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Pomona College Journal op Economic Botany 



Nature of Die-Back 



So far as present knowledge goes, die-back is not a fungus disease at all. There 

 have never been found mycelial threads or fruiting bodies which are characteristic 

 of the true parasitic fungi as well as of other members of this group. It is con- 

 sidered, then, a physiological disease similar to Gummosis, Mottled-leaf, Barren- 

 ness* etc., the nature of which are not often clearly understood. 



History and Distribution 



As was already stated the die-back disease was first described in Florida as 

 early as 1875, and is believed to have originated in that state. For some years 

 it was not known to exist in any other locality. Later it was found to have spread 

 very rapidly in that state until in 1907 Hume states that it could be found in prac- 

 tically every citrus orchard in Florida. 



It was introduced into California on nursery stock along with other disastrous 

 Florida diseases and first came under observation in San Diego county, being re- 

 ported by F. Austin, then secretary of the County Board of Horticultural Commis- 

 sioners, in 1896, although it was stated that the disease had been observed on the 

 same trees for six years, making its introduction approximately 1890. Since that 

 time it has been reported in the San Gabriel valley, along the foothills from High- 

 lands to Pasadena, and is now located in three orchards in Ventura county. 



Die-back was reported as existing in the citrus orchards of the Hawaiian Islands 

 in 1905, and in Arizona in 1908, so today it is present in practically every citrus 

 section in the United States, but has never been reported from South America, 

 Australia or from other citrus sections in the Far East. 



Appearance and Symptoms 



Trees affected with die-back usually present a very dark-green color, although 

 this is not always a sure case of die-back. In severe cases the tree remains dwarfed, 

 with a narrow, oblong shape, and may eventually die. Then, too, the top may be 

 very small, but composed of very compact foliage, due to an excessive production 

 of tender shoots. The dying back of the ends of the twigs and the formation of 

 numerous new shoots often give the trees a very regular and trimmed appear- 

 ance, but in advanced stages they appear very irregular and ragged. 



Twigs 



(Figure 29, A and C) 



Young — The young twigs on a badly affected tree first begin to turn yellowish 

 before the leaves are matured. The leaves may fall or not. Swellings or gum 

 pockets begin to appear, gum exudes, covering portions of the twigs and leaves 

 with a reddish-brown coat. 



Drooping — The ends of the twigs usually droop slightly at first. Later the tip 

 may curve upward, giving the end of the twig an s-shaped appearance. The end 

 dies back from four to six inches (Fig. 29 C). 



Gum Pockets — -(Fig. 29 A). The swelling appearing upon the young stems 

 between the leaves on the internodes or at the bases of the leaves at the nodes, are 

 caused by the formation of gum pockets within the stem. A cross-section of these 



