MABCH, 1920] PATHOLOGY 113 



772. Johnson, James, and R. K. Hautman. Influence of soil temperature on the root-rot 

 of tobacco. Jour. Agric. lies. 17:41-86. PI. 1-8. L919. — Authors' summary is as follows: 

 "(1) The root-rot of tobacco, caused by Thielavia basicola, is marked by the stunting of plants 

 in various degrees due to a reduced root system. The extent of tin damage is determined in 

 a large measure by the environmental conditions surrounding the roots ( >f the host. — (2) A 

 study of these environmental conditions is essential to the proper understanding of the occur- 

 rence and distribution of the disease in general and local areas, and to good judgment in rec- 

 ommendation for control measures. — (3) There seems to be no variation in the pathogenicity 

 of the root-rot fungus secured from different sources. The amount of disease is determined 

 entirely by the susceptibility of the host, the amount of infestation, and the soil environmental 

 factors surrounding the roots of the host. (4) The factors especially studied were the amount 

 of infestation in the soil, the soil moisture, soil temperature, soil reaction, physical structure, 

 and fertility. An analysis of these factors separately as related to root-rot frequently is very 

 difficult, if not impossible. Under normal conditions the end result in injury by root-rot is 

 the sum total of the favorable and unfavorable action of these factors on the disease. Some 

 of these factors are much more important than others. — (5) Other factors aside, the extent of 

 infection and injury from tobacco root-rot is directly proportional to the amount of infestation 

 of the soil. — (6) Root-rot is seemingly capable of developing in relatively dry soils. Increasing 

 the moisture content of the soil up to three-fourths of its water-holding capacity does not 

 materially increase root-rot. Saturated soils are, however, considerably more favorable for 

 the disease than unsaturated ones. — (7) The temperature of the soil is undoubtedly the most 

 important factor determining the extent of the root-rot of tobacco, other factors being equal. 

 The most favorable temperature for the disease ranges from 17° to 23°C. Below 15° the dis- 

 ease 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 suscepti- 

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

 factors. The results of tests of Wisconsic. 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 intro- 

 duced 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 para- 

 site 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 bad practice, although recovery from 

 the disease may occur. Such recovery, environmental conditions aside, is proportional to 

 the resistance of the type used." — Extensive experimental data are presented in detail in sup- 

 port of the conclusions. A bibliography of 27 titles is appended. — D. Reddick. 



773. Johnson, James. The influence of heated soils on seed germination and plant growth. 

 Soil Science 7: 10-103. PL 1-8.— See Bot. Absts. 3, Entry 854. 



