68 TRACE ELEMENT DEFICIENCY 



Frenching may also occur in another species of Aleurites, 

 A . montana, the mu-oil tree. 



It was found that frenching was not limited to alkaline soils but 

 was related to a low value of exchangeable manganese in the soil. 



In the earlier stages of chlorosis recovery could be brought 

 about in from 3 to 6 weeks by dipping the shoots in a 1 per cent 

 solution of manganese sulphate containing 1 per cent calcium 

 hydroxide and 1 per cent calcium caseinate spreader. 



Determinations by Reuther and Burrows (1942) of the photo- 

 synthetic activity of affected leaves and leaves which had 

 regained their normal colour by treatment with manganese 

 sulphate did not indicate any very significant increase in photo - 

 synthetic activity as a result of treatment, and it is suggested 

 that an environmental condition such as high leaf temperature, 

 solarization or stomatal closure might limit the rate of photo- 

 synthesis under field conditions in Florida. Reuther and Bur- 

 rows also point out that trees severely affected by frenching tend 

 to produce small leaves, so that the total photosynthetic 

 activity of the tree and consequently its production of new 

 material may be reduced by frenching. 



2. Diseases Attributable to a 

 Deficiency of Zinc 



So far there have been no records from Britain of plant diseases 

 attributable to a deficiency of zinc, but in America such diseases 

 may be serious. They largely affect fruit trees, but have also been 

 recorded as occurring in maize and some other herbaceous plants. 

 As with manganese deficiency, the first sign of zinc deficiency 

 is usually an intervenal chlorosis, but in trees this is generally 

 followed by very characteristic symptoms of abnormal growth 

 known as rosetting. In spring, instead of the development of 

 elongated shoots with normal-sized leaves distributed along the 

 length of the shoot, there develops a rosette of small stiff leaves. 

 According to the species affected, the disease is variously known 

 as rosette, little-leaf, mottle-leaf or yellows. Chandler (1937) 

 prefers to class all these conditions, including those of zinc 

 deficiency in maize, as one disease, which he calls zinc-deficiency 

 disease, although he points out that the evidence may not yet 



