-338 



Mosaic Disease of Potatoes. 



[July, 



there is evidence that the reduction is more marked. Under 

 average field conditions, affected plants appear to show a 

 decrease of 15 to 35 per cent, in yield as compared with healthy 

 plants. It is true that only a certain number of plants in 

 the crop are attacked, but even at a moderate estimate the 

 aggregate loss due to ^Mosaic in the midland, southern and 

 eastern counties of England must be very conside fable. In 

 gardens and allotments v^here local or home-saved seed is 

 used, a very dwarf form of the disease frequently occurs, and the 

 losses are much more serious. 



Mosaic is particularly troublesome to the potato breeder. 

 In certain districts of England it persistently attacks seedlings 

 in its most intense form, and may at times practically kill 

 out first year plants. 



Transmission of the Disease.— It has been clearly established 

 that Potato Mosaic is carried from season to season in the 

 seed tuber, and that diseased plants do not recover, their 

 progeny reproducing the disease each successive year.* It is 

 also known that Potato Mosaic is infectious, inasmuch as 

 healthy plants, if surrounded by, or grown in proximity to, 

 diseased ones, are liable to contract the disease and shov^ 

 it the following season in their progeny. 



The method by w^hich infection of healthy plants takes 

 place in nature is recorded in two papers recently published 

 in America.! It w^as discovered that, as in the case of Tobacco 

 Mosaic, the disease virus was conveyed by Aphides (" green- 

 fly ") w^hich fed on affected plants, the particular species 

 responsible in the State of Maine being chiefly the Spinach 

 Aphis {Myzus j^ersicce). Experiments proving this were 

 carried out both in the greenhouse and in insect-proof cages 

 in the open. If Aphides which had been sucking the juice 

 of diseased plants were introduced into the cages, infection 

 followed; if Aphides from healthy plants were introduced, no 

 infection followed. Where infection took place early in the 

 season, the mottling of the foliage developed during the same 

 season, but when the plants were inoculated later, the disease 



^' As a rule the whole progeny of a newly-infected plant shows the disease 

 the following season, but occasionally, perhaps in cases of late infection, a 

 few tubers escape and give rise to healthy plants. 



f Investigations on the Mosaic Disease of the Irish Potato, by E. S. Schultz, 

 D. Folsom, F. M. Hildebrandt, and L. A. Hawkins. Journ. Agr. Research 

 XVII, pp. 247-273, 1919. 



Transmission of the Mosaic Disease of Irish Potatoes, by E. S. Schultz, 

 and D. Folsom. Loc. cit., XIX, pp. 315-337, 1920. 



