Chapter X — 167 — Plant Diseases 



mycetes are commonly observed not only on the tubers of potatoes 

 (_Solanmn tiiherostiiu'), but also on the roots of turnips QBrassica ra^a) 

 and rutabagas (B. camfestris^; less commonly on the roots of radishes 

 CRaphanus sativus^; and occasionally on carrots QDmicus carota) and 

 parsnips QPastinaca sathm'). The eggplant (^Solanum melangena), 

 the weeds S. nigrum and Amaranthns retroflexus, and several other 

 plants, such as crucifers, may become scabbed when grown on scab- 

 infested soil. Afanasiev (6) obtained scab formation on radishes and 

 on sugar beets by a strain of S. scabies. Species of Streptoviyces were 

 also found bv other investigators in the leaves of strawberry plants, 

 causing such deformities as small-leaf, yellows, cauliflower disease, and 

 chlorosis (23). Infecting agents of the various plants are said to rep- 

 resent different species of actinomycetes, in addition to the S. scabies 

 that attacks the Irish potato. 



The possibility that an actinomyces is responsibile for gummiosis of 

 citrus associated with wood necrosis has also been suggested (140). 



Other Plant Actinomycoses:— In certain plants, actinomycetes pro- 

 duce associations with the root systems which are believed to comprise 

 specific relationships comparable to those of mycorrhiza. Peklo (333) 

 made a detailed study of the endophytes of the alder bush, Alnns glu- 

 tinosa, and of sweet gale, Myrica gale. The formation of pseudoparen- 

 chymatous tissues in the swelling of the roots of the alder was ex- 

 plained by the condensation of the plasma together with the mycelial 

 mass of the endophyte. Two species of actinomycetes, Stre-ptomyces 

 alni and S. myricae, were isolated. These organisms produced, in cul- 

 ture, swellings comparable to those formed by animal pathogens. The 

 significance of these associations for plant growth still remains to be 

 established. 



Methods of Control:— 



Influence of antagonistic orgawisws.— Millard and Taylor (299) 

 found that certain nonpathogenic organisms in the soil markedly re- 

 duce scab. Similar results have been obtained by other investigators 

 (90). The nature of the antagonistic organisms which repress the de- 

 velopment of the scab organism appears to vary greatly, and, therefore, 

 more than a single antagonist is involved. Goss (147) also ascribed an 

 important role to competing soil microorganisms in reducing potato scab. 

 Infection of tubers grown in sterilized soil with S. scabies could be 

 overcome by inoculation of such soil with an aqueous extract of un- 

 sterilized soil. 



The soil microflora exerts an important effect in reducing the ac- 

 tivity of the infectious agents which result in the production of scab. 

 Sandford (379) believed that the favorable effect of applications of 

 certain forms of organic matter in controlling potato scab was due to 

 stimulation of soil microorganisms which are injurious to the growth 



