342 



SCIENCE 



[Vol. LVI, No. 1447 



(thus differing from the chlorosis caused by 

 potassium defleieney in which the affected area 

 acquires a dull yellow oolor with a bronze or 

 copper overcast). The blanched tissues usually 

 do not die as quickly as in potassium deficiency 

 so that local specking or spotting of the leaf 

 is commonly wanting except in advanced stages 

 of the malady. In some instances, however, 

 local dying of the tissues between the veins 

 has been the first symptom. In plot tests con- 

 ducted in several tobacco-growing districts, in 

 which chemically pure sources of nitrogen, 

 phosphorus, potassium, calcium and sulfur 

 were used instead of the ordinary commercial 

 fertilizer materials, the symptoms of mag- 

 nesium deficiency have usually appeared where 

 sandy and sandy loam soils were used and 

 there was abundant rainfall. In all such eases 

 addition of sulfate or chloride of magnesium 

 to the fertilizer salts has prevented the chloro- 

 sis. In comparative tests, applications of the 

 so-called high grade or relatively pure com- 

 mercial forms of the sulfate and chloride of 

 potassium have resulted in severe chlorosis 

 while low grade sulfates and chlorides of po- 

 tassium containing considerable quantities of 

 magnesium, such as "double manure salt" and 

 "kainit," have prevented the disease. In some 

 cases use of the purer forms of sulfate of po- 

 tassium has resulted in severer chlorosis than 

 that caused by the chloride and, moreover, the 

 severity of the chlorosis has been proportional 

 to the quantity of sulfate of potassium used. 

 Symptoms of the disease also have been pre- 

 vented by applying dolomitic limestone to the 

 soil while comparatively pure calcite has been 

 ineffective. Certain organic fertilizer materials 

 of vegetable origin which are commonly used 

 as sources of nitrogen, notably cotton seed 

 meal, tobacco stalks and stems and barn ma- 

 nure, tend to prevent the disease. These ma- 

 terials contain appreciable quantities of mag- 

 nesium. Use of other common sources of fer- 

 tilizer nitrogen which contain little or no mag- 

 nesium, including nitrate of soda, dried blood 

 and especially ammonium sulfate, has favored 

 development of the disease. In pot cultures 

 this chlorosis is readily induced by applying a 

 nutrient solution containing all the usual plant 

 food elements except magnesium, using an ex- 



cess of the solution so as to produce a leaching 

 action on the soil. Moreover, the disease if 

 not too far advanced is readily cured by adding 

 magnesium to the nutrient solution. In view 

 of the relation of the sulfur supply to ihe 

 symptoms of magnesium deficiency it is worthy 

 of note that an inadequate supply of sulfur 

 per se results in a mild, diffuse type of chloro- 

 sis of tobacco, affecting all gi-een parts of the 

 plant and thus differing from the symptoms 

 of magnesium deficiency. Since more or less 

 sulfur is constantly added to the soil through 

 rainfall, while there is loss of magnesium 

 through the leaching action of the rain water, 

 it is to be expected that symptoms of mag- 

 nesium deficiency will be especially pronounced 

 in wet seasons. In dry seasons the likelihood 

 of relative deficiency of sulfur is increased. 

 Experiments with corn, a crop plant differing 

 widely from tobacco in many respects, show 

 that it also is subject to the "sand drown" 

 disease, the symptoms and characteristics of 

 the disease in corn corresponding rather close- 

 ly with those found in tobacco. It seems like- 

 Ij^, therefore, that other crop plants are sub- 

 ject to injury from an inadequate supply of 

 magnesium in the lighter, more sandy type of 

 soils. Apparently the quantity of magnesium 

 required in the fertilizer to prevent the symp- 

 toms of magnesium deficiency is small, prob- 

 ably less than 50 pounds per acre. These in- 

 vestigations suggest that the element mag- 

 nesium needs to be taken into account both in 

 the general problem of liming and in the 

 proper choice of commercial fertilizer ma- 

 terials for making up so-called complete fer- 

 tilizers. 



W. W. Gaener 

 J. E. McMtjrteet 

 E. G. Moss 

 Bureau op Plant Industbt, 



TJ. S. Department op Agricultuee 



TRANSFERENCE OF THE BEAN MOSAIC 

 VIRUS BY MACROSIPHUM SOLANIFOLII 

 It has been assumed by pathologists that the 

 virus of bean mosaic is transferred from dis- 

 eased to healthy plants by insects. No experi- 

 mental proof has been submitted to substan- 

 tiate this claim. The only satisfactory ex- 



