Apr. 31,1933 
Gummosis of Citrus 
227 
phuric, phosphoric, nitric, and lactic acids, and with potassium hydrate 
but failed to induce it with acetic acid or kerosene; and Fawcett {23) 
induced gum with nitric, sulphuric, acetic, citric, and phosphoric acids, 
copper sulphate, mercuric chlorid, and ammonium lactate but failed 
to induce it with carbolic acid. 
Among the chemical stimuli that have been seen to result in gum 
formation occasionally in Citrus orchards may be mentioned: (i) liquid 
hydrocyanic acid spilled on the soil too near the roots of trees; (2) hydro¬ 
cyanic acid gas us^ in fumigation; (3) spray mixtures containing copper 
sulphate not properly neutralized with lime, or containing other toxic 
substances; (4) ant poison containing arsenic. 
It has not been possible, however, to duplicate entirely by chemical 
stimuli any of the typical maladies produced by certain organisms. 
While the gum formation induced by chemicals may be slight or extensive, 
the stimulus is soon at an end and the wound usually begins to heal 
promptly without the long-continued, progressive killing and gumming 
characteristic of Pythiacystis and Botrytis gummosis. Much confusion 
and apparent differences in results regarding gummosis have arisen from 
a failure to distinguish between the severe types of gummosis induced 
through the agency of microorganisms and those more temporary and 
usually milder types brought about by chemical or other stimuli. While 
the nature of the gum exuded may be the same, the manner in which the 
accompanying injury develops is usually very different. 
ENZYMS AND GUM FORMATION 
Only a small amount of experimental work on the r 61 e of enzyms in 
Citrus gummosis appears to have been done. The results of the writer's 
experiments will be presented here merely as suggestive of certain possi¬ 
bilities in connection with this subject. 
Savastano {52) concluded that in the form of Citrus gummosis due to 
Bacterium gummis, a toxin secreted by the organism, spreads out con¬ 
siderable distances beyond the invaded tissue, stimulating the gum 
formation. In line with this view, Higgins (59) in the study of plum 
wilt, concluded that the fungus Lasiodiplodia irifiorae Higgins secreted 
a toxic substance which reacted directly or indirectly on the zymogen 
of the host cells smd brought about the production of a gum-forming 
enzym. Butler (ij), on the other hand, had concluded that enzyms 
had no part in gum formation in Citrus or Prunus. This view, it seems, 
was based mainly on the origin and histology of the gum pockets and 
not on microchemical tests or experiments to detect the presence of 
enzyms. (See criticism of this view by Wolf) (65). Floyd (ji) made 
certain microchemical tests with material taken from gum pockets on 
Citrus branches affected with exanthema and concluded that the enzyms 
present were probably hemicellulase and pectinase. 
To get definite information as to the possible influence of enzyms or 
other filterable substances on gum formation in Pythiacystis gummosis, 
the following experiments were carried out.^^ 
Experiment i.—O n October i, 1912, lemon bark containing Pythia¬ 
cystis gummosis lesions was cut from two large trees at Whittier, Calif., 
and 350 gm. of green material were ground, first in a meat grinder, then 
The assistance of H. D. Young, formerly of the Southern California Pathological laboratory, in carry¬ 
ing out these experiments is acknowledged. 
