274 



THE ACTINOMYCETES, Vol. I 



ment of the .s(;ab organi.sni. The ciuestion was 

 raised whether actinomycetes diminish in 

 numbers and whether the parasitic potato- 

 scab-producing strains tend to die in soil in 

 which no potatoes have been grown for a 

 long time. The actinomycetes were found to 

 remain fairly constant in their numbers and 

 in their percentage relation to the total num- 

 ber of organisms. The pathogenic types, how- 

 ever, were gradually reduced in numbers. 

 Tests were made b}^ planting a susceptible 

 potato variety in the soil; if the scab-produc- 

 ing strains were still present, scabbing would 

 result. The formation of russetted tubers sug- 

 gested the possibility that the scab organism, 

 after a long period of deprivation of its host, 

 was weakened in pathogenicity and produced 

 the true, deep scab only in rare instances. *S'. 

 scabies is also capable of causing necrosis of 

 subterranean stems of potatoes. The stems 

 may become girdled and rotted at the base 

 with vascular discoloration extending up the 

 stem six to eight internodes (Hooker and 

 Kent) . 



Sugar Beet and Mangel Scab 



Certain actinomycetes are also capable 

 of causing scab on various root crops, nota- 

 bly sugar beets and mangels. Kriiger was the 

 first to establish, in 1904, that the production 

 of scab on sugar beets is due to actinomy- 



cetes. Under the influence of the generic des- 

 ignation of. the actinomycetes used by Thax- 

 ter, he described several actinomycetes as 

 species of Oospora, namely, 0. crctacea, 0. 

 rosella, 0. intermedia, 0. tenax, 0. nigrificans, 

 and 0. violacea. All of these were, however, 

 typical actinomycetes, belonging to the 

 genus Streptomyces. Kriiger studied particu- 

 larly the type of scab known as "girdle" scab 

 of sugar beets. The strains of the organisms 

 he isolated were believed not to be identical 

 with the potato scab form of Thaxter. 



Various other isolations of actinomycete 

 cultures were made from beet scab. Some 

 of these cultures were found to be parasitic 

 but varying in the degree of virulence. Mil- 

 lard and Beeley recognized two distinct 

 types of mangel scab, the raised and the pit- 

 ted forms. The raised scab was subdivided 

 into the mound and knob types, which were 

 found to develop particularly on yellow- 

 skinned varieties of mangels. The pitted scab 

 was similar to the common scab of potatoes, 

 whereas the raised scixh was not formed from 

 the cambium of the ^'ascular rings, but re- 

 sulted from the proliferation of the pericycle. 

 A culture was isolated from mound scab 

 which reproduced the same type of scab in 

 artificial inoculation experiments; it was de- 

 scribed as A. tumuli. From pitted scab a 

 culture was isolated which also reproduced 



Figure 107. Mangel scab (licproduced from: Millard, W. A. and Beeley, F. Ann. Appl. Biol. 14: 

 311, 1927). 



