Bulletin 140, Dieback of Citrus Trees


used without injury to a grapefruit or a tangerine tree may in-
duce dieback in an orange tree.
 Of the different organic sources of ammonia, cottonseed
meal and tankage are considered in practice to be more active
in bringing on the disease. But tankage is not a uniform product
and may consist of a variety of materials. It is probable that
some tankages are much more active in bringing on dieback than
others. The guanos and dried blood are considered as next in
activity to cottonseed meal and tankage. Tobacco stems, sheep
manure and goat manure, raw bone and steamed bone, are the
least active.
 While these fertilizer materials may be active in bringing on
dieback when used in excessive quantities, their use is not to be
avoided under all conditions. When used in limited quantities and
under proper conditions they are excellent fertilizers for citrus
trees. Their effect is to produce growth; when they are used in
excess this growth is likely to be rapid, rank and sappy and
may be followed by the development of dieback. But, when used
in smaller amounts, especially on trees that are backward and
need stimulation, these fertilizers may produce an active but
more nearly normal growth. Experienced growers on the pine-
lands sometimes give their trees a rather liberal application of
cottonseed meal, tankage, or stable manure when they get into a
condition of slow growth, but care is used not to repeat it until
after a long interval.
 Many of the best brands of fertilizers on the market contain
limited amounts of ammonia from organic sources. When these
fertilizers are used in average amounts and under average con-
ditions the organic ammonia gives good results. But if the trees
are overfed with them they may be thrown into a dieback con-
dition. The mineral sources of ammonia, such as nitrate of
soda, sulphate of ammonia and nitrate of potash, are not known
to induce dieback even when used in excessive quantities. When
used liberally about trees that are already affected they may
for awhile accentuate the trouble. It is only rarely and for
short periods that ammonia should ever be entirely omitted from
the fertilizers used about dieback trees. In some cases a low
percentage of ammonia is desirable and in others the usual per-
centage should be used. In most cases the ammonia should be
derived entirely from mineral sources. The amount of fertilizer
used about dieback trees should not be large, but it should be
applied regularly-three times each year.