162 



Common Scab of Potatoes 



according to Waksman (20) would suffice to kill all Actinomyces present 

 with the possible exception of A. invulnerabilis. It might not however 

 preclude some bacterial infection. 



The potato sets were sterilised by immersion in one-sixth per cent. 

 Formaldehyde for two hours, and at the time of planting three sterilised 

 glass tubes of ^ in. bore were inserted obliquely into each pot for the 

 purpose of inoculation. 



The varieties of potatoes planted and the strain of Actinomyces with 

 which each was inoculated is shown in the following table: 



The first inoculation was made on July 30th when the plants were 

 about 10 ins. high, and two further inoculations at intervals of one month. 



The greatest care was taken throughout the experiment to keep the 

 greenhouse as sterile as possible and floor and benches were washed with 

 disinfectant each time the house was entered. 



Results of Inoculation. 



Before tabulating these, a distinction must be made between obvious, 

 typical scabs and spots or points of infection which could only be 

 ascribed to the inoculation after microscopic and cultural examination. 

 These spots appeared to coincide in every case with a lenticel. They were 

 brown to black in colour, of a diameter from 1 to 3 mm. and were very 

 numerous on all the inoculated potatoes. Plate cultures made from the 

 more conspicuous consisted of either all Actinomyces colonies or of 

 Actinomyces and bacteria, and where these were found the spots were 

 considered as definite infections and described as scab spots. Some 

 of the more minute spots, however, which were such as are very 

 commonly found in the lenticels of potato tubers gave bacterial colonies 

 only. 



