1919] Coit-Hodgson : Abnormal Shedding of Washington Navel Orange 305 



(fig. 6), and therefore the average configuration of the navels more 

 ragged and open, to what can we attribute this difference in the amount 

 of infection with Alter nariaf "We believe that this difference is easily 

 explained by a study of the mean maximum temperatures for the two 

 seasons. In table 2 these are shown for the years 1914—17 inclusive. 

 For the 1917 season, taking the months of January and February, we 

 see that they are about average for the last four years. March is four 

 or five degrees below the average, April still more, and even May is 

 below the average. June is several degrees above the average for the 

 last four years and July shows an average mean maximum temperature 

 of 101. 1° F, considerably above the average. In other words, the 

 early part of the season was cooler than usual and the bloom was 

 delayed a month or more. Coincident with the end of the blooming 

 period the weather changed radically and became very hot and dry and 

 continued so for at least three months. Conditions were unfavorable 

 for infection by Alternaria; its growth was inhibited although the 

 spores were present. In fact, the amount of drop due to Alternaria 

 in 1917 is practically negligible, and this is supported by the fact that 

 there were very few black rot oranges at Edison at harvest time. On 

 the other hand, the season of 1916 was noted as a relatively cool, 

 pleasant summer and as such was favorable for infection by Alternaria, 

 with the result that there were many black rot oranges. 



In this connection the question arises, why are not other citrus 

 varieties grown in these arid districts subject to infection by Alternaria 

 with a consequent, shedding and loss due to black rot? The answer 

 apparently lies in two facts : that other varieties are not so susceptible 

 to shedding, which will be discussed later, nor are they morphologically 

 adapted to infection by the fungus. Plate 39 shows the apical end of 

 a small Valencia orange highly magnified and it is evident that there 

 is no favorable entrance for the fungus spores. Plate 34 shows a 

 similar view of a Navel orange with very favorable conditions for the 

 lodgment of fungus spores. 



During the course of these investigations a great deal of time and 

 effort was devoted to attempts to ascertain by inoculation methods 

 whether the stimulus of Alternaria citri which manifested itself so 

 clearly in the change of color of the fruit might not also be the cause 

 of abscission of the young fruits. On account of several peculiar 

 difficulties inherent in this particular problem we have so far been 

 unable to secure conclusive results. The three most important of these 

 difficulties may be mentioned briefly as follows : 



