50 



Common Scab of Potatoes. 



[Apr., 



On these grounds, the idea arose that Scabbing was due to 

 the wounds inflicted on the swelhng tubers by sharp soil particles 

 with which they came into contact during their growth, and the 

 disease was thus long known as " Mechanical Scab." 



In America, however, so long ago as 1890, the w^ork of Thaxter^ 

 had shown that Common Scab — or Deep Scab, as he then called 

 it — was due to the attack of a fungoid organism now known as 

 Actinomyces scabies (Thaxter) Giissow. Although during 

 recent j^ears it had been assumed that English and American 

 Scab w^ere identical, Thaxter' s work had never been confirmed 

 in this country, and certain text books, still in common use, 

 even averred that " American. Scab," i.e., that produced by 

 Thaxter 's organism, was almost unknown here. Pethybridge, 

 ho^vever, carried out experiments in Ireland in 1914 and came 

 to the conclusion that there could be practically no doubt that 

 Scab was due to an organism. 3 



It thus appeared very necessary to repeat the American work 

 for Common Scab as we know it in England, and this has formed 

 one section of the Leeds investigation. The details of the work 

 are not included in the Report above mentioned, but it is hoped 

 they will be published shortly. 



The results showed Actinomyces scabies to be the cause of 

 Common Scab, and thus proved the American and Enghsh 

 diseases to be one and the same. From other experiments, it 

 was also shown that Scab was not produced by mechanical injury 

 from sharp soil particles and, with the combination of these 

 results, the problem of controlling the disease reduced itself 

 to the simpler one of fighting a fungoid pest, whose charac- 

 teristics could be studied at will in the laboratory. 



Remedial Measures. — The discovery of a remedy for the 

 disease formed the second section of t*he investigation. Experi- 

 ments in America on the prevention of Scab appear to have 

 consisted mainly of attempts to sterihse the soil by means of 

 fungicides such as copper sulphate, corrosive sublimate, &c. 

 but, these have not given any results of practical value. Sulphur 

 has also been tried by various investigators but with indifferent 

 success. 



The only previous English work on the su!)ject is that of Seton 

 and Stewart ,4 who carried out a series of experiments at the 

 TJniversitv of Leeds in the years 1904-7 and came to the con- 

 clusion that " whatever the cause of Scab may he. it would seem 

 that there is some relationship between the virulence of the 

 disease and the moisture holding capacity of the soil." 



