22 AN INTRODUCTION TO THE STUDY OF VIRUSES 
transmission of two plant viruses, those of potato yellow dwarf 
and sugar-beet curly-top respectively. These two viruses are trans- 
mitted by the leaf hoppers, Agallia constricta and Eutettix tenella. This 
extreme specificity of vector is carried a stage farther by the fact that 
there exist two strains of the potato yellow dwarf and sugar-beet 
curly-top viruses each with its own vector. Thus the New Jersey 
strain of potato yellow dwarf is transmitted by Agallia constricta, and 
the New York strain by Aceratagallia sanguinolenta. Apparently this 
specificity is absolute, and neither insect can transmit the other virus. 
Similarly the strain of beet curly-top in the U.S.A. is transmitted by 
the leaf-hopper, Eutettix tenella, but this insect cannot spread the beet 
curly-top in Argentina which has the leaf-hopper, Agalliana ensigera, 
as its specific vector. An analogous case exists with the virus of equine 
encephalomyelitis which occurs in two distinct strains, the Eastern and 
the Western. The vector of the Western strain is the mosquito 
Aédes aegypti which cannot normally transmit the other strain. 
Selective Transmission of Viruses from a Complex 
It is no unusual thing for a plant virus disease to be caused by two 
or more viruses acting together, and the insect transmission from such 
composite diseases frequently shows some curious anomalies; the 
following instances illustrate this and refer only to aphis-borne 
viruses— 
(1) Two viruses present but only one aphis-transmitted. 
(2) Two aphis-borne viruses present but they differ in their 
relationship to different aphis species; thus one species of aphis will 
pick up both viruses whilst another will pick up only one. 
(3) Separation of a virus complex by differential feeding times of 
the aphis vectors. 
The following examples may make these three types of selective 
transmission clearer. There is a common crinkle disease occurring 
in the potato plant caused by two viruses acting in unison. These 
two viruses are known as potato viruses X and Y, and whilst both 
viruses are sap-transmissible only one, potato virus Y, is aphis-borne. 
Therefore, whilst sap-inoculation reproduces the whole disease, aphis 
transmission results in the production of a disease caused by the Y 
virus alone. 
In the second instance, the component viruses are both aphis-borne 
but differ in their relationships with different aphis species. The two 
viruses of cabbage black ringspot and cauliflower mosaic often occur 
