 
of the trees may be an aid to the disease 
in its spread. Yiet these are not the 
causal factors. 
As yet no cause has been found for 
this disease. There is some evidence from 
field observations that it is transmissible, 
but attempts to transmit it by inoculation 
have given negative results. 
