1919] Coit-Hodgson : Ab)wnnal Shedding of H'ashington Navel Orange 307 



The REii.\Tiox of Abscission to the Environmental Complex 



It has long been noted that there exists a marked correlation 

 between climatic conditions and the prevalence and amount of the 

 June drop. This correlation has been discussed somewhat by the junior 

 author in another place^* and has been reflected in the general attitude 

 of growers who are prone to assign Juue drop to hot north winds, 

 sudden changes in temperature, and other causes, most of which are 

 climatic in nature. 



In order to ol)tain more accurate information in this i-egard an 

 investigation of the yield per tree in different citrus districts, where 

 all other factors except the climatic complex were comparable, was 

 carried out during the season of 1917. The results were striking and 

 show mast pronounced correlation between climatic conditions and 

 yield wlien all other factors such as orchard management, etc., are 

 fairly comparable. It was found that, assigning a yield of 100 per cent 

 to the district averaging the highest crop, which district is character- 

 ized by considerable summer heat but moderate atmospheric humidity, 

 the farther inlanil the district lies the smaller is the crop. This is 

 precisely the order in wliich the asperity of the environmental cdin- 

 plex is heightened, the atmospheric humidity decreasing and the 

 average summer temperature increasing. ^loreover, and more im- 

 portant, distance from the coast brings with it increasing liability 

 to sudden changes in the weather whicli react most unfavorably on 

 crops, particularly when in certain stages. The districts where these 

 climatic conditions are most .severe, namely, the Coachella Valley and 

 the southern San Joaquin Valley, show a yield of approximately 

 25 per cent of that of the most climatically favored district. At 

 intermediate stations the extent of the drop and consequently the size 

 of tlie crop is easily correlated with weather conditions during the 

 critical period. This was exemplified bj' the almost total loss of the 

 Navel crop in tlie district between Corona and Redlands in 1017. whrn 

 a dry north wind of unprecedented severity was accompaiiictl by maxi- 

 mum daily temperatures as high as 118°-120° F from June 15 to 17. 



This correlation between asperity of climatic conditions and amount 

 of crop, or what amounts to the same thing, the prevalence of dropping, 

 was very apparent in the orchard where our experimental work was 

 done at EdLson. The yield from the particular ten-acre tract used was 



•■"s TIo(]<»son, Robert W., Some Abnormal Water Relations in Citrus Trees of 

 the Ariii Southwest and their Possible Significance, Univ. Calif. Publ. Agr. Sei., 

 vol. 3 (1917), i>p. 37-54. 



