TWO FUNGI AS CAUSAL AGENTS IN GUMMOSIS 

 OF LEMON TREES IN CALIFORNIA. 



{Botrytis vulgaris and Pythiacystis citrophthora.) 

 By H. S. Fawcett, Plant Pathologist, State Commission of Hortculture, Whittier, Cal. 



Introduction. 



The term "Gummosis, " or lemon gum disease, will be restricted in 

 this article to that condition described by Professor R. E. Smith and 

 0. Butler, in Bulletin 200, of the California Experiment Station. It is 

 characterized by dying of areas of bark, accompanied by the exudation 

 of gum, usually somewhere on the trunk from above the bud union to 

 the forks of the branches. The word Gummosis, as used by Professor 

 Smith, and as used here, does not apply to the formation of gum in the 

 small branches, leaves or fruit, nor to mere gumming that is entirely 

 unaccompanied by dying of bark. Mere gum formation in citrus trees 

 may take place as a result of chemical stimuli or other causes, but this 

 is not included under the word gummosis. In some respects, it would 

 be better to use for this disease a term such as "bark rot," since the 

 dying of the bark rather than the formation of gum is what results in 

 serious injury and justifies the term "disease." Gummosis is a definite 

 disease, mere gum formation alone is not a sign of any one specific dis- 

 ease, but since misunderstanding is likely to arise in making changes in 

 names, the term Gummosis, expressing a definite result of the disease 

 rather than the cause, had best be retained. That at least two forms of 

 this disease are induced in healthy trees by two fungi (commonly known 

 in the packing-houses as the brown rot fungus and the grey fungus) 

 have been discovered as the result of a series of experiments during the 

 past year. 



The commonly held view that all forms of this disease in California 

 were physiological, and that they were due to some deranged condition 

 of the tree itself, brought about entirely by unfavorable soil, climatic 

 or cultural conditions, was a natural inference from the commonly 

 observed fact that the larger percentage of the cases of gummosis 

 occurred where unfavorable soil or cultural conditions existed, such as 

 poor drainage, soil above the bud union, excessive amount of water, etc. 

 So well was this relation between unfavorable conditions and gummosis 

 recognized by growers and those previously studying this disease that 

 fairly successful methods of dealing with it had been worked out by 

 them and put in practice by some of the most successful orchardists. 

 The work previously done by Professor R. E. Smith* and others in 

 obtaining a thorough understanding of the causal conditions (poor 

 drainage, soil above the bud union, etc.) served as an excellent basis on 

 which to carry further the work in determining some of the causal 

 agents (the fungi). It would seem now that the success of the methods 



♦Smith, R. B., and Butler, O., Gum Diseases of Citrus Trees, California Experiment 

 Station, Bulletin 200, 1907. 



