REPORT OF THE SOCIETY 



73 



available for tuber formation is reduced according to the severity of the disease 

 and the duration of it in the particular plants. The tubers are therefore )-educed 

 in size or in number, or both. In the case of late infection there is not likely 

 to be any noticeable difference in the tubers. Murphy has estimated that 

 severely diseased plants yield only 57.8 per cent of a crop as compared with 

 healthy plants of the same variet}' under the same cultural conditions. 



Infection 



Gussow has shown that by grafting the disease maybe readily transmitted, 

 acd Folsom et al. have obtained similar results in Maine. The writer in un- 

 published work has found aphids to be fertile sources of infection, and Folsom 

 et al. have carried on extensive experiments in Maine proving clearly that plant 

 lice, {Myzus persicce Sulz. and M acrosiphum solamfolii Ashmead) are carriers 

 of the disease from plant to plant. The writer has found that aphids are also 

 most important in the transmission of clover mosaic and it is probable that 

 they play a part in most mosaic diseases. 



Other potato pests, such as the Colorado potato beetle and the flea beetle, 

 are not found to be carriers of the disease. 



Experiments also indicate that the seed cutting knife is not important in 

 transmitting the disease. The same applies to contact in the field between 

 healthy and diseased plants. Soil has not been found to transmit the disease 



from one season to the next, but it 

 may be possible that volunteer plants 

 arising from diseased tubers left in the 

 soil will give rise to centres of infec- 

 tion the following season. 



Varietal Susceptibility 



Little is yet known of this phase 

 of the question and work on it is 

 difficult because of the carrying over 

 of diseased tubers which came from 

 apparently healthy plants. It is 

 known, however, that on the whole 

 the Irish Cobbler variety is less 

 susceptible to the disease while 

 Green Mountain is particularly sus- 

 ceptible. 



Control 



Three points in control are empha- 



T\g. 3 — A leaf of potato showinis- mosaic 

 mottling. Note at 1 a flea-bee' le hole 

 and at 2 a typical light green area. 



