1921.] Mosaic Disease of Potatoes. 3B5 



MOSAIC DISEASE OF POTATOES. 



A. U. Cotton. 



Mycologist to the Ministry of Agriculture and Fisheries. 



Although the disorder of potatoes termed Mosaic has only 

 recently been regarded as a specific disease, there is evidence 

 that it has been in existence for many years. At the present 

 time it is found in potato crops all over Em'ope and in North 

 America; and since the disease is distributed by means of the 

 seed tubers it probably occurs wherever the potato is grown. 

 Potato Mosaic has, up to the present, been studied more 

 thoroughly in the United States than elsewhere, and the 

 scientific information available is almost entirely derived from 

 investigations carried out in that country. 



Potato Mosaic in Britain. — Owing to the fact that Mosaic 

 symptoms have not hitherto been regarded as those of an 

 actual disease, it is not surprising to find that no general or 

 popular name for the disorder exists. It is clear, however, 

 that the terms " miffy " and " miffiness " in common use in 

 certain districts are usuall}', if not always, applied to potato 

 plants affected with Mosaic, and that IMosaic symptoms are 

 more or less known to observant growers. There are Scottish 

 growers who can recollect having observed the characteristic 

 mottling of Mosaic for forty years, and in all probability the 

 disease has been general in Britain for a much longer period. 

 It is also clear that what is spoken of in the >^outh of England 

 as " deterioration " in potatoes is at times only the result 

 of a general attack of Mosaic disease. 



The disease occurs in all parts of the British Isles, though 

 in varying intensity, and is frequently responsible for light 

 crops on farms, and, to an even greater extent, in gardens 

 and allotments. For this reason growers are advised to make 

 themselves acquainted with the appearance of Mosaic and 

 with the extremely important discoveries with regard to its 

 contagious nature (see p. 888^), since remedial measures can 

 only be adopted after the disease has become fully recognised 

 and the facts as to its method of spread appreciated. 



Mosaic Diseases in General. — Before describing the 

 symptoms of Potato Mosaic it may be advisable to record whafj 

 is known of Mosaic diseases in general. They constitute a. 

 group or class of diseases of an infectious nature, this being 

 proved by the fact that if the sap of a diseased plant is inocu- 



