Apr. 31 ,1923 
Gummosis of Citrus 
231 
mosis lesions, increased the severity of the disease when associated with 
Pythiacystis citrophthora but that Fusarium alone was not capable of 
initiating this gummosis. 
(9) Observations and inoculation experiments both indicated that the 
order of resistance of species and varieties to Pythiacystis citrophthora 
from highest to lowest, is, sour orange {Citrus aurantium Linn.), tri¬ 
foliate orange {Poncirus trifoliata Raf.), rough lemon (a resistant variety 
of Citrus limonia Osbeck), pomelo {Citrus grandis Osbeck), sweet orange 
{Citrus sinensis Osbeck), and common lemon {Citrus limonia Osbeck). 
The first is almost immune, the last very susceptible. 
(10) Inoculation of small roots of young trees indicated that common- 
lemon roots are somewhat susceptible but that sweet-orange, pomelo, 
and sour-orange roots are resistant. 
(11) Mai di gomma due to Phytophthora terrestria Sherb. has been 
shown to be similar to certain phases of Pythiacystis gummosis, especially 
to the form the latter takes at the junction of the main roots and ttunk 
of old orange trees in California. 
(12) Inoculations with Phytophthora terrestria and with Pythiacystis 
citrophthora^ under the same conditions, produced lesions which showed 
no characteristic differences. 
(13) Experiments have shown that in cases where excess of moisture 
and other contributing conditions cannot be entirely avoided the disease 
may be prevented largely by applying Bordeaux mixture or other 
fungicides to the trunks. 
(14) It has also been shown that the progress of the disease may be 
readily prevented by dissecting out the bark invaded by the causal 
organism and applying a fungicide. It was not found necessary to 
remove the bark in the outer gummous zone, since this bark would 
finally recover after the advancing fungus had been removed. 
(15) Botrytis gummosis of lemon trees is characterized in the early 
stages by a soft area of invaded bark killed to the wood, with exudation 
of gum on the trunk. Later this soft area becomes surrounded by a 
larger, firmer area in which only the outer layer of bark is killed, leaving 
a layer next to the cambium region alive. There is also, as in Pythia¬ 
cystis gummosis, a noninfected outer gummous zone from which copious 
gum exudation may take place. 
(16) A strain of Botrytis cinerea Lk. has been found commonly asso¬ 
ciated with this type of gummosis and has been isolated from numerous 
lesions. 
(17) Pure c;ultures of the fungus, as well as bits of the diseased bark, 
were found capable of inducing the disease when inoculated into cuts 
and other kinds of injuries on healthy lemon trees. 
(18) Attempts made to induce gum formation by various kinds of 
wounds on lemon tree trunks invariably failed when these wounds were 
kept clean and free from contamination with injurious organisms or 
chemical substances. 
(19) The experimental results show that the disease may be pre¬ 
vented by avoiding injuries to the bark and by using a fungicidal coat¬ 
ing on its surface. The treatment found effective consists in cutting 
or scraping away the dead bark, leaving as much of the live bark as 
possible, and painting the treated area with a fungicide. 
(20) Sclerotinia lihertiana is occasionally found associated with rapid 
dying of bark and copious gum exudation on trunks and roots. The 
