172 L. M. BLACK 



considerable number of the insects remained infective as long as they 

 lived. 



Additional instances of transovarial passage have been recorded; in the case 

 of rice stripe virus transmitted by Delphacodes striatella, Yamada and 

 Yamamoto (1955, 1956) have shown that the virus may pass transovariaUy 

 to 23 generations of progeny under conditions which indicate multiplication 

 of the virus in the vector. The insects varied genetically in their ability to 

 transmit; the greatest, least, and intermediate proportion of infective progeny 

 occurred among descendants of the active (susceptible) insects, inactive 

 insects, and crosses between the two, respectively. Nymphs from females 

 which had just acquired virus before ovipositing required an incubation 

 period before transmitting; those from infective females did not. As in the 

 case of rice stunt virus and clover club leaf virus, the virus did not pass 

 with the sperm and almost all progeny of infective mothers received virus. 



Transovarial passage of certain other viruses has been measured as 42 % 

 for rugose leaf curl virus (Grylls, 1954), 2-11 % for wound tumor virus, and 



1 % for New Jersey potato yeUow dwarf virus (Black, 1953a). 



Although the Californian aster yellows and the typical aster yellows of 

 eastern United States had been distinguished by their host ranges since 1932, 

 it remained for Kunkel, in 1955, to distinguish the two diseases on the basis of 

 their symptoms. This enabled him to demonstrate for the first time that the 

 presence of one plant virus in a leaf hopper may interfere with the transmission 

 of another. Colonies of Macrosteles fascifrons, allowed to acquire one of these 

 viruses during a feeding period of two weeks, were unable to transmit the 

 other virus when given ample opportunity to do so. The same cross-protection 

 was later demonstrated with individual leaf hoppers (Kunkel, 1957). This result 

 is in marked contrast to the earher findings of Giddings (1950) using strains 



2 and 3 of curly top virus; Giddings found there was no cross-protection. 

 Incidentally, this reaction in the leaf hopper provides a means, not previously 



available, for testing relationships between viruses and, in some cases, it 

 might provide the only means available at present. For example, Colladonus 

 geminatus is a vector of California aster yellows (Severin, 1934) and several 

 strains of cherry buckskin virus (Jensen, 1956) and it may therefore provide 

 a means for testing for cross-protection between these viruses. 



Maramorosch (1956) allowed nymphs of Macrosteles fascifrons to acquire 

 virus from aster plants with yellows for 2 days. He then anesthetized them, 

 surface-sterilized them, cut each of them into 10 to 12 pieces and placed the 

 pieces in a culture medium containing antibiotics. Virus was recovered from 

 the tissue pieces after mamtaining them for 10 days in the solution, but not 

 from the culture fluids, or from similar tissue fragments tested at the beginning 

 of the 10-day period. The experiment provided further evidence for the 

 multiplication of the virus in the tissues of the vector. 



