Waksman — 164 — Actinomycetes 



the studies on temperature were largely concerned with the effect on 

 growth of the potato prior to infection and were carried out in steril- 

 ized soils. In non-sterilized soils and with inoculation at time of tuber 

 formation, there was only little effect of temperature (147). 



The actinomycetes that cause scab of potatoes are distinct aerobes. 

 Although the spores are capable of germinating in the presence of only 

 a very low amount of oxygen, the subsequent development of the or- 

 ganisms requires a high oxygen concentration. Spore formation does 

 not take place in the absence of oxygen. The actual amount of oxy- 

 gen present, rather than the partial oxygen pressure, is the limiting 

 factor both for germination and for growth. An excess film of water 

 will retard germination of the spores on nutrient agar. 



Stem necrosis caused by fiotato scab.— The organism S. scabies is 

 also capable of causing stem necrosis. Brown, necrotic lesions are pro- 

 duced on the subterranean stems. These lesions originate at the len- 

 ticels or at points of emergence of stolons and secondary roots. In 

 advanced stages, the stem is girdled and rotted at the base with vas- 

 cular discoloration extending up the stem a distance of 6 to 8 inter- 

 nodes; similar effects are produced on the roots. Varieties of potatoes 

 resistant to tuber scab are also resistant to stem necrosis (179). 



Sugar Beet and Mangel Scab:— Besides producing scab on potatoes, 

 certain parasitic actinomycetes are capable of causing scab on root crops, 

 notably sugar beets. Kruger (244) was the first to establish, in 1904, 

 that the production of scab on sugar beets is due to a certain species of 

 actinomycetes. He described several organisms under the names of 

 Oosfora cretacea, O. rosella, O. intermedia, O. tenax, O. nigrificans, 

 and O. violacea, all of which were typical actinomycetes, as we recog- 

 nize them to-day, belonging to the genus Streptomyces. Kruger 

 worked with what he called "girdle" scab of sugar beets; he emphasized 

 that the strains of the parasites which he isolated were not identical 

 with the potato scab organism of Thaxter. 



LuTMAN and Johnson (271) isolated eight strains of actinomycetes 

 from beet scab. Five of these cultures were found to be pathogenic, 

 one being more virulent than the others. 



Millard and Beeley (297) recognized two distinct types of scab, 

 the raised and the pitted, on mangels. The raised scab was subdivided 

 into the mound and knob forms, which were found to develop particu- 

 larly on the yellow-skinned varieties of mangels. A marked difference 

 was found in the origin of the two major types of scab. Pitted scab 

 was similar to the common scab of potatoes. The raised scab was not 

 formed from the cambium of the vascular rings, but resulted from the 

 proliferation of the pericycle. A strain of an actinomyces was isolated 

 from mound scab which reproduced the same type of scab in artificial 

 inoculation experiments. This strain was described under the name of 

 Actinomyces tumuli. From pitted scab a strain was isolated which 



