DISEASES OF PLANTS 75 



into the trunks of the trees appears to be the most effective 

 treatment, but favourable results are also obtained by the use of 

 zinc sulphate as a soil dressing or as a spray in winter. No 

 improvement could be detected as a result of treatment with 

 salts of silver, nickel, cobalt, tin, cadmium,- mercury, iron, 

 copper, chromium, manganese, aluminium, molybdenum, sele- 

 nium, zirconium, uranium, strontium, tungsten and titanium 

 (Chandler, Hoagland and Hibbard, 1934, 1935). A number of 

 organic compounds gave equally negative results. 



Determinations of the zinc content of stems and leaves of 

 various fruit trees affected by, and free from, little leaf published 

 by Chandler, Hoagland and Hibbard suggest that the zinc 

 content of shoots from trees in orchards free from little leaf is 

 on the whole higher than in those of trees affected by little leaf. 

 Hoagland, Chandler and Hibbard (1936) have been able to 

 induce the symptoms of little leaf in young apricot trees grown 

 in water cultures from which care was taken to exclude zinc, 

 as far as practicable. 



Chandler, Hoagland and Hibbard were at first very reluctant 

 to attribute little leaf to actual zinc deficiencv. Their reasons 

 for this reluctance were the suddenness with which healthy trees 

 might begin to die from little leaf, the recovery of some trees 

 without any obvious improvement in the zinc supply, and the 

 fact that, whereas trees are susceptible to little leaf, annual 

 plants growing on the same soils are apparently free from any 

 symptoms of zinc deficiency. 



That simple zinc deficiency alone may not afford a complete 

 explanation of the cause of little leaf is suggested by the work 

 of Ark (1937). This investigator sterilized, by means of steam, 

 soil from orchards showing little leaf and found this treatment 

 very beneficial to maize and tomato. Also sand cultures of 

 maize were treated respectively with little -leaf soil and sterilized 

 soil. In the former the plants soon showed symptoms of zinc 

 deficiency (white bud, see p. 79), while the plants receiving 

 sterilized soil, or in sand without any soil, remained normal. 

 Further, from little-leaf soils he isolated two strains of bacteria 

 which when added to the artificial culture medium in which 

 maize seedlings were growing induced symptoms of white bud 

 which were removed by raising the content of zinc in the 



