1921.] 



Mosaic Disease of Potatoes. 



339 



only showed itself in the progeny the following year. On the 

 negative side it may be noted that no infection followed when 

 biting insects, such as Flea-beetles or Colorado Beetles, were 

 used; neither was there any evidence that the disease was con- 

 tracted from the soil. 



Susceptibility of Varieties. — During 1920 the Ministry 

 undertook a preliminary survey of the distribution and 

 intensity of Potato ]\[osaic in England, and observations were 

 also made on the relative susceptibility of the different 

 varieties. It w^ould be premature as yet to generalize from 

 the information obtained, but the following may be taken as 

 holding good. It was found that, though the disease occurred to 

 a limited extent in almost all varieties, both early and main- 

 crop, there was a very marked variation in the percentage of 

 infection commonly present as well as in the intensity of the 

 attack. 



Under field conditions four varieties stood out above 

 others as being specially susceptible. These were Golden 

 Wonder, Langworthy, Burnhouse Beauty and Tinw^ald Per- 

 fection. In these varieties, affected plants to the extent of 

 30 per cent, and upw^ards of the crop were commonly found 

 in all parts of the country, whilst in some fields 75 per cent, 

 or even 100 per cent, show^ed the disease. It should be noted 

 that the first two varieties mentioned, although of good 

 quality, are w^ell known to be particularly light croppers; and 

 there can be little doubt that this peculiarity is due to the 

 very general .presence of Mosaic disease. Other varieties which 

 showed the disease in some quantity were Arran Chief. The Ally, 

 Dargill Early, and King Edward. It is obvious that these 

 badly-infected stocks should, if possible, be eliminated and 

 cleaner stocks worked up. 



Measures of Control. — The two points to be clearly grasped 

 are : — 



(a) Infected plants do not recover, but carry the disease from 

 season to season by means of the tubers, (b) By reason of 

 aphis attacks the disease is transferred from infected plants 

 to healthy ones and asserts itself more promimnitlv the fol- 

 lowing season. 



The following preventive methods should therefore be 

 observed : — 



(1) Seed tubers should not bo saved from diseased plants, nor 

 from plots or fields where the disease is present in any 

 qimntity ; 



D 2 



