DISEASES OF PLANTS 59 



and this occurred whether the solutions also contained some 

 other trace element such as boron, zinc, cobalt, copper, etc., or 

 not. With culture solutions containing 1 part of manganese in 

 50 x 10 6 , the symptoms developed suddenly in about 8 weeks. 

 On renewing the culture solution, including the manganese 

 supply, new healthy growth took place, but the symptoms of 

 deficiency again appeared after about 4 weeks. Recovery again 

 took place after a second renewal of the solution. With culture 

 solutions containing 1 part or more of manganese in 10 x 10 6 

 no symptoms of grey speck appeared, the solutions being 

 renewed after 10 weeks. 



Reference has been made above to the fact that grey speck 

 tends to occur on plants growing on alkaline soils containing much 

 humus. This has led to suggestions that the disease might be 

 associated with excess of calcium ions or with certain organic 

 compounds in the soil. Water-culture experiments carried out 

 by Samuel and Piper to test these possibilities yielded no sup- 

 port for such views. Culture solutions containing calcium ions 

 in various degrees of excess, or various organic substances 

 (humus, sucrose, glucose, starch, cellulose), in no case induced 

 symptoms of manganese deficiency in oats growing in them 

 provided manganese sulphate had been added to the solutions. 



That grey speck disease is the direct effect of manganese 

 deficiency has been disputed by Gerretsen (1937). He points 

 out that Lundegardh (1932) had recorded that the manganese 

 content of affected plants might be higher than that of healthy 

 ones; indeed, he gave values up to 420 p. p.m. of manganese in 

 affected plants and down to 1 p.p.m. in healthy plants. This is 

 certainly contrary to general experience, and Samuel and Piper 

 found that about 14 p.p.m. was the minimum amount of 

 manganese likely to be present in healthy Algerian oats at the 

 flowering stage. 



Gerretsen states that when a soil which had borne a crop 

 showing symptoms of grey speck was sterilized with formalin, oat 

 plants subsequently grown on it were free from grey speck, 

 although the manganese content was the same as before and there 

 had been no increase in either water-soluble or exchangeable 

 manganese. On reinfecting such sterilized soil with 10 per cent 

 of the original soil grey speck again appeared on oats grown on 



