CONTRTBTTTIONS FEOM THE WTSLEY LABORATORY. 



875 



disease makes no further progress, sliallow scabs or scars are left when 

 the spore-ball powder has fallen away. This sometimes gives rise 

 to curious appearances (fig. 101 b). A series of stages may be found 

 from such superficial scabs or scars to cankerous scabs and the more 

 deeply-seated injuries (figs. 100 d and 101 f). 



The various scabby appearances are extremely liable to be mis- 

 taken for a superficial scab of obscure origin, sometimes called " brown 

 scab," which is very generally distributed in Britain. The tuber in 

 fig. 101 E, photographed immediately upon removal from the soil, repre- 

 sents this scab as it is typically developed in the North of England. 

 The stages of development of brown scab, indicated by the letters 

 a to d, taken in order, differ entirely from those brought about by 

 S'pongospora, but, although the mode of development of the scab 

 differs in the two cases, it is by no means an easy matter, under certain 

 circumstances, especially when the brown scab is of a more or less 

 " corrosive " character, to distinguish between them. The Spovgo^pora 

 scab very frequently possesses a ragged margin formed by the ruptured 

 skin of the tuber (fig. 101a (a)), and it does not possess the rugose 

 appearance characteristic of brown scab. When the tubers have been 

 rubbed or rolled in the pit or store the distinguishing marks frequently 

 become obliterated. In this condition even a microscopic examination 

 may fail to distinguish the cause. 



In 1910 the Transvaal Department of Agriculture issued a Bulletin'" 

 dealing with " the recent discovery of the disease known as ' Corky 

 Scab ' in English potatos shipped to South Africa for planting pur- 

 poses." It is stated that " up to the present * corky scab ' has not 

 been observed in South African potatos. It is a pest against which 

 growers in this country will have to contend in the near future 

 unless a more uniform and rigid system of inspection of all oversea 

 potatos is undertaken by the several South African Colonies than is at 

 present in vogue." Since it is a very difficult matter to distinguish 

 between the scabby forms of potato canker which generally result 

 from a slight attack of Spongospora and the usually superficial brown 

 scab, even when only a short time has elapsed since the removal of the 

 potatos from the soil, it would be still more difficult to determine 

 ; whether the disease were present or absent after a consignment of 

 potatos had arrived at a Colonial port. It is pointed out below 

 (page 384) that potatos may or may not be affected with Spongospora, 

 OT at all events but slightly, even though the soil be thoroughly infected 

 by this parasite. Infection depends upon inter-associated conditions of 

 soil and climate, and upon the particular variety of potato grown. 

 Spongospora therefore may be carried upon the surface of tubers, 

 which are not themselves attacked. There can only be a certainty that 

 botatos do not carry infection when they are selected from districts in 

 vhich the disease does not exist. 



The powdery scab stage is followed by canker. When the spore- 



* J. B. Pole Evans, Trans. Dept. Agr., Farm. Bull 110 (1910). 



