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



SUMMARY 



1. Citrus trees as grown in the interior valleys of the arid south- 

 west are subject to an environment entirely abnormal to them in their 

 natural habitat. 



2. Moreover, the principal variety grown in these regions, the 

 Washington Navel orange, is itself decidedly erratic and unstable. 



3. Among other troubles incident to the abnormal climatic con- 

 ditions is that heavy dropping of the young fruits, with consequent 

 light crops, known popularly as the June drop. 



4. A study of the shedding has established the fact that it con- 

 stitutes true abscission, involving the separation of living cells along 

 the plane of the middle lamellae. 



5. Exhaustive investigations as to the stimulus or stimuli responsible 

 for the abscission have narrowed them down to two: a fungus, Alter- 

 naria citri E. and P., and climatic conditions. 



6. It is considered highly probable that a certain varying per cent 

 of the drop, occurring relatively late in the season, is brought about 

 by the stimulation of this fungus, which is also responsible for a black 

 rot of those infected fruits which remain on the trees to maturity. 



7. This fungus is of very wide distribution and infection of the 

 young fruits is made possible through the peculiar structure of the 

 navel orange. 



8. The amount of infection is dependent upon weather conditions 

 and the more or less fortuitous configuration of the navel end of the 

 young fruits. 



9. On account of the peculiar manner of infection and the rela- 

 tively small amount of shedding due to the fungus, spraying will 

 probably not pay for the labor and materials involved. 



10. By far the greater part of the shedding, which occurs earlier 

 in the season, is due to a stimulus to abscission arising from daily 

 water deficits in the young developing fruits, resulting from the 

 asperity of the climatic complex to which the trees are subject. 



11. The principal factor in causing these abnormal water deficits 

 lies in the fact that citrus trees are not adapted to withstanding the 

 heavy water loss incident to the desert conditions under which they 

 are grown. The amplitude of stomatal movement is small and cutic- 

 ular transpiration very high. 



12. It is further believed that under the prevalent clean cultivation 

 practice, the soil temperatures during a part of the day are so high as 



