502 SECTIONAL TRANSACTIONS.— K. 



vector concerned was rare, difficult to find, or nocturnal in habit. This 

 theory seems insufficient, however, to account for the fact that there are 

 no fewer than fifty-two plant viruses which spread without insect agency or 

 by unknown means. For some of these viruses, no doubt, insect vectors 

 will be found in due course. Nevertheless, it is now known that other means 

 of natural spread exist and some of these are discussed. In the case of 

 Nicotimia Virus XI (tobacco necrosis virus) it has already been stated that 

 this virus is both water- and air-borne and further experiments on these 

 lines are outlined. Murphy and Loughnane have recently expressed the 

 opinion that Solatium Virus I (potato virus X) spreads by mechanical 

 contact alone and this statement is examined in the light of field experiments 

 on the spread of this virus carried out at Cambridge during the past five 

 years. 



Dr. R. N. Salaman, RR.S. — Protection against virus diseases of the 

 potato (10.30). 



The solution will vary with the character of the district. The factors to 

 be considered are : (a) The virus content of our seed potatoes ; {b) the 

 conditions favouring the insect vectors ; (c) the existence of infective foci. 



Methods of Attack. 



(i) The breeding of virus-resistant varieties. — The only success so far is 

 the production of variety No. 41956, immune to the X virus, in U.S.A. 

 It is hoped to breed immune varieties from wild species but no virus- 

 resistant species have so far been determined ; to convey the immunity 

 when found to an economically valuable potato is a long and difficult task. 



(2) The maintenance of stocks tolerant to virus diseases. — Unconsciously 

 we have been following this course for the last hundred years. The results 

 are not impressive. We have no varieties completely tolerant to either 

 Leaf Roll or the Y virus, though Great Scot is semi-tolerant to both. A few 

 show some degree of tolerance to Y. All our popular stocks are highly 

 susceptible to both. Progress along these lines with our existing material 

 is unlikely. 



(3) Protection by means of vaccination. — In America all varieties are 

 infected with some mild strain and in consequence are protected against 

 the virulent strains of X. In England such is not so general, hence our 

 varieties are liable to acute X infection. We have isolated a non-virulent 

 strain of the X virus which, used as a vaccine, protects against other forms 

 of the X virus. With the Y virus, weak forms protect against the severer, 

 but the former are not yet weak enough to be of practical use. 



(4) The raising and growing of virus-free stocks. — The Potato Virus Re- 

 search Station at Cambridge has isolated and maintained stocks of our 

 chief varieties in a state of freedom from virus disease or, where that is 

 impossible, such as are infected by only a virus of low virulence. These 

 return the highest yields. If isolated from infected stocks, they will con- 

 tinue to do so indefinitely. 



Their multiplication is intended to follow a rotation beginning in the 

 Hebrides and/or coastal districts of West Scotland ; thence to the large 

 seed-growing areas of East Scotland and, finally, to the potato-raising 

 districts of England. Each year virus-free seed would leave Cambridge 

 for the ' islands,' and seed from these would pass to the mainland. The 

 cycle of rotation would be five or six years from the liberation of virus-free 

 Cambridge seed to its growth in England. Combined with this scheme 

 should go a nation-wide eradication of bad and infected stocks. 



