1921.] 



Common Scab of Potatoes. 



49- 



COMMON SCAB OF POTATOES. 



W. A. Millard, B.Sc, 

 Adviser in Mycology, University oj Leeds. 



Every farmer and gardener knows the uncertainty with which, 

 despite every precaution, he looks forward to the harvesting of 

 a sound potato crop. 80 many are the fungoid diseases which 

 attack the potato that it often seems barely possible for the 

 plants to escape infection from one or more of them. It is true, 

 that, in the case of some of the worst di-seases, certain protective 

 measures may be taken which give a fair guarantee of immunity, 

 but in the case of many others, which, though perhaps less 

 generally destructive, are often very serious, escape from attack 

 is very much a matter of luck with the majority of growers. 



Two of the most harmful diseases which come into this cate- 

 gory are Corky and Common Scab. Both are widely distributed, 

 to a large degree seasonal in their outbreaks, and each is 

 frequently responsible for great loss in many potato crops, 

 which, before lifting, appeared free from disease. 



A certain amount of experimental work has already been 

 carried out on Corky Scab, and the results of this together with 

 the treatment recommended are embodied in Leaflet No. 2B2. 

 A detailed description of the two diseases, which bear a certain 

 superficial resemblance to each other is given in the same leaflet. 



In the case of Common Scab, little information has been avail- 

 able, and no treatment could be recommended with any certainty 

 of success. An investigation of the disease, extending over a 

 number of years has, therefore, been carried out at the University 

 of Leeds, and it is hoped that the results obtained may be of 

 service to those growers, w^ho up to the present have been prac- 

 tically at its mercy. 



A reportl of the experiments has been recently published by 

 the University of Leeds and the Yorkshire Council for Agri- 

 cultural Education, and the present article is therefore intended 

 to give a short account only of the disease with especial reference 

 to the remedial measures found effective for it. 



Common Scab is found in its most virulent foi-ui on liglit sandy 

 or sharp gravelly soils, and a photograph of the disease as it 

 occurs on such soils is given herewith (Fig. 1) ; it appears to 

 a lesser extent on heavier soils and is practically unknown on 

 peat soil. It is frequently associated with the presence of ashes 

 in the soil. 



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