CONTROL OF VIRUS DISEASES 93 
proximity of seed and mother beets which overwinter and are usually 
heavily infected with virus and aphis. 
It is important, therefore, to separate the root crop and the mother- 
beet seed crop as far as possible and a scheme has been recently put 
forward for growing the “steckling” beets in another part of the 
country where sugar beets are not grown and where the likelihood 
of virus infection is less and then transporting them the following 
season to the sugar beet area. 
It is a good general rule to avoid, whenever possible, the planting 
of a susceptible variety near to other varieties which are known to 
act as symptomless carriers of a virus. This applies particularly to 
raspberries where the well-known variety Lloyd George is a carrier 
of the mosaic virus; to strawberries of which Royal Sovereign is 
almost invariably virus-infected without showing symptoms, and to 
hops and dahlias. 
3. Plant Breeding for Virus Resistance 
Breeding new varieties of plants for resistance to virus infection 
is a hopeful line of attack and is one which has already had some 
success. The resistance may vary in kind and in degree but the end 
result is what matters. For example, much work has been done, both 
in this country and in the U.S.A., on the development of virus- 
resistant potato plants. The American potato known as 41956 is 
apparently completely immune to infection with the common potato 
virus X. On the other hand, potato varieties have been produced 
which are so susceptible to a given virus that they are killed outright. 
This is also a type of immunity since the virus is destroyed with its 
host; this kind of reaction is called field immunity. Sometimes a plant 
may be made resistant to a virus disease because it has been rendered 
unpalatable to the insect vector by reason of hairiness of the leaves or 
thickness of cuticle. Other outstanding examples of the production 
of virus-resistance in crops are the P.O.J. strains of mosaic-resistant 
sugar cane, the American varieties of sugar beet (U.S. Nos. 1, 33, and 
34) which have a fair degree of resistance to curly-top and strains of 
cotton of Sakel type which are resistant to the leaf-curl disease. 
Much of the success of this kind of breeding work depends upon 
the occurrence of a naturally immune or resistant individual plant so 
that a factor of resistance from it may be incorporated in a new line. 
Again, it is possible to incorporate a character of resistance from 
one plant species in another by cross-breeding. An interesting example 
