256 Journal of Agricultural Research voi. xvii, no. e 



TRANSMISSION BY APHIDS 



The fact that plant diseases are frequently carried by insects is well 

 recognized. In this connection the work of Allard (z, p. 626) on the 

 mosaic of tobacco is of special interest. This writer showed that the 

 virus of tobacco mosaic is readily carried by the common green peach 

 aphis, or spinach aphis {Myzus persicae Sulz.). More recently McClintock 

 and Smith (5) have demonstrated that the spinach-blight, which ap- 

 parently is a virus disease, also is transmitted by plant lice, the pink 

 and green potato aphis (Macrosiphum solanijolii Ashmead) and the 

 spinach aphis both acting as carriers. Doolittle's work (2) with cucum- 

 ber mosaic is also worthy of mention in this connection. 



FIELD EXPERIMENTS WITH INSECT CAGES 



From findings of these writers it has seemed possible that the mosaic 

 of potato might be spread by some insect. To study this question an 

 attempt was made to grow plants in the field under cages that were 

 supposedly insect-proof. These were 22 by 30 by 36 inches, covered 

 with cheesecloth, one side being arranged so that it could be opened — a. 

 type that was used also by McClintock and Smith (5, PL 5 and 6). 

 Potatoes were planted about 14 inches apart, so arranged that one 

 cage covered two hills. During the season of 1917 at Presque Isle, Me., 

 potato plants were grown throughout the season under these cages and 

 observations made from time to time on their condition as regards 

 mosaic. It was found that the percentage of mosaic in the cages was 

 practically the same as that to be found in the same stock planted in the 

 adjoining plots. However, since the disease may be acquired in one 

 season without showing the symptoms until the tubers develop their 

 shoots the following season, it was necessary to continue the comparison 

 through 1 91 8. It was then found that not more than 5 per cent of the 

 tubers from healthy plants caged in 191 7 were mosaic, the lowest season- 

 to-season percentage of increase on record for lots grown on the experi- 

 mental plots. 



Tubers from plants grown under the cages in 191 7 and not showing 

 the characteristic mottling of the disease during the season were selected 

 for planting in the cages in 191 8. On account of the poor quality of the 

 cheesecloth obtainable in the second season the cages were not insect- 

 proof; and within them there were found, at the end of the season, 

 considerable numbers of aphids as well as some insects of other kinds. 

 However, since the dispersal of aphids probably was checked more or 

 less by the cages, tubers were reserved for the 191 9 season for comparison 

 with uncaged lots. 



