328 Journal of Agricultural Research voi.xix.No.7 



results might be interpreted as suggesting that some insect besides 

 aphids was a deciding factor, it is possible for the aphids observed in the 

 cages late in the 191 8 season to have come from a very few which did 

 not carry mosaic, and as yet no other insect is known to transmit mosaic 

 of potato. 



EXPERIMENTS WITH APHIDS 



Small colonies of the peach aphis were brought from Orono May i on 

 radish and mosaic potato plants. Both increased while feeding on these 

 plants. On June 7, when the vines were from i to 4 inches high, 9 caged 

 plants of 3 tuber units were treated with aphids from radish plants, 

 about 150 to each hill. These 3 units were regarded as controls, since 

 the aphids had lived for a number of generations on radish plants and 

 were supposed to be free from a mosaic virus. Three caged plants of a 

 fourth tuber unit were treated with aphids from a mosaic potato plant; 

 2 plants were left in each hill and the aphids, about 100 to a hill, were 

 introduced on leaves on a stick thrust into the soil near each hill. On 

 June 30 and July 5, when aphids were very numerous, these 12 plants 

 were sprayed with a solution of soap and nicotine sulphate. The plants 

 used came from the Gilbert stock, already described as exceptionally 

 healthy. On July 28 the fourth tuber unit was slightly mosaic in some 

 branches of i hill, and by August 9 it was dead, as the result of excessive 

 aphid infestation. The 3 controls remained healthy until dug on 

 August 26. 



On June 17, nine half-tuber sets from stock caged in 191 8 were planted 

 under three cages. On June 28, when the vines were from i to 3 inches 

 high, the plants were treated with aphids from mosaic plants; several 

 hundred aphids were introduced by each hill with the stick method de- 

 scribed above. They were sprayed on July 5 and 8. On August 9 one 

 hill showed some mosaic. When dug on August 26, this hill was all 

 mosaic, while two other hills — one in the same cage — were each mosaic 

 in the upper leaves of one stalk. The untreated plants from the other 

 nine half tubers were grown in the field and remained healthy through- 

 out the season. 



Four tuber units of the Gilbert stock, comprising 12 hills, were treated 

 on July 12, when the plants were large enough to press against the tops 

 of the cages. The first unit was treated with hundreds of aphids from 

 radish plants and the others with aphids from mosaic potato plants. 

 In the latter case several thousand aphids were left on the diseased 

 leaves and stems in a flower-pot saucer set at the base of each of the 

 first and third hills, whence they dispersed within a few days. The 

 first tuber unit remained healthy throughout the season. The other 

 three were still healthy on August 9, but when dug on August 26 two 

 hills were mosaic, each in the upper leaves of one branch of a stalk. 



