84 Journal of Agricultural Research voi. x\ai, No. 2 



being equal. The most favorable temperature for the disease ranges 

 from 17° to 23° C. Below 15° the disease is less marked, and above 

 26° the severity is gradually reduced, until at about 29° or 30° it has 

 little or no influence. At 32° practically no infection occurs even in 

 the most heavily infested soils. Soil temperature records in the field 

 for four seasons indicate that occurrence of the disease under practical 

 conditions is determined primarily by soil temperature. 



(8) The disease is checked by very high soil acidity. Heavy infection 

 can occur, however, in soils showing a considerable acid reaction. The 

 results depend a great deal upon the susceptibility of the variety used 

 in the test, the amount of infection, the soil temperature, and on other 

 factors. The results of tests of Wisconsin tobacco soils indicate that the 

 use of acid fertilizers will not reduce infection by T. basicola. Although 

 alkaline soils are more favorable to disease than very acid ones, the use 

 of lime on infested soils may not necessarily reduce the yield due to 

 increased infection from T. basicola. 



(9) The amount of organic matter present or introduced into the soil 

 does not play a very important part in the amount of infection. High 

 organic matter content, however, no doubt favors increased infestation 

 and aids the fungus to persist in the soil. Where heavy inoculation is 

 made, infection apparently occurs more readily in pure sand than in the 

 presence of organic matter, but under conditions unfavorable for the 

 parasite the amount of infestation is more rapidly reduced in soils lacking 

 in organic matter. 



(10) Clay soils as such seemingly are no more favorable for infection 

 than sand, and under certain conditions possibly less so. Clay may, 

 however, favor the persistence of the parasite in the soil, and may 

 actually favor infection because of increased danger of saturation with 

 water and because of the occurrence of lower temperatures than in sandy 

 soils. 



(11) Increasing the fertility of infested soil by pure chemicals is likely 

 to cause increased stunting of growth rather than increased growth, 

 especially if too high a concentration of soil solution results. Fertilizers 

 applied to heavily infested soils under practical conditions seem to be 

 largely wasted except in seasons in which such high temperatures result 

 that the disease is held in check. 



(12) Field observations and limited laboratory experiments seem to 

 show that infested soils when compacted are more favorable for the 

 disease than loose, open soil. 



(13) Transplanting infected seedlings to an uninfested field is a bad 

 practice, although recovery from the disease may occur. Such recovery, 

 environmental conditions aside, is proportional to the resistance of the 

 type used. 



