76 
FLORIDA STATE HORTICULTURAL SOCIETY. 
and parasitic organisms. But this is only 
a mental picture, for, as everyone knows, 
these conditions are never fully realized 
in any orange grove. A tree is continual¬ 
ly being subjected to conditions tending 
to directly injure it, or to slowly sap its 
vitality. It is subjected to improper 
amounts and kinds of plant-food; too 
much or too little moisture; too great or 
too sudden changes of temperature, such 
as frosts or burning heat; wind storms; 
improper kinds of soil; and also to a host 
of visible or invisible enemies that prey 
upon it from the outside. Many of these 
conditions are to a large extent under the 
control of the grower. With proper care 
and due expenditure, the plant-food and 
the moisture may be largely controlled; 
but the changes of temperature, the wind 
storms, and the nature of the soil, are 
mostly beyond the limits of man’s power 
to change. Parasitic enemies, as far as 
they are understood, are also largely un¬ 
der control of the grower; but there are 
many whose habits are not yet sufficiently 
known for us to get them under intelli¬ 
gent control. 
When, because of unfavorable condi¬ 
tions or attacks of parasites, a tree or any 
part of it ceases to function properly and 
begins to weaken, we say it is diseased. 
If we leave out of account the insect 
troubles, the diseases of citrus trees may 
be divided into two classes. (i) The 
physiological diseases due mainly to mal¬ 
nutrition, and (2) the diseases due to par¬ 
asitic organisms. In the first class, or 
those presumably due to mal-nutrition, 
are dieback, melanose, yellow spotting 
and frenching. In the second class, or 
those due to, or probably due to parasitic 
organisms are withertip, scaly bark, scab, 
gumming and blight. It is of this latter 
class that this paper will treat. The 
causes of some of the diseases here de¬ 
scribed have not yet been proved, but 
since their symptoms and mode of action 
are similar to those undoubtedly due to 
fungi or bacteria, they are included here. 
Most of the parasitic fungi that cause 
diseases in trees are invisible forms, or 
at least their seed-like bodies or spores are 
too small to be detected with the unaided 
eye. Just as the higher plants have dif¬ 
ferent ways of distributing their seeds, 
so these lower plants, the invisible fungi, 
have different means by which their 
spores are scattered. The seeds of the 
thistle and dandelion are carried by the 
wind to great distances; the seeds of the 
oxalis are shot out violently from the 
pods; the seeds of the mistletoe are car¬ 
ried by birds, and those of the burs by 
other animals. If we had microscopical 
eyes we would find that with these little 
fungus plants there are also various.meth¬ 
ods of spore distribution. Some spores 
are light and powdery, are blown off and 
float like dust in the air; others are moist 
and waxy and stick to the bodies of in¬ 
sects, the feet of birds, and the like. 
One kind of spore has hygroscopic 
movements, by which it moves on drying 
and being moistened again. Some are 
even shot out into the air as invisible dust 
particles. 
Most of the kinds of fungi, of which 
there are nearly 80,000 species described, 
are entirely harmless to a citrus tree, and 
many are even useful in keeping insects 
in check; but there are a few evilly inclin¬ 
ed individuals that make a great deal of 
trouble for the orange growers. It is 
only within comparatively recent times 
