Chapter X — 1 59 — Plant Diseases 



gcnic tendencies, but, it no host is present, they may lose them. If a 

 suitable host is provided, the pathogenic habits are reacquired, a milder 

 form of scab, such as russetting, appearing first. 



In contrast to the concept of multiple species, some investigators 

 believe that not only is the potato scab organism a single species, but 

 that S. scabies can cause root necrosis in seedling plants representing 

 many families, notably those of wheat, pea, soybean and radish, as well 

 as a variety of others (178). 



WoLLENWEBER was the first to recognize that more than one species 

 of actinomyces is capable of causing potato scab; difi^erent types of scab 

 were believed to be caused by different organisms (Table 42). No 

 infection tests on potatoes were made with these cultures. Since most 

 of the species thus described are similar and since large numbers of 

 saprophvtic organisms are commonly found in the soil, and are closely 

 associated with scabby surfaces, the pathogenicity of these species is 

 open to question. Wollenweber's descriptions were incomplete and 

 the experimental data were limited. 



Millard and Burr (298) isolated 24 cultures of actinomycetes 

 which were said to cause potato scab. Three of these cultures were 

 obtained in duplicate, and only one was found to be identical with 

 Thaxter's original form. The type of scab produced by the various 

 cultures was believed to be correlated with the specific nature of the 

 organisms. The organism identified as S. scabies produced the deep 

 scab and was capable of attacking the roots and stolons of the potato 

 plant. 



The description of the organisms by Millard and Burr was based 

 on growth in glycerol synthetic solution: 



I. Star-like colonies which persist in suspension or cling to the sides of tube: 



1. Deep pigment produced in nearly all media; tyrosinase reaction positive; 

 solid curd formed in brom-cresol milk S. clavifer 



2. Deep pigment produced in protein media only; tyrosinase reaction posi- 

 tive S. pmhriatus 



3. Pale pigment produced in nearly all media; tyrosinase reaction negative 



S. carnosus 



4. No pigment (or only trace) produced on artificial media; growth on 

 saccharose and dextrose media covered with minute craters S. craterifer 



II. Heavy surface growth with abundant aerial mycelium: 



1. Gray aerial mycelium on nutrient potato agar not abundant; fern-like 

 outgrowths produced from margin on saccharose synthetic agar 



S. gracilis 



2. Aerial mycelium on nutrient potato agar abundant, white; gelatin lique- 

 fied, without pigment S. fraecox 



3. Liquefaction of gelatin accompanied by production of pigment 



S. setonii 



4. Carnelian red pigment in calcium malate glycerine agar; abundant aerial 

 mycelium produced on nearly all media S. fraefecundus 



III. Fair surface growth with some aerial mycelium: 



^ 1. Produces pigment (often green) on all solid media S. viridis 



2. Produces pigment (yellow) on all synthetic media S. flavus 



