298 University of California Publications in Agricultural Sciences [Vol. 3 



about the navel end and drops from the tree when about one to two 

 inches in diameter, or even larger. The remainder persist to maturity, 

 the disease coming into evidence at picking time, in transit, in storage, 

 or not until in the hands of the consumer. 



Early in 1916 our attention was directed to the fact that on dissec- 

 tion a relatively large number of the shed fruits and fruits about to 

 drop were found to have a discolored area under the navel end. In 

 many cases a dark colored, gummy mass was present, although in others 

 the tissue immediately under the navel was only slightly discolored 

 (pi. 35). In some fruits there was no evidence of any such spot or 

 area. A few of the dropped fruits were sterilized in mercuric chloride 

 (1-1000) and placed in small moist chambers. To our surprise these 

 cultures showed practically 100 per cent infection with an Alternaria. 

 Other cultures were made with the same results. Therefore we con- 

 cluded that it was well within the realm of possibility that the June 

 drop was due to the same fungus causing black rot and decided to 

 investigate the matter more thoroughly. 



The fruits had reached a size of one or two centimeters and the 

 blooming period was entirely over, precluding any investigation as to 

 the source and manner of infection in 1916. Therefore our efforts in this 

 direction during 1916 were confined to attempts to determine, if pos- 

 sible, the extent of the infection. Cultures of many hundreds of 

 shed fruits, and fruits about to fall, from many districts of the state 

 were made both by the method above described and by inserting a 

 piece of tissue from the discolored area into slanted tubes of Shear's 

 corn meal agar. The cultures uniformly showed a high percentage of 

 infection with Alternaria. A few cultures were then made using 

 healthy green fruits picked from the trees. The percentage of infec- 

 tion was small. Still later in the season dropped fruits from four to 

 five centimeters in diameter (pi. 35) were collected from districts as 

 far apart as Oroville in the Sacramento Valley and El Cajon near 

 San Diego. Cultures made from these fruits showed practically 100 

 per cent infection. 



Although the number of cultures made was too small to justify 

 a broad generalization, the work done in 1916 was sufficiently 

 productive to form tbe basis for a working hypothesis which was 

 advanced as a theory to account for the June drop of Washington 

 Navel oranges. Other experimental work under way had indicated 

 the presence of certain abnormal water relations between the young 

 fruits and the leaves immediately behind them, which phenomenon 



