JW OF Amm RESEAM 
VoL. XXIV Washington, D. C., Aprh, 21, 1923 No. 3 
GUMMOSIS OF CITRUS* 
By Howard S. Fawcett ^ 
Professor of Plant Pathology, Citrus Experiment Station, College of Agriculture, Univer- 
sity of California 
PART I.—GUMMOSIS DUE TO PYTHIACYSTIS CITROPHTHORA 
INTRODUCTION 
The purpose of Part I is to present the results of an investigation, be¬ 
gun in 1912, into the nature, causes, and manner of development of cer¬ 
tain types of gummosis of Citrus trees. In the three parts of this paper 
the term gummosis will be employed in the broader sense, in which it 
applies not only to the process of gum formation but also to the diseases 
ot pathological effects in which gum formation is one of the conspicuous 
features. When employed in connection with specific diseases, supple¬ 
mentary terms will serve to show its modified meaning. 
Part I will deal mainly with the causal relation of Pythiacystis citro- 
phthora Sm. and Sm. to one of the most widespread and destructive 
forms ef Citrus gummosis in California. The relation of another fungus, 
Phytophthora terrestria Sherb., to a similar disease, mal di gomma, in 
Florida is also included. Part II will present the results of an investiga¬ 
tion into the relation of Botrytis cinerea Lk. and other fungi to other 
types of gummosis in Citrus. The last section of this paper. Part III, 
will deal with gum formation as such, the conditions influencing its 
foimation, ancl its relation to diseases. The results of investigations on 
the control of some of these diseases will be discussed in a bulletin to be 
issued from the California Agricultural Experiment Station and will 
therefore receive only a brief mention in this paper. A preliminary 
report regarding control has already been published by Fawcett, (24,26). 
Previous investigators had come to the conclusion that all gum dis¬ 
eases of Citrus trees in California originated independently of micro¬ 
organisms, according to Smith and Butler (56). It was held that these 
diseases were largely autogenous in their nature and were frequently 
induced through the effects of certain climatic or soil conditions alone. 
It now appears evident that these environmental conditions can not 
by themsdves initiate all the severe forms of gummosis in Citrus earlier 
attnbuted to them, although many of these factors are found to play, 
as they do in most parasitic diseases, an important r 61 e as contributing 
1 Accepted for publication June 29, 1922. Paper No. 91, University of California, Graduate School of 
Tropical Agriculture and Citrus Experiment Station, Riverside, Calif. 
* This investigation was started in the spring of 19x2 under the auspices ci the State commission of horti¬ 
culture and ifvas trac^erred to the University erf California in the fall of 1913. The author wishes to acknowl¬ 
edge the encouragement given by Dr. A. J. Cook and Prof. R. E. Smith at the beginning of the work, 
the assistance rendered by J. D. Culbertson, J. A. Prizer, and many other Citrus growers. Acknowl¬ 
edgment is flilso due to Bruce Douglas fcH* laboratory assistance during a portion of the time of the 
inve^igation and to th(»e members of the staff of the College of Agriculture who aided by many suggestions 
and criticisms. 
Journal of Agricultural Researdi, 
Washington, D. C. 
acw 
(19*) 
Vol. XXIV, No. 3 
Apr. 21, 1923 
Key No. Calif.-jj 
30617—23-1 
