158 Common Scab of Potatoes 



Later, however, the work of Krainsky(6), ConD(7) and particularly 

 of WaksmaD and Curtis (8) showed that the characters of A. chromogenus 

 were those of a group rather than of a species. Finally, in 1919, Drechsler (9) 

 made an exhaustive study of the morphological character of the genus 

 Actinomyces and concluded that it should be j^laced in the Hyphomycetes 

 as a Mucedinous group. He therefore re-named Thaxter's organism 

 Actinomyces scabies (Thaxter — Giissow). 



Thaxter's original work was only slowly recognised in this country 

 and this was possibly due in part to our ingrained belief in the theory 

 of Mechanical Scab. We may attribute it, however, with more credit to 

 ourselves, to the fact that from Thaxter's own account and the photo- 

 graphs which accompanied it, it was by no means certain that the disease, 

 which he called "Deep Scab" was the same as the "Common Scab" 

 familiar in this country. So much indeed was this the case that Massee (lo) 

 in 1910, describes the American Scab and says : " I have but rarely observed 

 this disease in this country." 



The writer now realises that Thaxter's "Deep Scab" was the form 

 of Common Scab which he has here named "pitted" and as already has 

 been pointed out, this type of Scab is not the most common in this 

 country. Later American writers have published confirmatory accounts 

 of Thaxter's work in which photographs of scabbed potatoes appear that 

 might well replace our own photograph of typical Scab in Fig. 1. Un- 

 fortunately, however, no details of the inoculation experiments appear 

 proving the pathogenicity of the organisms isolated and no photographs 

 of the results of any such experiments. Lutman and Cunningham (ii), 

 for example, having isolated species of Actinomyces from scabbed 

 potatoes from various sources, state that these agreed, with a few minor 

 differences, with Thaxter's organism and add "all were found capable 

 of producing scab on inoculation." It seems very surprising that in 

 such an early stage of our knowledge of Scab no details or photographs 

 of these successful inoculations should have been given. 



Again, McKinney(i2) says: "The writer has studied three strains of 

 the scab organism isolated from scabby potatoes grown in three localities 

 in this state, all of which are pathogenic upon growing tubers." No data 

 of the experimental proof of this statement are given, however, although, 

 in this case, the omission is excusable on the grounds that the paper deals 

 only with the nomenclature of the Potato Scab organism. 



There appears indeed to be no complete confirmation of Thaxter's 

 work, which he himself called "preliminary" in either American or 

 English literature. 



