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bark. The disease is most conspicuous in the largest roots, but the 

 smallest fibres, upon close examination, are seen to be similarly af- 

 fected. In cases where the woody cylinder is blackened before the 

 adjacent bark shows injury the smaller feeding roots, which pass 

 from the diseased wood through the still healthy bark die. They 

 are infected by or they infect the wood of the central cylinder. 



When all the roots of a plant are diseased death is naturally 

 more rapid than when a few only are affected, and in such cases the 

 progress of the decay of the stem is cut off by the death of the plant. 

 Thus in the plants most badly diseased the disease does not reach as 

 high as in plants less affected. The wood midway up or near the top 

 of such plants shows no blackening, barely a slight yellowish tinge 

 is evident. (N. C. E. S. B. 188.) 



Spread of the Disease. Corroborative evidence from tobacco 

 growers seems to establish definitely that this disease increases in 

 violence upon a given field year by year after the first infection. In 

 the first year of its occurrence in a field, a few plants only may be 

 affected. The next year that tobacco follows on this field larger 

 regions are affected. A third year without protracted rest from to- 

 bacco growing would probably in every case bring on deplorable 

 conditions. 



The germs grow and multiply in the affected plants. Upon the 

 death and disintegration of the plant they are liberated in the soil 

 where they seem to be able to live for considerable time. The im- 

 mensity of their number in a diseased plant furnishes inconceivable 

 hosts, so that even a few diseased stems, roots, or leaves in the field 

 will stock the soil plentifully with the germs. Infected plants or 

 soil in which infected plants have been or which bears parts of in- 

 fected plants, can therefore convey the disease to healthy fields. 



Instances have been cited in previous pages where the contagion 

 has been spread by WASHING from higher land to lower, so, too, 

 may it be carried by any means which can convey soil from a sick 

 to a healthy field, notably through TOOLS, which have been used on 

 diseased soil. Though apparently reasonably clean, such tools, if 

 they bear even a fraction of a spoonful of infected soil, may carry 

 hundreds of germs and thus start an epidemic in the field next 

 cultivated. The HOOFS of animals or the FEET of laborers, may in a 

 similar way bear the disease-laden soil. WIND passing over an in- 

 fected field may pick up broken bits of sick tobacco plants or germ- 

 laden soil, and convey these to healthy fields. Infection by wind, 

 however, seems to be rare, possibly because of the germicidal action 

 of the sun's rays upon the surface layer of soil upon which the wind 

 must act. It is still an unanswered question whether the germ can 

 live through the heat of the curing-house. If it can, an additional 

 means of dispersal obtains in the manure made from the refuse 

 stalks and stems derived from such plants. 



It is hopeless to attempt to cure a plant after it is once diseased. 

 Prevention must be relied upon. With the knowledge that the dis- 

 ease is caused by living germs rational methods may lie adopted 

 to prevent its spread. Any means which can carry soil from a dis- 



