230 
Journal of Agricultural Research 
Vol. XXIV, No. 3 
produced by the host tissue itself; otherwise it would be difficult to ac¬ 
count for the gum formation induced by such a variety of chemical sub¬ 
stances and agencies. In Pythiacystis and Botrytis gummosis, for ex¬ 
ample, there would seem to be two possible explanations of the manner 
in which the gum formation is brought about: (i) A substance secreted 
directly by the fungus sets in motion the gum-forming enzym or (2) a 
substance resulting from the interaction of parasite and host furnishes 
the means by whi^ the gum-forming enzym is brought into action. 
In case of the action of chemicals, such as corrosive sublimate and 
sulphuric acid, the substance might act also in either of two ways analo¬ 
gous to that given for the parasite, either directly in the same way as a 
secreted substance, or indirectly through a substance produced by the 
interaction of the plant tissues and the introduced chemical. The experi¬ 
ments with sterile filtrate from diseased and healthy bark previously de¬ 
scribed, taken in conjunction with the work of Floyd, Higgins, and others, 
appear to show that whatever may be the indirect factors in gum forma¬ 
tion, the immediate cause or stimulus to its formation is a filterable sub¬ 
stance of the nature of a heat-sensitive enzym. It is evident that many 
important phases of the subject of gum formation in Citrus remain for 
further investigation. 
SUMMARY 
(1) A destructive form of Citrus gummosis attracted attention first 
in the Azores in 1834. A similar gum disease appeared in Italy as early 
as 1863; in Portugal, 1865; in Australia, 1867; in Spain, 1871; in United 
States of America, 1875; and in most other Citrus regions before the 
year 1890. 
(2) Pythiacystis gummosis, the most widespread and destructive 
type of Citrus gummosis in California, is characterized by copious 
exudations of gum and dead patches of bark on the trunk and main roots. 
The gum may arise not only from the margin of the invaded area but 
also from a large contiguous, outer, noninvaded zone. 
(3) It has t^en shown that the disease with all its usual symptoms 
may be readily transmitted to healthy trees by inoculation with bits of 
bhfk tissue cut from the advancing margins of killed regions of bark. 
It is not transmitted, however, by tissue from surrounding outer gum- 
mous zones or by killed tissue not recently invaded. 
(4) Cultural tests have shown that live mycelium of Pythiacystis 
citrophthora Sm. and Sm., formerly known as the cause for lemon brown- 
rot, is present in this narrow band or fringe at the advancing edges of 
the killed region of bark (invaded zone) but is absent or dead elsewhere. 
(5) Numerous inoculations with pure cultures of Pythiacystis ci-. 
tropkthora into healthy trees under various conditions have shown that 
this fungus is capable of inducing gummosis with all the characteristic 
symptoms of naturally occurring cases. The fungus has been reisolated 
from many lesions after it has remained in the bark from i to 11 months. 
(6) Lemon fruits affected by brownrot due to Pythiacystis citrophthora 
have been shown to be capable of inducing the same type of gummosis 
as that produced by the fungus from gummosis lesions. 
(7) The organism appeared to die out more readily and the lesions 
became self-limited more quickly after inoculation in branches or large 
roots than after inoculation in the trunk. 
(8) A limited number of inoculation tests gave some indication that 
Fusarium sp., found commonly in connection with Pythiacystis gum- 
