Sept. IS, 1919 hwestigations on Mosaic Disease of the Irish Potato 249 



but 58 per cent of the latter and concluded that in New Brunswick, 

 Canada, the yield is reduced about i| bushels for every i per cent of 

 mosaic present. Reduction in yield reported by collaborators of the 

 Plant Disease Survey {11) ranges from 5 to 30 per cent. Comparative 

 results secured by the writers in northern Maine will be discussed later 

 in connection with the questions of hill selection and roguing. 



The preceding statements refer to the comparative yields of healthy 

 and entirely diseased lots and so may seem to be somewhat inapplicable 

 to conditions where a large number of the plants are not diseased and 

 where these may possibly be able to make up for the deficiency of 

 affected plants by making better growth at their expense. However, the 

 writers have found that often, in the absence of any control measures, a 

 healthy lot of a susceptible variety will show symptoms of the disease 

 in some hills the next year after being grown near to diseased stock 

 and will thereafter from year to year have a larger percentage of hills 

 affected. 



APPKARANCE OF THE DISEASED PLANTS 



Some of the symptoms will be described here, although the subject 

 has already received considerable attention (<5, 7, 8, 10). On Green 

 Mountain or Bliss Triumph potatoes, the leaves of affected plants are 

 characterized by mottling (PI. A; B; 25), which is produced by the 

 presence of light green areas on the foliage. These areas may occur on 

 any part of the leaf; they may include or adjoin sections of the larger 

 veins or not come in contact with them. The light green patches vary 

 greatly in shape, being punctate, elongate, circular, angular, and irregular. 

 Considerable variation in the degree of paleness may be seen even in the 

 same small discolored patch, from a barely discernible fading of the green 

 to an almost pure yellow. The abnormal spots differ in distinctness of 

 outhne, usually in proportion to the degree of discoloration. Their 

 dimensions seldom exceed a few millimeters. Their frequency varies, 

 usually becoming greater as the disease progresses and thus giving to the 

 general appearance of the leaves a much lighter color than that of healthy 

 foliage. In the more severely affected plants the foliage may become 

 spotted with brown flecks of dead tissue. Furthermore, in the more 

 advanced stages the foliage presents a characteristic crinkled or corru- 

 gated appearance. In these stages the diseased plants are frequently 

 dwarfed because the stems, the leaf petioles, and leaf blades are con- 

 siderably shortened or reduced in size. 



The symptoms as described above are not so marked in certain other 

 varieties — for example, in Blue Victor, Early Rose, Irish Cobbler, Pearl, 

 White Bliss, Carmen, Early Dix, Netted Gem, Peach Blow, Portuguese 

 Purple, and Spaulding Rose. In the first five named, decided rugosity 

 is a characteristic of the disease. 



