TOBACCO DISEASES AXD THEIR CONTROL. 



19 



growth of tobacco does not ordinarily result from unfavorable soil 

 or weather conditions alone, and consequently such fields commonly 

 indicate the presence of root-rot or in some cases other parasites. 



The rapid recovery of stunted tobacco following a change in 

 weather conditions is also characteristic of root-rot. Infested soils 

 do not ordinarily favor a gradual and continuous growth throughout 

 the season, but are affected by short periods of drought and cool as 

 compared with warm weather followed by rains, so that the develop- 

 ment of the crop occurs in short intervals rather than as uninter- 

 rupted growth. 



The extent of the damage from black root-rot ranges from complete 

 crop failures to imperceptible losses even in the presence of consider- 

 able evidence of the parasite on the roots, the latter condition re- 

 sulting from the power of the tobacco plant to recover from injury 

 to the root system. More commonly the reduction in yield varies 

 between 25 and 50 per cent. 





Fig. 13. — Field showing- the growth of susceptible and resistant strains of White 

 Burley tobacco with respect to black root-rot. The ordinary strains of White Bur- 

 ley (left side) failed to grow where the resistant strain (right side) made a nor 

 mal crop. 



Cause. — Black root-rot is caused by the fungus Thielavia basicola. 

 This parasite is also capable of attacking a considerable number of 

 other plants, usually, however, to a lesser degree than tobacco. Varie- 

 ties of tobacco differ markedly in their susceptibility to the disease. 

 This fungus produces in great abundance three kinds of spores upon 

 the roots. The presence of one of these forms, the chlamydospores, 

 is responsible for the jet-black appearance of some of the infected 

 roots. 



Thielavia is capable of living over from year to year in the soil 

 and may, in fact, persist for several years even in the absence of the 

 culture of its common host plants. This parasite does not readily 

 spread from field to field except through the transfer of infested soil 

 or the transplanting of infected plants. With continuous culture of 

 tobacco most soils become heavily infested with Thielavia. The ex- 

 tent of the damage from this fungus depends upon the extent of soil 

 infestation, the susceptibility of the variety grown, and the environ- 

 mental conditions to which it is exposed in the soil. 



Conditions favoring the disease. — Low soil temperatures during the 

 growing season are especially favorable to serious damage from black 

 root-rot (PL V, fig. 1). Since such temperatures are most likely to 

 occur in the early part of the growing season it is at this time that 



