BLACK ROOT-ROT OF TOBACCO 



127 



1925 the plots wliich had previously had a timothy cover crop jaelded more 

 than plots on which a cover crop had never been grown. The average in- 

 crease of plots which had had timothy cover crop o\er plots whicii had liad 

 no cover crop was 17 per cent. The increase following timothy was greater 

 on limed than on imlimed areas. An examination of roots showed that after 

 the omission of timothy cover crop for one year there was no significant 

 difference Ijetween the amount or severity of brown root-rot on plots with 

 and without timothy cover crop. The depressing effect of timothy cover crop 

 on tobacco seems to disappear if the cover crop is omitted for bat a single 

 year. 



Discussion of the Five-Year Experiment as a Whole 



Occurrence of Thielavi-a in Tobacco Soils and Rate of Infestation 



This fungus probably occurs to a limited extent in all of our soils. It has 

 been shown that it occurs on tlie roots of nearly a hundred other species of 

 plants besides tobacco (6). Many of these are common cultivated and weed 

 plants. Further investigation would undoubtedly show that there are many 

 other hosts. But even though its host plants were not present — they do not 

 seem to be necessary — it apparently lives indefinitely in the soil on dead or- 

 ganic matter. In tlie examination of tobacco roots from fields which appeared 

 quite normal and healthy above ground there are usually found occasional 

 lesions of black root-rot. The reason that most Connecticut Valley tobacco 

 fields do not suffer severely from black root-rot is not the absence of tlie 

 fungus, but the maintenance of a soil condition which is unfavorable for its 

 rapid propagation. Soil reaction is probably the most influential factor. An 

 extensive survey of Massachusetts tobacco soils lias not been made, but those 

 which have been tested have been fairly acid with a reaction ranging between 

 pH 4:5 and 5.8 except where they have been limed within a few years. With- 

 in this range there seems to be very little spread and the degree of infesta- 

 tion is light. But when raised above this range by application of iime or 

 other alkaline substances, conditions are made more favorable to growth of 

 Thiclavia and the soil becomes more heavily infested. Increase in degree of 

 infestation however is a slower process than has been commonly supposed. 

 Most investigators of the disease have assumed that when acidity was re- 

 duced the disease would immediately become serious. In our own experi- 

 ments it was not until the second year after liming was started that a high 

 infestation was secured. We have seen a number of cases where growers 

 ha\'e applied lime or wood ashes and the effect the first year was an increase 

 in the yield of tobacco. It was only in the second or third year after liming 

 that the trouble became serious. Hutcheson and Berger (4:7) noticed in 

 their experiments that the first effect of lime was beneficial but its ultimate 

 effect was to reduce the yield. It has been the common experience of those 

 who have tried to produce the disease by soil inoculation that their first at- 

 tempts were unsatisfactory. Since infection of the roots is local, i.e., the 

 fungus does not travel far from the point where it enters the root, the amount 

 of damage is in direct proportion to the number of ])laces at which the root 

 lias been infected by the fungus. The degree of soil infestation must there- 

 fore be hitch before the disease becomes serious. 



