Pflanzenkrankheiten, 297 



cient strength to destroy the peroxidase, but leaving little or no 

 free peroxid in the Solution. Such Solutions retain their infections 

 properties for a long time, — Solutions containing 1 part formal- 

 dehyde in 800 parts of Solution gave infection. Stronger concentra- 

 tions were no longer infectious, although giving intense reactions 

 for peroxidase. — If virus Solutions are heated, the infective prin- 

 ciple is more quickly destroyed than the peroxidase. — It is not 

 readily destroyed by ether, Chloroform, toluene or carbon tetra- 

 chloride; it is highly resistant to low temperatures. "When frozen to 

 a temperature of —180° C. with liquid air, its infectious properties 

 were not weakened. — Since it has been shown that the mosaic 

 disease of tobacco does not occur in the absence of infection, nei- 

 ther enzyms nor other normal constituents in the sap of healthy 

 plants can be considered responsible for the disease. A specific, 

 particulate substance, probably an ultra microscopic parasite of 

 some kind, is the cause of the disease. 



Van der Lek (Bennekom). 



Briggs, Lyman J., C. A. Jensen and J. W. Mac Lane. Mottle- 

 leafof citrus trees in relation tosoilconditions. (Journ. 

 Agric. Research. VI. p. 721—739. PI. H, XCVI, XCVII. 1916.) 



„Mottle-leaf" is a term applied in California to a mottled or 

 spotted condition of the leaves of Citrus trees, due to the disappea- 

 rance of Chlorophyll from parts of the leaf, the portions farthest re- 

 moved from the midrib and larger veins being first affected. The 

 advanced stages are accompanied by a marked decrease in the size, 

 quality and jield of the fruit. The principal conclusion ofthisinves- 

 tigation — based upon a study of 130 orange groves and 45 lemon- 

 groves — is that the mottling is definitely correlated with the low 

 humus content of the soil, the mottling diminishing as the humus 

 content increases. Orchards fertilized with organic substances (f. i. 

 Stahle manure) usually showed less mottling than groves supplied 

 principally with commercial fertilizers. Groves which for some years 

 had received only the „complete" fertilizers in general use were 

 badly mottled. A study of the data by Statistical methods shows 

 that approximately one-half of the mottling can be accounted for on 

 this basis. The incorporation of organic matter with the soil in such 

 a manner as to be accessible to the roots during its decomposition 

 is indicated as a promising treatment for mottle leaf. 



Van der Lek (Bennekom). 



Hawkins, Lon A., The disease of potatoes known as 

 „leak". (Journ. Agric. Research. VI. p. 627-639. PI. XC. 1916.) 



From the experiments carried out in this study, it seeras that 

 potato leak is most coramonly caused by Pythiuni DeBaryaniim. 

 This fungus was obtained 49 times in 61 attempts; in inoculation 

 experiments it produced a rot typical in all appearances to the 

 potato leak. This fact is not surprising when the work of earlier 

 writers is taken into account (Sadebeck, DeBary, Ward, Ed- 

 son). Pythium DeBaryanurn was found in soil samples taken from 

 various parts of the delta potato region. Infection apparently takes 

 place in the field \iy some of this infected soil getting into wounds 

 made in digging. No cases of infection were observed where the 

 skin of the tuber was unbroken. That this fungus should cause so 

 much damage to potatoes in the San Joaquin delta region is 



