Chapter X —161 — Plant Diseases 



to the same strain of the pathogenic organism. The change in type 

 of scab produced by a certain strain of organism was explained by the 

 above investigators as due to a change in virulence or in the conditions 

 which favor growth of the particular organism. 



Afanasiev (6) investigated seven parasitic cultures of the scab- 

 producing organism isolated from three different types of scab, namely, 

 the common, deep, and russet forms. Individual potatoes were found 

 to show two or even all three of these types of scab. It was suggested, 

 therefore, that the difference in the scab exhibited by cultures of para- 

 sitic actinomycetes was one of degree of pathogenicitv rather than of 

 type of organism concerned. All parasitic cultures were able to utilize 

 sucrose and raffinose, whereas most of the saprophytes were unable to 

 use these two sugars. The growth of both parasitic and saprophytic 

 actinomycetes was similar on all media containing different nitrogenous 

 compounds, with the exception of urea. All parasitic and some sa- 

 prophytic cultures failed to grow on a medium to which 0.5 per cent 

 of urea was added. This was found to be due to the toxicity of the am- 

 monia, which was produced as a result of decomposition of urea. The 

 conclusion was reached that the ability of parasitic cultures to utilize 

 sucrose and raffinose, their inhibition by ammonia, and their abilitv 

 to produce a melanin pigment in a tyrosine medium could be utilized 

 to differentiate the parasitic actinomycetes from the saprophytic soil 

 actinomycetes. 



In a study of the correlation of the cultural characteristics of actino- 

 mycetes and scab development, Taylor and Decker (413) isolated 

 143 cultures, of which 128 were non-acid-fast and 15 were acid-fast or 

 partly acid-fast. The last group did not hydrolyze starch, and 13 of the 

 15 did not liquefy gelatin, though the others did. Sixty-six of the 

 non-acid-fast produced a dark -brown surface ring of growth on milk; 

 of these, 61 caused typical potato scab. Within the "dark-brown ring" 

 group, variations occurred in the total amount of scab produced, the 

 lesions varying from shallow to deep pits. The work of Schaal (387) 

 on the variation and the physiological specialization of the scab-pro- 

 ducing actinomycetes has been discussed previously (p. 76). 



Persistence of scab organism in so/Z.— The parasitic organisms caus- 

 ing potato scab are capable of persisting for a long time in the soil. 

 They thus form a part of the microbiological population of the soil. 

 The parasite has been found (272) even in virgin soils. Pratt (347) 

 obtained a higher percentage of scab on potatoes planted in virgin land 

 in Idaho than on the same land following other crops. He suggested 

 the latter produced some effect upon the soil which resulted in a re- 

 duction of the parasitic forms. In a survey of the distribution of the 

 potato scab in Western Nebraska, Ggss (146) obtained 67 per cent 

 scab on one virgin soil and 46 per cent average scab on 61 fields never 

 before planted to potatoes when all were planted with the same lot of 

 apparently healthy, treated seed potatoes. 



