1919] Coit-Hodgson: Abnormal SIu tiding of Washington Navel Orange 29!) 



will be discussed more fully in a later section. Briefly, the theory 

 advanced was that excessive transpiration from the leaves caused 

 water together with enzymatic solutions secreted by the fungus in the 

 navel end to be drawn back through the vascular system of the young 

 fruits through the pedicel and thus provide the stimulus to abscission. 34 

 That there is no mechanical difficulty involved in this theory was 

 borne out when by means of dyestuff solutions it was demonstrated 

 that the vascular system running to the navel or secondary orange 

 traverses the central pith or core of the primary fruit, which thus 

 serves as receptacle and stem to the smaller fruit (fig. 1). 



Fig. 1. Structure of the Navel orange. The central pith containing fibro- 

 vascular bundles acts as the stem of secondary fruit. 



Further evidence tending to support this theory lies in the fact 

 that black rot is much more prevalent in the interior valleys than in 

 the coast regions. In fact, there seems to be a certain correlation 

 between the amount of black rot and the amount of drop. The reason 

 for the greater prevalence of black rot in the hotter, more arid districts 

 was not uncovered until later; this will be brought out in another 

 section. 



Alternaria citri, Ellis and Pierce 



During the winter of 1916 a careful study of the alternarias 

 obtained in our cultures was made and disclosed the fact that although 

 there were several strains of Alternaria obtained, one particular type 

 rather easily recognizable after a little practice, was by far the most 



3* Coit, J. Eliot, and Hodgson, E. W., The Cause of June Drop of Washington 

 Navel Oranges, Univ. Calif. Jour. Agr., vol. 4 (1916), p. 10. 



