Apr. 21,1923 
Gummosis of Citrus 
229 
parasitic organisms and gum formation 
Although there are various agencies .or diverse stimuli that contribute 
directly or indireptly to gum formation in Citrus, the more serious and pro¬ 
gressive types of gummosis (in California at least) appear to be initiated by 
Sie invasion of certain parasitic fungi. With Pythiacystis gummosis, for 
sample, the bark and cambium region are first invaded and killed by the 
fungus which appears to advance most rapidly in the inner bark adjacent 
to 5 ie, cambium. Soon after a small portion is killed by the initial in¬ 
vasion of the fungus, an influence is exerted bringing about gum forma¬ 
tion in the. xylem region surrounding the dead portion. After some 
time gum begins to form rapidly wiliiin the xylem for long distances 
vertically, but usually for only short distances laterally, from tiiQ invaded 
zone. The parasite, therefore, must stimulate such gum formation in 
an indirect manner, through setting into action some substance which 
passes put into the conducting system, and which moves probably in 
the sieve tubes as well as in the wood vessels. This gummy degenera¬ 
tion within the outer gummous zone usually does not kill the bark nor 
prevent: its recovery, provided the parasite does not invade it later. ^ A 
thin outer layer of wood, not penetrated nor killed deeply by the parasite, 
becomes infiltrated with gum, and occasionally gum pockets are formed 
within it but usually near the cambium. 
The seriousness of excessive gum formation in itself has been overem¬ 
phasized by some investigators because it was erroneously held that the 
death of the bark was Sie usual cause of gum formation instead of 
being the result of fungus invasion. Gum formation appears to diminish- 
rather than to increase the seriousness of the diseases. As has been 
previously mentioned, the excessive gum formation in connection with 
Pythiacystis gummosis is frequently accompanied, or is soon followed, 
by a stoppage in the further advance of the invading parasite. It is 
highly probable that the infiltration of tissue by gum in the outer gum¬ 
mous zone tends to hinder the further advance of the mycelium into this 
tissue* In Diplodia gumming, Barle and Rogers (21) state that their 
observations have convinced them that gum flow in Citrus serves as a 
“protective device,** and in plum wilt, Higgins (jg) concluded that the 
gum served to hinder the advance of the organism by being foimed in its 
path of advance. In the disease due to Sclerotinia libertiana on Citrus 
twigs, C. O. Smith {^4) states that with the appearance of jthe gumming 
of the twig the further enlargement of the lesion is checked. 
The gum in connection with Pythiacystis gummosis lesions, in addition 
to being a probable hindrance to the organism, serves in dry weather as 
a seal or covering to the wood beneath the killed bark, thus preventing 
the entrance of wood-destroying organisms and preventing excessive 
(hying-out of the exposed wood. The gum being soluble in water, how^j 
ever, it is of little value for this purpose after the rains appear. 
brief generalization on gum formation 
The explanation of gum formation itself, which seems to fit in best 
with the known facts and results obtained with Citrus, is that a substance 
of an enzymatic nature may be brought into action through various in¬ 
strumentalities, such as substances acting under the influence of invading 
organisms, introduced or induced chemical stimuli, etc. It appears to 
be evident that the enzym immediately responsible for gum formation is 
