98 R. MARKHAM 



B. Chemical Properties 

 The virus has not been investigated very thoroughly as yet. The phosphorus 

 content is about 3.1 %, indicating that the virus has some 35 % of nucleic 

 acid, presumably of the ribose type. 



IX. The Turnip Yellow Mosaic Virus 



There are several viruses which cause mosaics in turnip; one of these, 

 which has many interesting characteristics, causes an extremely bold mottling. 

 This virus, the turnip yeUow mosaic virus, is fairly widespread in Europe, 

 and causes serious disease in turnips, cauliflowers, savoy cabbages, and other 

 cruciferous plants of economic importance. It is a virus which is present 

 in the host plants in fairly large concentrations, as much as 4 grams per 

 liter of sap having been isolated under somewhat exceptional circumstances, 

 and it is spread about imder natural conditions by the turnip flea beetle 

 {Phyllotreta sp.). This method of spread is in itself unusual, and is dependent 

 upon the organization of the digestive system of the beetles coupled with 

 the resistance of the virus to digestion (Markham and K. M. Smith, 1949). 



A number of strains of the virus are known, all having similar effects on 

 the host plants. One complication which has not been resolved (and which 

 has not been published before) is that all stocks which are being investigated 

 as the "type" strain of this virus at the moment may have originated from 

 a different isolate from that originally described. This was due to a mixing of 

 two isolates of the virus by accident in 1951. The present cultures resemble 

 the original isolates closely, but may in fact differ slightly. 



Besides the "type" virus, cultures of other isolates are maintained in 

 England, but have not been investigated thoroughly so far. 



A. Purification 



The virus is best cultured on Chinese cabbage or turnip plants, in which 

 it produces its characteristic yellow mosaic (Fig. 11). The rate at which the 

 virus multiplies is slow, and appears to increase as the plants begin to suffer 

 from malnutrition. Consequently the plants should be grown in small pots 

 and be allowed to become pot-bound. They are harvested about 2 months 

 after infection, and then ground, and the sap expressed. 



The sap is clarified by the addition of 300 ml. of 90 % (v.v.) ethanol to 

 each liter of sap, the flocculent precipitate of plant proteins spun off, and the 

 supernatant liquid, which is yeUow and shghtly opalescent, has a half 

 volume of saturated ammonium sulfate in water added. Crystallization of 

 the virus begins in a few minutes, and is complete in 4 or 5 hours. At the 

 same time, strongly birefringcnt crystals, possibly of calcium sulfate, are 



