1920.] 



Potato Leaf-curl. 



289 



which become unduly warm in summer ; or it might be due 

 to a sudden check owing to the occurrence of a dry spell 

 during the period prior to ripening. The fact that seed saved 

 from the southern, drier and warmer parts of England is more 

 subject to leaf- curl than Scotch or Irish seed, as well as the 

 circumstance that curl in the south may sometimes be largely 

 avoided by lifting the crop of tubers for seed before maturity, 

 appears to lend some support to this view. It is believed, 

 however, that these conditions are secondary, and that the 

 primary cause must be sought in an entirely diiierent direction. 



The Dutch plant pathologist Ouanjer has proved that leaf- 

 curl can be commimicated from one plant to another by 

 grafting ; and he is of opinion that the disease is due to the 

 presence of a virus which is capable of being inoculated into 

 the plant. Exactly how this infection comes about in nature 

 is not known. Insect bites may possibly carry the virus. In 

 any case it appears that not only does infection take place 

 more extensively in dry, warm climates, but also that infected 

 plants suffer much more severely in such cUmates than under 

 cooler and more congenial conditions. Further, it appears 

 from the researches carried out in Holland, and also in Canada, 

 that healthy plants growing in close proximity to diseased 

 ones are very Hable to become infected. 



Control Measures. — As the^ disease is perpetuated by means 

 of the " seed " it is of the utmost importance that no seed 

 should he saved from affected plants. Xot only should tubers 

 from dwarfed or distinctly curled plants be rejected for seed 

 purposes, but also those from all plants showing curling of the 

 lower leaves. 



In cases where potatoes are specially grown for seed purposes 

 a definite practice should be made of roguing the crop each 

 season for leaf-curl. This should be done early (June or 

 July). If this course were followed a higher level of seed- 

 quality would be obtained, and the infection of sound plants 

 from the diseased ones, which, according to recent investiga- 

 tions, is very hable to take place, would be prevented. 



In the case of gardens and small holdings it is not advisable 

 to save any seed from crops on the lighter soils in the drier 

 and warmer parts of^ the country. It is well known that 

 potatoes grown in such areas develop leaf-curl extensively, 

 and that even crops from once grown " Scotch seed [i.e., seed 

 saved the first season after being received from Scotland), 

 showed marked deterioration and reduced yield. In all such 

 localities fresh, northern seed should be obtained each year. 



