SOIL AS HABITAT FOR PATHOGENIC MICROORGANISMS 811 



only the geographical distribution of the disease, but also its seasonal 

 severity. 43 



The case of onion smut illustrates the possible importance of a specific 

 factor of soil environment in determining the possible geographical 

 range of soil parasites. This smut is a persistent soil born fungus, 

 Urocystis cepulae, which is each year distributed throughout the 

 United States on smutty onion sets. It infects the seedling onions 

 only at low temperatures, being totally inhibited at the higher soil 

 temperatures, 28°C. or above. As a result, although established in all 

 the northern onion districts where onion seed is planted in cool soil in 

 spring, it is unknown in the southern states, Texas and Louisiana, 

 where the seed is planted in the autumn when soil temperature is so 

 high as to inhibit infection. Different soil born parasites are affected 

 very differently by environmental factors. Thus high soil temperatures 

 stimulate the development of the Fusarium "yellows" disease of the 

 cabbage and low temperatures inhibit it. By contrast the Thielavia 

 root rot of tobacco is checked in warm soils and is seriously injurious 

 only in cool soils. Jones further points out this seasonal contrast by 

 citing evidence from two successive summers of which the one, 1915, 

 was very cool, with a mid-summer soil temperature averaging about 

 5°C. lower than that of the succeeding summer. In the cool summer the 

 Thielavia root rot of tobacco was unusually severe whereas the cabbage 

 remained relatively free from disease. The succeeding year with its 

 warm mid-summer period brought disaster to the cabbage crop because 

 of the yellows disease, whereas the tobacco was free from root rot 

 even on old "tobacco sick" soils. In general high soil temperatures 

 favor the vascular Fusarium diseases, including flax wilt, F. lini, u tomato 

 wilt, F. lycopersici, 45 and cabbage yellows, F. conglulinans.™ High 



43 Jones, L. R. The relation of environment to disease in plants. Amer. 

 Jour. Bot., 11: 601-609. 1924; Soil temperature as a factor in phytopathology. 

 Plant world, 20: 229-237. 1917; Experimental work on the relation of soil tem- 

 perature to disease in plants. Trans. Wis. Acad. Sci., XX: 433. 1922. 



44 Tisdale, W. H. Relation of soil temperature to infection of flax by Fusarium 

 lini. Phytopathol., 6: 412. 1916. 



45 Scott, I. T. The influence of hydrogen ion concentration on the growth 

 of Fusarium lycopersici and on tomato wilt. Missouri Agr. Exp. Sta. Res. Bui. 

 64. 1924. 



46 Gilman, J. C. Cabbage yellows and the relation of temperature to its 

 occurrence. Ann. Mo. Bot. Card., 3: 25-S4. 1916; Tisdale, W. B. Influence of 

 soil temperature and soil moisture upon the Fusarium disease in cabbage seed- 

 lings. Jour. Agr. Res., 24: 55-86. 1923. 



