Microbiol:9 



you who are interested, may I suggest the excellent review by yood 

 and Tveit (38). They conclude that no clear picture emerges from 

 the very considerable work which has been done with the several root 

 pathogens just named. In brief, there is no doubt but that the 

 saprophytic microflora of roots is antagonistic to various pathogens. 

 This antibiosis has been demonstrated in numerous experiments. Never- 

 theless, the real importance of this antibiosis at the root surface 

 is still to be assessed. 



Some recent work by Harley and Waid (39) illustrates the need for 

 caution in the interpretation of rhizosphere data. They determined 

 numbers of fthizoctonia and of Trichoderma on root surfaces of beech 

 seedlings grown in varying intensities of daylight. A plot of their 

 data shows Rhizoctonia to decrease linearly at roughly a U^ degree 

 angle with increasing percentage of daylight, and Trichoderma , to 

 increase linearly, also at roughly a U5 degree angle. Because Tricho- 

 derma is well-knovm as a soii. fimgus which can inhibit many other 

 fungi in culture, one can interpret the inverse relationship between 

 numbers of Rhizoctonia and Trichoderma as antibiosis. Harley and 

 Waid recognized that sampling and growth problems coiild well be 

 involved, as Trichoderma over-grows very rapidly in culture. They 

 then determined how greatly the presence of Trichoderma led to vinder- 

 estimation of Rhizoctonia by enumerating this latter fungus on plates 

 not bearing Trichoderma . They found that Trichoderma had only slight 

 influence on growth of Rhizoctonia . They believed that the only 

 permissible conclusion was that the condition of the beech seedlings 

 determined the nature of the root surface populations. 



Such data serve to temper our enthusiasm when we look at information 

 concerning f'out microfloras and root parasites. I do not mean to 

 infer that biological control does not function at the root surface. 

 There are a number of excellent reports showing that it does. There 

 is much left to be discovered. Garrett (UO) has made a forceful 

 suiranarizing statement, adroitly side-stepping the unknown: 



"Interference by other soil microorganisms with the para- 

 sitic activity of a root-inhabiting fungus is greatest at 

 the root surface, where the soil microflora is greatly 

 intensified, and is also changed in composition, by root 

 excretion and other activities of the living root. The 

 rhizosphere and the root surface microfloras thus appear to 

 constitute the root's outermost barrier against invasion by 

 pathogenic fungi." 



To What Extent Can We Alter the Root Surface Microflora ? 



Some further insight into the ecology of the rhizosphere can be gained 

 by noting the influence of soil, seed, or plant treatments on the 

 quality or quantity of the rhizophanal microflora. I shall end my 

 discussion by commenting briefly upon the extent to which the root 



