20 BULLETIN 1256, U. S. DEPARTMENT OF AGRICULTURE. 



the effects of the disease are most evident. Periods of warm weather 

 following, which are usually accompanied by dry conditions, exag- 

 gerate the stunting but permit new roots to grow out into warm, dry 

 soil without decay, hence permitting a rapid recuperation of growth 

 as soon as sufficient soil moisture is supplied (PL VI, fig. 1). Occa- 

 sionally, however, a season as a whole is relatively cool, and crops 

 remain stunted throughout the season on infested soils. On the other 

 hand, during exceptionally warm seasons excellent crops may be 

 produced on heavily infested soils. 



The injury from black root-rot will, of course, also vary with a 

 number of other factors of an environmental nature in the soil, some 

 of which are not fully understood. It is certain that highly acid 

 soils tend to reduce the disease and that soils alkaline or moderately 

 acid in reaction or soils made less acid by the application of lime are ■ 

 more likely to favor the disease. Again, there is some evidence to 

 show that more damage from the disease occurs on soils which are 

 compact than in the 'same soils kept loose. Within ordinary limits 

 the amount of water in the soil does not seem to affect materially the 

 extent of the disease, though poorly drained wet soils may favor 

 disease largely because of their lower temperature. 



Control. — Black root-rot can be adequately prevented in the seed 

 beds by selecting land for beds which has not been in tobacco or 

 tobacco beds for several years. If such a change of seed-bed soil 

 can not be made, soil sterilization, preferably steam sterilization, 

 should be resorted to. Do not transplant infected plants to the field, 

 since such plants will not only fail to start growth properly but in 

 case of planting on new tobacco land serve as a source of introduc- 

 tion of the parasite, which will affect later tobacco crops grown on 

 the land. 



The most evident wa} T of avoiding black root-rot in the field is by 

 repeatedly changing the land for the tobacco crop, since continuous 

 culture favors root-rot, whereas soils not planted to tobacco for 5 to 

 10 years are generally relatively free from the parasite. The fre- 

 quency with which this change of land should be made, however, will 

 depend largely upon the susceptibility to the disease of the variety 

 grown, the time of introduction, and the rate of the increase of the 

 parasite in the soil. Land may become too heavily infested for Bur- 

 ley tobacco in one or two years, whereas certain fields in the Con- 

 necticut Valley, where shade-grown Cuban or Broadleaf are grown. 

 frequently permit continuous culture, although in certain seasons 

 high percentages .of damage from Thielavia occur. 



No recommendations for crop rotation with tobacco which will 

 safely meet all conditions can be given. Not only do the above-de- 

 scribed factors vary the case, but the actual rest period from tobacco 

 which is required and the crops to be grown in rotation in ordinary 

 farm practice vary in the different growing districts. As a general 

 principle, however, heavily infested soils should be rested four to 

 eight years and should not be cropped in the meantime to other good 

 host plants of Thielavia, such as beans, eowpeas, or soybeans. Corn, 

 small grains, and most truck crops are immune to the disease. 



Another means of control, which can not yet be used to its fullest 

 advantage, lies in the use of disease-resistant varieties or strains 

 (fig. 12). The representative types of tobacco now grown in the 



