DISEASES OF PLANTS 65 



cotyledons. These necrotic spots generally only affect coty- 

 ledonary tissue, although occasionally the plumule may be 

 affected. Pods containing exclusively healthy seed, and 

 pods containing only diseased seed may occur on the same 

 plant, and pods are even found containing both healthy and 

 diseased seed. Externally the plant may appear quite normal, 

 although sometimes mild chlorosis or mottling of the younger 

 leaves may be present. In this country it occurs particularly 

 in Romney Marsh where it appears to be limited to alkaline 

 soils (Heintze, 1938). In Holland, Ovinge (1935) generally found 

 it on alkaline and relatively new polder soils. 



The fact that he found peas affected with marsh spot growing 

 near oats w T hich had developed grey speck led Pethybridge 

 (1936) to suspect that marsh spot might be attributable to the 

 same condition as grey speck, that is, to manganese deficiency. 

 This view was supported by the finding of Lohnis (1936) that 

 peas affected by marsh spot contained somewhat less man- 

 ganese than healthy peas, and by the experiences of Ovinge 

 (1938) in Holland and of Lewis (1939) in this country who found 

 that the application of soluble manganese salt either as a soil 

 dressing or a spray was effective in reducing the incidence of 

 marsh spot. Also Heintze found that among the Romney Marsh 

 soils those on which marsh spot occurred contained less salt- 

 soluble manganese (manganese extracted by a normal solution 

 of magnesium nitrate or calcium nitrate) than those on which 

 peas were free from the disease, the difference in the extractable 

 manganese being related not to the total manganese content, 

 but to the acidity or alkalinity of the soil. 



Determinations of the manganese content of different parts of 

 healthy peas and of peas affected by marsh spot made by Glass- 

 cock and Wain (1940) show a considerably lower manganese 

 content in the diseased seed. The peas examined were of the 

 variety Harrison's Glory, the diseased sample having been 

 obtained from Romney Marsh and the healthy peas from 

 Folkingham in Lincolnshire. The results are summarized in 

 Table III. 



