Apr. 21,1923 
Gummosis of Citrus 
195 
The previous views regarding Citrus gummosis in California, based in 
part on investigations on Pythiacystis gummosis, are fairly well indicated 
by the following quotation from Smith and Butler (56): 
The lesions, ulcers, or afiE^ected areas produced, are not primarily the seat of the 
trouble. They represent rather the effect of what may be called a general constitu¬ 
tional derangement showing itself by external outbreaks or symptoms at whatever 
points may chance to be most susceptible. What may be called a primary 
weakness exists back of the visible symptoms, and in this weakness can be sought 
the fimdamental cause and nature of the disease. It is, therefore, not necessary to 
identify a parasite, or strikingly evident climatic or soil conditions, to account for 
diseases of this class. It may be said here without extended discussion, that 
in no case have we been able to recognize or demonstrate the presence of any fungus, 
bacterium, or other parasitic organism as the cause of any form of citrus gum disease. 
Some of the paramount conditions mentioned by Smith and Butler as 
contributory to the occurrence of gum disease are the accumulation of 
soil against the trunk, excessive moisture where poor drainage or careless 
irrigation e^dsts, the heaping of manure against the trunk, low budding, 
lack of loose aerated soil and sweet orange stock on heavy soils. It is 
now known that these conditions are favorable for infection by the 
fungus and the* * development of the disease.^ 
Previous investigations and observations on Citrus gum diseases had 
led, therefore, to three general hypotheses; (i) That gum diseases were 
brought about by organisms capable of infecting and invading the tissue 
under certain contributing conditions; (2) that gum diseases were due to 
wounds or other external stimuli other than microorganisms acting 
immediately on the parts affected; (3) that gum diseases arose auto- 
genously being due to internal derangements of the host brought about 
by or without the influence of certain factors of the environment acting 
on the host as a whole. 
PYTHIACYSTIS GUMMOSIS 
SYMPTOMS 
Pythiacystis gummosis, with its associated rot of the fruit, is probably 
the most widespread and destructive of the Citrus gum diseases. The 
most striking features of the disease on the common lemon,® which is 
the most susceptible form, are copious exudations of gum and large dead 
patches of bark on the trunk, followed by yellowing and dropping of 
leaves. On old sweet-orange trees and other partially resistant forms, 
the dead patches are usually smaller and show a greater tendency to 
become self-limited. 
In the earlier stages of the disease (PI. i. A; 3, A; 5, A) the extent to 
which the bark is invaded by the fungus can only be determined by lightly 
scraping the thin surface of bark in the vicinity of the exuding gum 
until the green color is seen. The margin between the invaded tissue 
and the sound bark then shows only as a difference in color, the normal 
green shading off gradually into a drab. The bark is not softened but 
remains firm. After a considerable time it becomes sunken and begins 
to crack longitudinally. 
^ In this connection it should be mentioned that Smith is5) was also among the first to accept the conclu¬ 
sions to which the first experimental work by Fawcett {24) led, although these were opposed to the 
views which he had formerly held. 
* The names of species and varieties of Citrus will be used in accordance with Swingle {63), as follows: 
common lemon, Citrus linumia Osbeck; rough lemon, a horticultural variety of C. limonia Osbeck; sweet 
orange, C. sinensis Osbeck; sour orange, C.grandis Osbeck; citron, C. medica Linn; trifoliate orange); Po»- 
cirus trifoliata Raf. The word lemon when used alone will refer to the common lemon and the word orange 
to the sweet orange. 
