BLACK ROOT- ROT OF TOBACCO 



131 



of loss during two of the years were not so high on tlie inoculated as on the 

 iininoculated plots, but the reverse was true the third year. Certainly there 

 was not, on the whole, sufficient evidence from either the yield data or the 

 examination of the roots to prove that timothy iiad any influence on the pre- 

 valence or severity of black root-rot. 



In 1925, after one year's omission of cover crops, the plots which had had 

 timothy cover crops yielded 17 per cent more than the plots which had never 

 had cover crop. It would appear from this that the depressing effect of 

 timothy cover crop quickly disappears and is replaced by a beneficial effect 

 when the cover crop is not sown for one year. This increase following timothy 

 after an interval of one year was greater on limed than on imlimed plots. 

 In this connection it is interesting to consider the previous depressing effect 

 of timothy on limed versus unlimed plots. For the three years previous to 

 1925, the average reduction in yield associated with the use of timothy cover 

 crop was 5 per cent on limed plots and 10 per cent on unlimed plots. 



When the roots of the plants on the timothy plots were compared with 

 those on the adjacent plots without cover in years previous to 1925, it was 

 found that many of the laterals — especially the lower ones — were brown and 

 dead. The symptoms were quite typical of the disease which is now com- 

 monly called brown root-rot. The disease was not entirely lacking in any of 

 the plots but was certainly more pronounced on the timothy plots. On the 

 timothy plots which had been limed, black root-rot was also present, and it 

 was not always possible to separate the influence of the two; but on the un- 

 limed plots the brown root-rot was unmistakable. Brown root-rot, then, is 

 apparently one of the causes of depressed yields on the timothy plots. It is 

 known that timothy in rotation encourages this disease, and it is not surjiris- 

 ing that we should now find the same effect from timothy used as a cover 

 crop. This, however, is not a satisfying explanation, because we do not yet 

 know what brown root-rot is. Whether or not it is caused by soil organisms 

 or by toxins or malnutrition has never been determined. We are not even 

 sure that all the tobacco-sick soil troubles which we include xmder the title 

 of brown root-rot are the same. The only characters they have in common 

 are the presence of some dead brown roots and consequent reduced growth. 



Pure Cultttue FiXPERUiENTs on the 

 Relation of H-iox Concentration to Growth of Thielavia 



From the preceding, it is apparent that the adverse effect of iime on tobacco 

 is due to the favorable influence of an alkaline reaction on tlie growth of 

 Thielavia. In order to see whether this conclusion is supported by growth 

 of the fungus in pure culture, synthetic culture media (Czapek's) were pre- 

 pared and adjusted to the following reactions: 1.5, 2.0, 3 05, 3.45, 3.8, 3.9, 4.25, 

 4.55, 5.35, 6.05, 6.2, 7.2, 7.9, 8.55, 8.9, 9.6, 9.9, 11.05, 12.1, and 12.9. The reac- 

 tions were determined both colorimetrically and electrometrically. Three 

 series of cultures were made, one in tubes and the others in Petri dishes. 

 Records of measurements and notes on growth were made at intervals of 3 

 or 4 days. Briefly summarized, the results were: On all cultures between 

 6.05 and 12.9 the growth consisted of a dense black mat of mycelium on the 

 surface and dark hyphae under the agar. Both chlamydo.spores and conidia 

 were produced in great abundance. Within this range no difference in char- 

 acter or quantity of growth could be observed. Apparently the reaction was 



