RELATION OF ENVIRONMENTAL FACTORS TO CITRUS 
SCAB CAUSED BY CLADOSPORIUM CITRI MASSES 1 
By George L. Peltier, formerly Plant Pathologist, Alabama Agricultural Experi¬ 
ment Station, and Agent, Bureau of Plant Industry, United States Department 
of Agriculture, and William J. Frederich, formerly Assistant Pathologist, 
Bureau of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
In connection with citrus canker investigations conducted by the writers, an 
opportunity was afforded to observe the behavior of citrus scab caused by 
Cladosporium dtri Massee under the weather conditions prevailing in southern 
Alabama for four seasons. In the following pages an attempt is made to corre¬ 
late field observations with studies made under controlled conditions in the 
laboratory and greenhouse on the pathogene, on the host plant, and on infection 
and development of the disease. 
HISTORY AND DISTRIBUTION 
Scab attacks the young fruit, leaves, and occasionally the shoots of Citrus 
plants. The effect of scab on the fruits is the most important aspect of the 
disease from an economic viewpoint, in that scabby fruit either results in lower 
market value or in cull fruits. 
What probably constitutes the earliest record of scab was found by Lee (5) 2 
on a herbarium specimen of Citrus nobilis Lour, collected at Nagasaki, Japan, 
in 1863. He states that the early occurrence of this disease in Japan and the 
general distribution Of citrus scab throughout the Citrus-growing regions of 
South China indicate that this disease may be indigenous to the Orient. 
Reinking (7, 8), in a plant-disease survey of southern China found scab 
generally prevalent in this region, which substantiates Lee’s conclusions. Con¬ 
siderable work has been done on scab in Japan, as it is there one of the more 
important diseases of Citrus, but owing to the inaccessibility of the Japanese 
literature it is difficult to obtain much information regarding the history of scab 
in that country. No doubt, as in the case of the citrus canker, Japan has 
indirectly served as the center for the distribution of scab to some of the coun¬ 
tries in which the disease is now present. For several decades large exportation 
of Citrus nursery stock to various parts of the world has been made from several 
Japanese ports. Thus it is possible that Japan served as the center for scab 
distribution to Australia, the Gulf Coast section of the United States, and South 
Africa, as it did in the case of canker. So far as is known, no scab has ever been 
reported from California and the Philippine Islands, although there is every reason 
1 Received for publication February 16,1924. Published with the approval of the Director of the Alabama 
Agricultural Experiment Station. The paper is based on cooperative investigations between the Office of 
Crop Physiology and Breeding Investigations, Bureau of Plant Industry, U. S. Department of Agricul¬ 
ture, and the Department of Plant Pathology, Alabama Agricultural Experiment Station. Presented 
before the thirteenth annual meeting of the American Phytopathological Society, Toronto, Canada, 
December 28 to 31, 1921, and abstracted in Phytopathology 12:57. 1922. Since that time two papers 
dealing with certain phases of the work discussed have appeared, to wit: 
Winston, J. R. 
1923. citrus scab: its cause and control. U. S. Dept. Agr. Bui. 1118, 38 p., illus. 
Tanaka, T. 
1923. A BRIEF HISTORY OF citrus scab in japan. Phytopathology 13:492-495. 
s Reference is made by number (italic) to “Literature cited,” p. 254. 
Journal of Agricultural Research, 
Washington, D. C. 
( 241 ) 
Vol. XXVIII, No. 3 
Apr. 19,1924 
Key No. Ala.—10 
