296 Pflanzenkrankheiten. 



follows when the virus is sprayed upon the leaves and subsequently^ 

 rubbed in. 



3. Infection is more likely to foUow if the virus is inoculated 

 into the leaves at more than one point. 



4. Cutting across the midrib at the base of the leave, or seve- 

 ring all the larger veins on one or both sides of the midrib, does 

 not prevent the final dissemination of the virus from distant points 

 of inoculation in the leaf to other leaves on the plant. 



5. Thorough washing with soap and water serves to remove the 

 virus from the hands for all practical purposes. 



6. Thorough steam Sterilisation of the soil of the seed bed com- 

 pletely destroys any virus of the mosaic disease of tobacco which 

 may be present in the soil. 



7. The disease does not necessarily follow when large quantities 

 of mosaic-diseased material are introduced into the soil so that the 

 roots of healthy tobacco plants must ultimately reach it. Infection 

 from such material appears to depend upon injury to some portion 

 of the root System which allows the virus to enter at these points. 

 It is possible that root parasites may sometimes produce this injury 

 to roots and root hairs. 



8. The species Nicotiana glauca is susceptible to the mosaic 

 disease of tobacco, although visible Symptoms of the disease may 

 be very slight. The sap of such plants, although apparently but 

 little affected by the disease, so far as visible Symptoms are con- 

 cerned, is highly infectious to healthy plants of Nicotiana tahacum. 



9. Some species of aphids may become active carriers of the 

 infective principle of the mosaic disease. tJnder field conditions the 

 large plant louse Macrosiphum tabaci which sometimes becomes 

 very common on solanaceous plants, appears to be an efficient car- 

 rier of the infective principle of the disease. 



10. Red Spiders and white flies do not appear to be actively 

 concerned with the transmission of the disease. 



Van der Lek (Bennekom). 



Allard, H. A., Some proper ties of the virus ofthe mosaic 

 disease of tobacco. (Journ. Agric. Research. VI. p. 649—674. 

 PI. XCI. 1916.) 



In an earlier paper the writer has adduced evidence to show 

 that the mosaic disease of tobacco is not produced by simply cut- 

 ting back or otherwise subjecting plants to unfavorable conditions. 

 Now he gives the results of a careful and detailed study ofthe pro- 

 perties of the virus of this disease. He secures data which do not 

 lend Support to the physiological origin of the disease, and indicate 

 that infection does not depend upon the presence of oxidases or 

 peroxidases, but upon an infective principle which is not a normal 

 constituent of the sap of healthy plants. Among others the following 

 facts are to be mentioned: The infective principle is retained by 

 the Livingstone atmometer porous cup used as a filter and also 

 by powdered talc. Although the filtrates may show intense peroxi- 

 dase reaction, they do no longer produce infection. — The peroxi- 

 dase can be seperated from the infective principle by precipitating 

 it by means of alcohol, filtering and redissolving in water. This 

 Solution gives intense peroxidase reactions, but no longer produces 

 infection. — By the addition of different quantities of hydrogen 

 peroxid to the virus, it is possible to find a concentration of sußi» 



