60 TRACE ELEMENT DEFICIENCY 



it and the dry weight of the plants was reduced to 59 per cent 

 of that of plants grown on the sterilized* soil, while the man- 

 ganese content of the affected plants was reduced from 51-5 to 

 19-3 p. p.m. Some very striking results were given by sand- 

 culture experiments. Plants were grown in sterile sand and in 

 the same sand infected with 5 per cent of so-called 'diseased' 

 soil. The plants in the sterile sand were all healthy, had a mean 

 dry weight of 436 mg. and a manganese content of 15-0 p. p.m., 

 whereas those in the infected sand all showed typical symptoms 

 of grey speck, a mean dry weight of only 216 mg., but a man- 

 ganese content of 26-6 p. p.m. 



Oats grown in sterile water-culture solutions containing very 

 small quantities of manganese showed no symptoms of grey 

 speck, although the plants were stunted and might contain less 

 than 10 p. p.m. of manganese. But when the solutions were 

 inoculated with a root tip from an affected plant or with bacteria 

 isolated from affected roots, the symptoms of grey speck deve- 

 loped strongly. Again, the addition of 0-001-0-002 per cent of 

 Germisan, a germicide, to the culture solution of non-sterile 

 plants kept the plants healthy even when the manganese con- 

 tent was low, whereas without the addition of the Germisan the 

 plants were badly affected. 



These facts are held to indicate that grey speck is related to 

 the presence of micro-organisms. According to Gerretsen the 

 roots of affected plants always show signs of microbiological 

 disintegration. It is suggested that alkaline products are pro- 

 duced in the roots by the infecting micro-organisms and that 

 these products are carried in the transpiration stream to the 

 leaves, where they produce the grey spots. 



Gerretsen therefore concludes that it is necessary to dis- 

 tinguish between the direct physiological effect of manganese 

 deficiency which is a retardation of growth, and the symptoms 

 of grey speck disease which are related to the infection of the 

 roots by micro-organisms. The capacity of the root to resist 

 parasitic attack by micro-organisms is indeed held to depend 

 on the manganese content, but if the roots are maintained 

 sterile healthy plants are produced in presence of a very small 

 supply of manganese so that the manganese in the plant is only 

 from 5 to 35 p. p.m. 



