W. A. Millard 163 



No. of pot Infection 



1, 2, 6, 10 In each of these pots tlie tubers showed typical scabs from 3 to G mm. in 

 diameter. In Pot 2 many of these were covered with the deHcate greyish 

 white myceHnm so characteristic of the disease. In addition, large numbers 

 of scab spots were present. 



3, 5 No mature scabs present, but the tubers were covered with scab spots. 



8, 9 These two crops were lifted 5 weeks before the others. No .scabs were present. 

 A few spots were shown by the May Queen tubers but these were not examined. 



4, 7 Controls. Potatoes clean and glossy. In Pot 7 a few minute spots were present 



on the tubers but were found to be of bacterial origin only. 

 11 One or two small but typical scabs of diameter from 4 to 7 mm. were present 



on most of the tubers. This control pot was included in the series in order to 

 ascertain the extent to which scabbing would occur normally in pots. The soil 

 was known to produce scab badly in the open. It would appear, therefore, 

 from the results that the conditions of pot culture are unfavourable to scab 

 production and this is probably due to lack of aeration. The degree of scabbing 

 produced in the inoculated pots cannot therefore be taken as any indication 

 of the virulence of the Actinomyces strains under normal conditions. 



Photographs of the scabbed potatoes from Pots 2 and 6 and of the 

 clean potatoes from the control Pot 4 are given in Figs. 2, 3 and 4 

 respectively. 



From these experiments, therefore, it will be seen that out of the 

 seven strains of Actinomyces tested, five proved to be pathogenic and 

 the negative results given by the remaining two may have been due to 

 the early ripening of the potato varieties (May Queen and Witch Hill) 

 concerned. The positive result in Pot 10, where an Actinomyces strain 

 isolated from a Dalhousie potato was used as inoculum for British Queen 

 potatoes shows that this strain is not specific to one kind of potato and 

 this is probably true of the other strains also. 



The experiments thus fully confirm Thaxter's original work and 

 show its applicability to types of scab in this country. They show, in 

 addition, that Common Scab may be produced by various members of 

 the Actinomyces group exhibiting considerable differences in culture. To 

 what extent these different members may be considered as belonging 

 to a single species or species group is however a matter needing further 

 investigation. Some work on the subject is now being carried out and it 

 is hoped to publish the results of this in due course. 



The writer wishes to express his thanks and indebtedness to Professor 

 R. S. Seton for the kind interest he has taken in the work, to Mr S. Burr, 

 Demonstrator in Agricultural Botany at this University, for his valuable 

 assistance, and to Mr J. Manby, University Photographer, for the photo- 

 graphs. 



