48 Journal of Agricvltural Research voi. xvii. no. 3 



AMOUNT OF INFESTATION 



With diseases in which the injury from infection is confined to rela- 

 tively small local areas on the host, it is to be expected that the amount 

 of damage done is in some measure proportional {o the number of infected 

 areas which occur. Although it is evident that under the most favorable 

 conditions, T. hasicola may spread a considerable distance over a single 

 root from one source of infection, more commonly the diseased portions 

 are confined to areas of from i to 5 mm. In either case it is evident 

 that relatively few infections may cause no appreciable stunting in 

 growth, where a larger number of infections in proportion to the size of 

 the root system may produce a rapid check in growth, owing largely to 

 the mechanical reduction of the feeding area, and possibly also in some 

 degree to the formation by the fungus of toxic substances injurious to 

 normal metabolism. The latter possibility appears not to hold, however, 

 since the host is rarely killed by the parasite. In fact, the plant appears 

 to receive a stimulus toward the formation of new roots to replace those 

 lost by disease, and in many instances the youngest leaves of infected 

 plants possess a deeper green color than healthy plants. The new roots 

 are at a greater disadvantage than the original ones, if they are formed 

 at or near the region of disease, since frequently they are obliged to pass 

 through small local areas of high infestation before reaching the deeper 

 layers of soil. 



It is a well -recognized fact that infestation increases under field con- 

 ditions at an enormous rate once it is present or introduced to a soil 

 which is being cropped to a highly susceptible host. This is illustrated 

 in the practice in certain tobacco-growing areas of growing only one or 

 two crops of tobacco and then laying the land aside for other crops for 

 a long time. That this practice has been empirically developed, in the 

 Burley section at least, as a result of the rootrot, seems unquestionable, 

 in view of some unpublished results obtained in that section during 

 the last three years. 



The influence of the amount of infestation on the amount of disease 

 might be illustrated in a number of ways. The simplest method appeared 

 to be the mixing of varying quantities of infested and uninfested soil 

 and transplanting into this mixture a susceptible variety of tobacco. 

 The soil selected for this purpose was from the old tobacco field on the 

 Station farm at Madison, on which tobacco had been grown continu- 

 ously for 10 to 12 years, together with soil of the same type from a neigh- 

 boring plot located not more than 2 rods away, but never having been 

 used for growing tobacco. The fertility of the two soils naturally would 

 not be the same, but the soil from the tobacco field because of heavy 

 applications of fertilizers was the more fertile of the two from a chemi- 

 cal standpoint. The soils, after having been thoroughly screened, were 

 weighed out and mixed in the proportion shown in Table 1. Two sepa- 



