;j32 MASS. EXPERIMENT STATION BULLETIN 22:) 



optimum throughout this entire range. But l)elow this it was otiierwise. At 

 .5.3.5 the rapidity of growth began to decrease. Below 5.35, all growth on the 

 surface ceased and even under the surface it was progressively more scanty 

 down to 3.05. Below this, gi-owth ceased altogether. The production of 

 chlamydospores also decreased progressively with the scantiness of growth 

 until there were none at all at 3.05. A few conidia were produced even to 

 the most acid point where mycelium grew. 



Since in pure culture the fungus will grow even at reactions which are more 

 acid than any tobacco soils, it is pertinent to ask why we do not have loss 

 from root-rot below 5.5. The answer is probably to be foimd in the difference 

 between conditions which the fungus encounters in a sterile nrtihcial medium 

 and what it encounters in its natural medium, the soil. In tlie latter case it 

 meets keen competition from other soil organisms and it is only the strong 

 vigorous growers that get the available food and thrive. But, as was noted 

 above, Thielavia at 5.35 is beginning to lose its vigor and at more acid ranges 

 it dwindles to nothing. When in its weakened condition due to unfavorable 

 reaction it is being crowded out Iiy other organis.ms better adapted to acid 

 conditions, it is not strange that Thielavia should lie able to make but little 

 headway at 5.35 or lower. Incidentally it should be kept in mind that Tliie- 

 lavia does occur sparingly on roots m more acid soils, and the lesions are just 

 as pronounced but are not numerous enough to cause nuich loss. The critical 

 point at which vigor of growth begins to diminish is thus lower than (i.Oo 

 and higher than 5.35. Thus it nuist fall somewhere in tiie region which we 

 have called the critical region, and the results from the pure culture tests 

 support those of the field tests. 



SUMHIARY 



1. The inunediate effect of liming an acid soil was to increase tlie yield of 

 tobacco. 



2. If suiRcient lime is added, black root-rot is ultimately (iromoted, and 

 there is consequently a reduction in yield. 



3. Black root-rot is present in most soils but is more injurious in nearly 

 neutral soils because the causal fungus, Thlelavio hasicola, grows more raj)- 

 idly and vigorously in a nearly neutral medium. 



4. In the field under experimentntion, black root-rot caused little or no 

 loss in soils more acid than pH 5.6. It caused severe loss on all soils less 

 acid than pH 5.9. It should be understood that this critical region between 

 pH 5.6 and 5.9 is an intermediate zone which may be shifted to a somewhat 

 higher or lower position by varialions in temperature and compactness of soil, 

 and by differences in the degree of infestation of the fungus in the soil. 

 Growth of the fungus in pure culture on media confirmed the results in the 

 field. 



5. As the soil becomes less acid above pH 5.9, the loss from black root-rot 

 increases. The upper limit of alkalinity has not been determined. 



6. After repeated applications of lime brought the soil reaction to a point 

 favorable to the fungus, the omission of lime for two years, with tobacco grown 

 continuously meanwhile, did not result in any immediate reduction in the 

 severity of black root-rot. 



