DISEASES OF PLANTS 77 



by injection of crystals of the salt in holes bored in the trunk 

 and then sealed, and by spraying the foliage with various sprays 

 containing zinc sulphate. In all cases a favourable result was 

 obtained. Johnston is of opinion that there are probably several 

 kinds of mottle leaf affecting Citrus which are due to different 

 causes, and he proposes to designate the type which responds 

 to treatment with zinc as little leaf, thus bringing the terminology 

 into line with that used for the similar condition found in 

 deciduous trees. 1 



In Florida the chlorotic condition of Citrus trees is known as 

 frenching. This is presumably the same disease as the mottle 

 leaf described by Johnston, and affected trees respond favour- 

 ably to the application of zinc sulphate. Thus Satsuma orange 

 trees showing symptoms of frenching were treated by Mowry 

 and Camp (1934) with a soil dressing of zinc sulphate, and as a 

 result showed signs of complete recovery. 



Whether mottle leaf or frenching is a zinc-deficiency disease 

 in the strict sense is not clear. Johnston, indeed, stated that 

 mottle leaf does not appear to be a case of soil deficiency and 

 suggests that the zinc may act as an antitoxin. 



Bronzing of Tung Trees. The condition of tung trees (see 

 p. 67) known as bronzing was first noted in 1930 by Newell, 

 Mowry and Barnette as occurring in trees in Florida growing on 

 soils containing large amounts of phosphate. Later observation 

 has shown that bronzing is not confined to such soils. 



The disease usually appears first in the late spring or early 

 summer or even later. The first symptoms are the appearance 

 of a bronze colour in a number of leaves, together with a defor- 

 mation of the terminal leaves of the shoots. With the develop- 

 ment of the dark bronze colour in the leaves necrotic spots 

 develop and parts of the leaves die so that these appear ragged. 

 Ultimately, a twig may lose many or all of its leaves. After the 

 first appearance of the disease in a tree the severity of attack 

 generally increases rapidly. New loaves are successively smaller 

 and become more deformed, the internodes fail to develop 



1 Already in the earlier of the papers by Chandler, Hoagland and 

 Hibbard cited above they had included (JUrus fruits and walnut along 

 with deciduous fruit trees as liable to little leaf. They also showed (1934) 

 the favourable effect of zinc on orange trees affected in this way. 



