72 Journal of Agricultural Research voi. xvii, no. 2 



On the other hand, the practical application of such a recommendation 

 is doubtful on account of the variations in seasons as to temperature and 

 general growing conditions. In general the farmers must transplant to 

 the fields when the seedlings are of proper size, a matter which usually 

 can not be predetermined very effectively for more than two or three 

 weeks. With steam-sterilized seed beds closer approximations can be 

 made, and seed may be sown two to three weeks later than normally, 

 with fair certainty of obtaining plants by June 20 to 30. Planting later 

 than July i, however, is no more certain of giving satisfactory final 

 results than early planting. 



During the season of 191 7 a planting experiment was carried out, 

 with the hope of getting some data on this subject. Seedlings were 

 transplanted at intervals of one week from June 1 1 to July 23 on infested 

 and uninfested soil. Unfortunately, the White Burley variety was used 

 on heavily infested soil, and the season being relatively cool throughout, 

 no appreciable difference in yield occurred on the infested soil. On the 

 uninfested soil, however, the advantage was all with the early-set tobacco; 

 a gradual decrease in size and value occurred in the later plantings. A 

 wide range of obser^'^ation has convinced us that, other conditions being 

 alike, early-planted tobacco on uninfested soil usually is considerably 

 safer than late-set tobacco on either infested or uninfested soil. 



Before leaving this subject another point of more scientific interest 

 should be considered: Why are tobacco roots most seriously attacked 

 by T. basicola at from 17° to 23° C. and practically not at all at a tem- 

 perature of 30° C. ? Several hypotheses may be formulated. The sim- 

 plest explanation would be that the resistance of the roots to the parasite 

 is modified at different temperatures, high susceptibility occurring from 

 17° to 23° and practical immunity at 30°. At first sight a tenable 

 theory seemed to be that the increased vigor of root formation at higher 

 temperatures sufficed to overcome the destructive effects of the disease. 

 On the other hand, the action of temperature variation may be regarded 

 as modifying the ability of the fungus to grow in the soil or to attack 

 the host. On the basis of some preliminary experimental results, the 

 latter theory seems to be the most probable. 



It should be noted, however, that the behavior of the parasite in cul- 

 ture does not correspond entirely with its behavior on the host as regards 

 temperature relations. Gilbert (12) found the following critical tem- 

 peratures for growth: Minimum 7° to 8° C, optimum 30°, maximum 

 34° to 37°. The determinations of the writers have given figures very 

 much the same as these. The temperature most favorable for infection 

 does not therefore agree with the optimum for growth in culture. 

 On the other hand, the optimum growth in culture is obtained at 29° 

 to 30°, where the organism is apparently ineffective as a parasite. It 

 is not possible, therefore, to draw any decisive conclusions as to the 

 behavior of the fungus as a parasite from its behavior in artificial culture 



