Florida Agricultural Experiment Station


of these factors are bound to play an important role as con-
tributing conditions which favor infection of the trees by the
causal parasites. He has demonstrated that at least six types
of gummosis in California, including the most common and
destructive forms, are caused by parasitic organisms.
 In Florida, however, but little is known concerning the cause
of gummosis. While some forms of it unquestionably are caused
by the attack of certain parasitic organisms, others appear to be
initiated by totally different causes and to be aggravated by the
subsequent invasion of the tissues by various organisms. The
relation of a fungus, apparently identical with Diplodia natalen-
sis, to the gumming and death of citrus twigs and branches in
Florida was reported by Fawcett and Burger in 1911. This or-
ganism was isolated from a number of gumming areas on citrus
trees and was shown by inoculation on healthy trees to produce
copious gumming. In 1913, Fawcett reported having repeatedly
isolated this fungus from the discolored wood at different depths
under gumming areas on trees in widely separated localities.
He also showed that certain other fungi, when inoculated into
citrus trees, will produce some gumming, but he found none that
equalled Diplodia in the amount of gum produced.
 Stevens, who later worked on gummosis in Florida for a num-
ber of years, isolated and studied a number of fungi and bacteria
from gumming areas of citrus trees and found Phomopsis citri
to occur with the greatest frequency. However, his inoculation
experiments with this organism, Diplodia natalensis, and tissues
from gumming areas gave negative results. Gumming was in-
duced in a number of cases but the wounds soon healed with but
little gumming of the tissue, especially when the inoculations
were made on young trees. Inoculations on old trees gummed
more freely and much larger areas of tissue were killed in most
cases. However, typical gummosis areas did not develop from
any of these inoculations. Therefore, Stevens questioned whether
any organism is directly responsible for the initial stage of
gummosis. However, he concluded that, in the later stages of
the disease after the cracks had formed and enlarged in the bark,
Diplodia, Phomopsis, and other fungi probably were responsible
for the gradual enlargement and aggravated condition of gum-
mosis areas. While it is evident that such organisms do fre-
quently invade gummosis lesions and aggravate the trouble, the
senior writer has observed a number of instances where active