76 TRACE ELEMENT DEFICIENCY 



medium or by the injection of a zinc salt into the stems. Addition 

 of one of the bacteria to cultures of peach and walnut also 

 resulted in symptoms very similar to little leaf, symptoms which 

 were prevented by the presence of zinc. Altogether Ark's 

 observations are verv reminiscent of those of Gerretsen on the 

 relation of micro-organisms to the grey-speck disease of oats. 



Chandler (1937) suggests that if little leaf is the result of a 

 simple zinc deficiency this might be brought about by the pro- 

 duction in certain soils of a flourishing micro-flora that absorbs 

 zinc in large quantities. The effect of sterilization, by killing 

 this flora, would thus be to leave more zinc available for higher 

 plants rooted in the soil. Alternatively, Chandler suggests that 

 soil organisms might excrete, or yield on dying, substances 

 which combine with zinc to form insoluble zinc compounds 

 and so render it non-available. In these circumstances the effect 

 of the sterilization treatment might be to break up the insoluble 

 zinc compounds and release the zinc in a soluble form. 



Mottle Leaf (Little Leaf Type) or Frenching of Citrus. 

 The disease of Citrus trees known as mottle leaf in California has 

 been described in detail by Johnston (1933). The rfame is derived 

 from the fact that yellow areas arise between the veins of the 

 leaves giving these a mottled appearance. These chlorotic areas 

 enlarge, and as fresh leaves develop these are progressively 

 smaller until in severe cases they may be only an inch long, with 

 chlorophyll developing only at the basal end of the midrib. In 

 extreme cases dieback may follow, ultimately resulting in the 

 death of the tree. The root system is also affected, the smaller 

 rootlets first and the larger ones later, until in severe examples 

 only large roots with a few rootlets remain functional. All 

 species and varieties of Citrus can be affected by the disease. 

 The severity of the affection is increased by extremes of high or 

 low temperature. 



Since mottle leaf of Citrus and little leaf of deciduous trees 

 often occur in the same orchard, and since Chandler, Hoagland 

 and Hibbard (1932. 1933) had found that application of zinc 

 sulphate was a cure for little leaf, Johnston tried the same treat- 

 ment for mottle leaf of Citrus and found that this resulted in 

 restoring the normal green condition to mottled trees. The zinc- 

 sulphate was applied as a circle of salt on the soil round the tree, 



