Florida Agricultural Experiment Station


 The phosphoric acid in fertilizers is not known to have any
effect, either beneficial or detrimental, upon dieback. The more
common organic sources, such as raw bone and steamed bone,
are excellent for use in fertilizers where the effect to be derived
from a small amount of organic ammonia is desirable.
 Potash is believed to have a balancing effect upon the action
of ammonia on growth. When the market permitted, a high
percentage was used in fertilizers for dieback trees. The organic
sources of potash, such as tobacco stems, may be favorable to
dieback when used in quantity, but the amounts that are con-
tained in fertilizer mixtures are usually not sufficient to induce
injury under ordinary conditions. They should be used with care
on low moist soils well supplied with organic matter.
 Hardwood ashes and other forms of lime, such as air-slaked
lime and ground limestone, are frequently applied as an antidote
for dieback. In some cases they appear to be beneficial and in
others they are without effect.
 EXPLANATION OF THE CAUSE OF DIEBACK
 It is seen that dieback may occur in trees growing in a variety
of locations and under many different conditions. In a study of
the affected trees the symptoms appear to be exactly the same
under all these conditions.. The only difference noticed was in
the degree of effect and in the predominance of certain symp-
toms. This suggests that there must be a cause for the disease,
that is common to all. It is probably connected with certain bac-
terial transformations in the soil. Dr. C. B. Lipman of the Uni-
versity of California suggests the hypothesis (Science, Vol. 39,
p. 728, 1916, and Florida Grower, June 3, 1916) that the disease
is caused either by "a lack of available nitrogen in any form
resulting in nitrogen starvation," or a weak nitrifying power
of the soil and a normal ammoniafying power resulting in the
enforced assimilation by the citrus tree of ammonia instead of
nitrate, with the result that the tree will give evidence of poi-
soning."
 The results of the writer's studies lead him to believe that
dieback is the result of the action of some chemical agent upon
the growing cells of the plant parts. The results of his experi-
ments do not confirm Lipman's suggestion that it is due to the
injurious action of ammonia, neither does he agree with Lipman
that it can be due to nitrogen starvation. Nitrogen starvation is
well known to be shown by a yellowing of the foliage. The foliage
of dieback trees often exhibits an intense green color that is very