266 



THE ACTINOMYCETES, Vol. I 



Wollenweber made a comprehensive study 

 of the organisms concerned in the causation 

 of potato scalx He was the first to suggest 

 that more than one actinomycete has the 

 capacity to cause scab. He also emphasized 

 that different types of scab are caused by 

 different organisms. Xo infectiv^ty tests on 

 potatoes were made, however, with the iso- 

 lated cultures. Further, Wollenweber's de- 

 scriptions were incomplete and the exper- 

 imental data presented in support of his 

 assumptions were limited. Since most of the 

 cultures isolated from scabby potatoes thus 

 described are similar to the large numbers of 

 saprophytic organisms commonly found in 

 the soil and on surfaces of potatoes, the 

 pathogenicity of the species belie\'ed to cause 

 scab is open to (juestion. 



Millard and Burr isolated from potato 

 scab a large number of cultures of actinomy- 

 cetes believed to be responsible for the causa- 

 tion of the disease. Only three of these cul- 

 tures were obtained in duplicate, and only 

 one was found to be identical with Thaxter's 

 original isolate. The latter produced the 

 deep form of scab and was capable of attack- 

 ing the roots and stolons of the potato plant. 

 Millard and Burr believed that different 

 types of scab were caused by different or- 

 ganisms. They used glycerol synthetic solu- 

 tion as the basis for growth of their cultures. 



The mere isolation of an organism from a 

 scabby potato is no proof, however, that the 

 organism is capable of causing the disease. 

 This has been emphasized by Decker and 

 numerous others. 



DeBruyn studied in detail the specific na- 

 ture of scab-producing actinomycetes. He 

 employed the method described by Kiess- 

 ling. Sap of different varieties of potatoes is 

 added to synthetic media and inoculated 

 with actinomycetes. Since the pH of the sap 

 changes with the ripeness of the tuber, the 

 growth of the organisms in such media is 

 parallel to the pH change. The cultures made 

 their best growth in the presence of sap of a 



susceptible variety; no growth occurred in a 

 medium to which the sap of 3^oung tubers of a 

 resistant variety had been added. DeBruyn 

 was able to confirm the ideas of Wingerberg 

 and Kieszling concerning the existence of 

 physiological scab resistance. Four types of 

 scab were recognized: (a) deep, (b) tumu- 

 lus, (c) common, and (d) superficial or russet 

 scab ; each type was found to be caused by a 

 different species. All of them are now recog- 

 nized as belonging to the genus Streptomyces. 



Leach et al., however, could not confirm 

 the reports of the multiplicity of scab-pro- 

 ducing actinomycetes. On the basis of their 

 studies, they emphasized that scab lesions 

 are a result of a reaction between the patho- 

 gen and the host tissue; they may be influ- 

 enced as much by the host as by the patho- 

 gen. They recognized, however, the existence 

 of pathogenic races of the organism. Cocchi's 

 results, on the other hand, tended to sup- 

 port, at least partly, Millard and Burr's con- 

 clusions; two of the three strains he isolated 

 were identified as S. flavus and S. clavifer. 



Taylor and Decker isolated 143 cultures 

 of actinomycetes of which 128 were nonacid- 

 fast and 15 partly acid-fast. Starch was hy- 

 drolyzed by all of the nonacid-fast isolates 

 and by none of the partly acid-fast isolates. 

 The ability of the cultures to produce a dark 

 brown surface ring of growth on milk was 

 characteristic of 66 isolates, all nonacid-fast. 

 Of the remaining 77 cultures, 71 produced 

 on alkaline reaction; four formed a hard, 

 acid curd; one showed no visible change in 

 reaction in the presence of abundant growth ; 

 one did not grow at all on milk. 



Thirteen of the acid-fast isolates ga\e mi- 

 croaerophilic growth on agar-shake tubes. A 

 dense ring of colonies developed beneath the 

 surface of the agar, at depths of 5 to 10 mm. 

 As the cultures grew, additional rings de- 

 veloped beneath the original one. With the 

 nonmicroaerophilic isolates, no growth was 

 produced below a depth of o mm; any sub- 

 surface developm(Mit was continuous with 



