W. A. Millard 157 



The scabs increase to au indefinite size, two or more often coalescing, 

 but, generally, before a diameter of about 4 mm. has been reached in 

 any individual scab, its covering skin ruptures and a shallow depression is 

 exposed. Very quickly, however, the base and edges of the scab become 

 thickened with layers of cork laid down by the potato in its attempt to 

 cut off the disease from the underlying tissue. 



The mature scabs vary considerably in general appearance. In some, 

 the shallow depression formed in the early stages of the disease is never 

 afterwards raised to the surface by the subsequent formation of cork 

 and the affected potato presents a pitted appearance. In others, the 

 scabs are raised by the abundant cork formation and stand out above 

 the surface of the tuber in knob-like projections. These two forms of 

 Scab, which we may call "pitted" and "raised" respectively, appear 

 to be the outstanding types of the disease when it occurs in its most 

 virulent form. 



The commonest form of Scab in this country, however, is inter- 

 mediate between these two extremes (Fig. 1). It is slightly raised and 

 is also characterised by an irregularly concentric series of wrinkled layers 

 of cork arranged around a central core or depression. 



These different types, together with others showing greater or lesser 

 variations, will be considered more fully in a later paper. 



Cause of Scab. The earliest reference to Common Scab is found in 

 Loudon's Encyclopaedia of Agriculture (1825) and reads as follows : " Scab, 

 that is to say, the ulceration of the surface of the tubers, has never been 

 explained in a satisfactory manner. Some attribute it to the Ammonia 

 from the dung of the horse, others to alkali, and certain others to the 

 use of wood ashes in the soil." 



In 1884, the idea was introduced by W. G. Smith (i) that Scab was 

 produced by the irritating action of sharp or gritty particles in the soil 

 on the swelling tubers. Much support was lent to this theory by the 

 fact that Scab was certainly most prevalent on soils of a light gritty 

 nature and by the generally accepted belief that ashes produced Scab. 



In 1890, however, an organism was isolated in America from a form 

 of Scab known as "Deep Scab" by Thaxter(2) who proved it to be the 

 causative organism of the disease, and gave it the name "Oospora 

 scabies." The nomenclature of the fungus and its place in the systematic 

 scale have since passed through many changes. In 1912, Cunningham (3) 

 placed it in the genus Streptothrix. In 1914, Gussow(4) transferred it 

 to the genus Actinomyces, and Lutman and Cunningham (5) believed it 

 to be identical with Actinomyces chromogemis (Gasperini). 



Ann. Biol, ix 11 



