68 THE DEPARTMENT OF AGRICULTURE. 



although many theories have been advanced regarding the cause, none of these 

 theories have been accepted as yet. 



Economic Importance. Leaf -roll (Fig. 72) is a serious disease because it reduces 

 the yield to about one-third of the normal, and also greatly reduces the percentage 

 of marketable tubers (Fig. 73). 



Fig. 73. Potatoes on the left the product of the leaf-roll plant in Fig. 72 ; those 

 on the right the product of the healthy plant (photo by P. Murphy). 



Control When a crop is bady infected with leaf-roll it is better to secure seed 

 from districts free from the disease. Spraying and treatment of the potatoes is of 

 no use. 



MOSAIC (Fig. 74). 



This disease has for some years now been attracting the attention of plant 

 pathologists in Europe and America. This disease is also very prevalent in some 

 parts of Canada, but little in evidence in British Columbia. 



Symptoms. Plants affected with mosaic bear an abnormal condition of the 

 foliage, as characterized by the spotted or mottled appearance of the leaves, portions 

 of which are lighter green in colour ; the discoloration is not very clear in sunlight, 

 but in shade it is very marked. The leaves may be puckered, particularly when the 

 disease is in the advanced stage, and the lower leaves sometimes fall off, leaving the 

 stem bare. No other marks will be found on any parts of the affected plants or on 

 the tubers. The tubers are normal looking and sound and their keeping or eating 

 qualities are not impaired. This disease can be recognized most easily in July, 

 although it may be observed by careful examination at any time during the growth 

 of the plant. Like leaf-roll, it is inadvisable to leave the examination for mosaic 

 until late in the season, as other diseases may be attacking the plant or the plant 

 dying off. . 



The yield of potatoes is not reduced to the same extent as in leaf-roll, seldom 

 exceeding 50 per cent, of the crop. 



