334 AI^NUAL REPORT SMITHSONIAN INSTITUTION, 1940 



yellows virus is known to occur in more than 180 separate plant species. 

 Some viruses are known to affect only 1 species of plant, but by far 

 the greater number are known from more than 1 host. 



Needless to say, the symptoms of the diseases caused by these viruses 

 vary greatly, from the alternate light and dark green patches of the 

 well-named "mosaic" diseases, to the bizarre patterns of a group of 

 viruses generally known as the ringspot viruses because of the ring- 

 shaped spots, often showing a pattern of concentric rings, which ap- 

 pear on the foliage of affected plants. 



The insects concerned in the spread of viruses are almost all sucking 

 insects, and of these, the aphis or plant lice are the worst offenders, 

 with the leaflioppers another very important group. The discovery 

 that the thrips are also carriers of virus disease is of much interest 

 because thrips feed by first tearing the leaf tisue and then sucking 

 the exuding juices; the aphis and leaf hoppers penetrate deeply into 

 leaf tissue, but the thrips are shallow feeders. 



WORLD-WIDE EXTENT OF THE PROBLEM 



The occurrence of insect-transmitted diseases of plants is world- 

 wide, and wherever agriculture is practiced, from the wheatlands of 

 the noith to the equatorial Tropics, the problem presents itself in one 

 way or another. Even greenhouses are not exempt; in fact,. because 

 of their usually protected conditions they sometimes give opportunity 

 for a disease-bearing insect species to survive far out of its normal 

 geographic range. Specific problems are often regional because the 

 insects concerned have a restricted geographic range, whereas others, 

 because the insects are cosmopolitan, are world-wide. An example 

 of the first type is the transmission of curly top of sugar beets and 

 other plants by the sugar beet leafhopper. Koughly speaking, this 

 insect is confined to the great semiarid regions of the intermountain 

 region and California. Years before it was known that a serious 

 disease of tomatoes was caused by the same virus which causes curly 

 top of sugar beets and was transmitted by the same insect, it was 

 realized that the geographic range of the two diseases was the same. 

 Isolated from the curly top of the western United States by a whole 

 continent, and similarly restricted to a specific insect's range, is a 

 disease of sugar beets in Argentina which is so similar to curly top as 

 to be practically indistinguishable, yet it is transmitted by another 

 species of leafhopper and appears to be a distinct disease. 



An example of the second type is that of spotted wilt of tomato. 

 This virus disease, which is transmitted by certain species of thrips, is 

 now known to be world-wide although the disease has been described 

 under several names in various places. It was first recognized in 

 Australia; later in English greenhouses. Independently conducted 



