Apr. 31 . 2933 
Gummosis of Citrus 
213 
The lesions produced by the inoculations with the two fungi were 
identical in general appearance. The characteristic manner of killing 
the bark, the formation of the outer noninvaded gummous zone, and 
the exudation of gum were the same with both fungi. Cuts without 
inoculation healed without producing disease. 
In addition to these inoculations, a number of other comparative 
inoculations with pure cultures of Pythiacystis citrophthora and Phy- 
tophthora terrestrial using fruits affected with the two fungi, were made on 
the large main roots and at the crown of old orange trees. In nearly 
all the successful inoculations of this kind, lesions resulting from the 
two species of fungi could not be distinguished clearly one from the 
other and resembled true mal di gomma or footrot lesions as they occur 
in Florida. Most of the lesions became self-limited in three to four 
months on old orange trees, after enlarging to a maximum of 8 by 10 cm., 
but some made with Pythiacystis citrophthora progressed longer and 
-developed large patches, in one case spreading 25 cm. and killing a large 
main root. 
CONTROL OP PYTHIACYSTIS GUMMOSIS 
More rational control methods based directly upon a knowledge of 
the cause and development of the disease and upon the results of many 
•experiments growing out of this knowledge now became possible. 
Previous control methods, though based largely on a different explana¬ 
tion of the cause, were partially successful because they frequently 
removed entirely the conditions contributing to infection and develop¬ 
ment of the disease, which conditions were formerly thought to be the 
sole cause. 
The most successful method of prevention for new plantings is obviously 
the use of the resistant sour-orange stocks budded high. This method 
was employed in Italy after the orchards were killed out by gummosis 
?and has since been used largely in Florida and California. To prevent 
gummosis on susceptible stocks or on low-budded trees, the method now 
in common use in California on heavy soils is to pull back the soil from the 
base of the trunk, thus exposing the top of the first main roots and making 
circular ridge to exclude irrigation water from standing in contact with 
the trunk of the tree. As an added preventive on soils especially subject 
to gummosis, the base of the trunk is painted with Bordeaux paste or 
oth^ noninjurious fungicide. In one experiment with Bordeaux used in 
this way on about 100 acres of lemons the number of new cases decreased 
from 123 in 1912 to 16 in 1914. By a similar treatment in another 
locality with 23,837 lemon trees the number of new gummosis outbreaks 
decreased from 727 in 1912 to 113 in 1914 (from 3.5 to 0.5 per cent). In 
a third experiment with 560 lemon trees, one-half of which was treated, 9 
per cent of those receiving Bordeaux spray two previous years developed 
disease, against 21 per cent of those not so sprayed. 
The treatment of trees after they are diseased consists in removal of 
the bark tissue from the invaded zone and the application of a fungicide 
to kill out the fungus in small bits of tissue possibly left behind and to 
prevent reinfection (PI. 3, B—D). No attempt need be made to cut 
beyond the large outer gummous zone, since it has been shown that this 
does not contain the invading parasite and will recover rapidly after the 
inciting cause for the gumming has been destroyed. 
w The detailed results of experiments in prwention and control will be presented in a bulletin irom 
-the University of ^lifomia Agricultural Experiment Station. 
