("AISA'I'IOX t>l' I'LANT 1 )ISI':ASI':S 



273 



iiifcstcd soil ;it pi I .").() lo .").!. 'riicrc \\;is no 

 not icc.-ihic iii('iH\-is(> ill soil acidily diiriiii;' ;i 

 .")8-(l;iy period. Jii artificial luodia, howcxcr 

 the lii-ow til of (•(M'taiii haclcria made condi- 

 tions uiil'a\()rahl(' tor the ,<;ro\vth oi S. sca- 

 hicii. The conclusion was reached that when 

 scab is controlled in some soils hy ii,reeii ma- 

 nure crops, this may he due to the "antibi- 

 otic" (luaiities of eeitain predominant soil 

 microorj>;anisms. 



Kenlvnight attemi)ted, unsuccessfully, to 

 control scab through tlu^ antagonistic ac- 

 ti\'itics of other microorganisms. Addition of 

 organic matter to the soil tended to aggra- 

 \ate the development of scab except in ex- 

 treme cases of scab infection. The findings of 

 Millard and Taylor that green manure was 

 effective in controlling scab in the presence 

 of S. praccox were not confirmed. When this 

 and other species of actinomycetes were in- 

 troduced into scab-infested soil in green ma- 

 nure and in ^•arious other media no control 

 was obtained. KenlCnight thus agreed with 

 Ctoss, who also failed to obtain control of 

 scab with S. praecox. 



McCormick observed that S. praecox was 

 antagonistic on solid media to <S'. viridis and 

 S. intermedins, but not to certain other para- 

 sitic actinomycetes. This observation sug- 

 gested specific differences in the action of the 

 antagonist. Xo control of scab was obtained 

 with B. megaterium, an organism antago- 

 nistic to certain actinonwcetes. Pseudomonas 

 fluoresceus, which was antagonistic to certain 

 actinomycetes and to Trichoderma lignorum, 

 reported to be antagonistic to many fungi, 

 exerted no effect on the scab organism. 



Daines reported (1937) that Trichoderma 

 Jignorum produces a diffusible substance 

 which is toxic to S. scabies in an artificial 

 liquid medium. Because of the rapid destruc- 

 tion of this toxic principle by aeration at the 

 pH of potato soils and because of remo^•al of 

 the toxic material from solution by charcoal 

 and by the soil itself, the eflBcac}^ of this 

 fungus in combating potato scab was con- 



sidered as doubtful. Although Ti iciiodrrma 

 might be of some assistance in this capacity 

 in poorly aerated soils whicii po.sse.ss low ad- 

 sorpti\-e capacities, its fum^tion in many 

 soils was (luestioiied. 



The physical as well as the biological en- 

 vironment in many cultivated soils was con- 

 sidered a strong barrier against the establish- 

 ment of the Trichoderma. When introduced 

 into a r)-(lay-old *S'. scabies culture, Tricho- 

 derma was strongly inhibited by the scab. A 

 soil-inhabiting bacterium was also found to 

 produce a material that was toxic to Tricho- 

 derma and to the actinomycete alike. Daines 

 argued that "in such complex physical, 

 chemical and biological environments, as are 

 afforded by soils, these antagonistic relation- 

 ships may be modified or even entirely de- 

 stroyed." 



KenKnight argued that the absence of 

 proper biological controls in the abo\'e ex- 

 periments may have been due to failure to 

 establish the introduced organisms in the 

 soil, or to the failure of these organisms to 

 show antagonism toward any or all of the 

 parasitic actinomycetes under soil condi- 

 tions. He found an analogy between the in- 

 troduction of organisms not well suited to 

 the soil conditions and efforts to obtain a 

 stand of wheat by planting the seed in un- 

 broken prairie sod. He believed that Kies- 

 shng's mixed cultures of bacteria that had 

 the capacity to control scab, except for one 

 type of scab caused by a species other than 

 S. scabies, suggested that if practical control 

 of potato scab is ever obtained by biological 

 means in soil infested with several parasitic 

 actinomycetes, it will be with mixed cultures, 

 because it appears unlikely that a single 

 organism will be found that is antagonistic 

 to all strains of the scab organism. 



The relation of soil fungi to the de\-elop- 

 ment of potato scab has been studied further 

 by Pratt (1918) and by others. 



Detailed studies ha\-e been made of the re- 

 lation of the soil population to the develop- 



