Waksman — 160 — Actinomycetes 



3. Seldom produces pigment and then poor; forms a decided clot in brom- 

 cresol milk S. wedmorensis 



IV. Scant or no surface growth, but some bottom growth: 



1. Produces color changes in brom-cresol milk: 



a. Tyrosinase reaction positive; no aerial mycelium on nutrient potato 

 agar S. scabies 



b. Tyrosinase reaction negative; produces aerial mycelium on most solid 

 media : 



a. Abundant aerial mycelium on saccharose synthetic agar 



a. Good growth with abundant aerial mycelium on egg albumen 



agar S. temiis 



h. Poor growth with scant aerial mycelium on egg albumen agar 



S. -iiiarginatus 



b. Scant aerial mycelium on saccharose synthetic agar; no true aerial 

 mycelium on any media, colonies often show dark centers 



N. sahnonicolor 



2. Produces no color changes in brom-cresol milk: 



a. Facultative anaerobe; no true aerial mycelium or only traces on any 

 media; colonies frequently show dark centers S. maculatus 



b. Obligate aerobe; aerial mycelium arises centripetally on the colonies 



S. coroniformis 

 V. No growth in glycerine solution; starch not hydrolyzed: 



1. Good growth in brom-cresol milk with characteristic color changes 



S. spiralis 



2. Poor growth in brom-cresol milk with coagulation, no hydrolysis and 

 no color change S. saiwpsonii 



The reports of Wollenweber and Millard that different species 

 of actinomycetes are responsible for the various forms of potato scab 

 were investigated in considerable detail by deBrltyn (50), who em- 

 ployed for this purpose the method of Kieszling (218). The sap of 

 different varieties of potato is added to a synthetic medium and inocu- 

 lated with the various cultures of actinomycetes. Since the pH of the 

 sap changes with the ripeness of the tuber, the growth of the organisms 

 in the above medium was parallel to the fH change. Best growth 

 was obtained in a sap-medium of a susceptible variety; no growth oc- 

 curred in a medium to which the sap of young tubers of a resistant 

 variety had been added. The ideas of Wingerberg (511) and Kie- 

 szling concerning the existence of physiological scab resistance were 

 thus confirmed. DeBruyn recognized four types of scab, namely, 

 deep, tumulus, common, and superficial or russet scab. Each type of 

 scab was found to be caused by a different actinomyces species, all be- 

 longing to the genus Stre-ptomyces. 



Other investigators, however, notably Schlumberger, Berkner, 

 Goss and Afanasiev, considered the differences in the type of scab 

 to be due to the severity of attack, rather than to differences in the 

 species of the organism concerned. The severity of attack is controlled 

 by external conditions, varietal reactions and virulence of the organism. 

 Attention is to be directed, in this connection, to the fact that even 

 deBruyn recognized that different varieties of potato react differendy 



