812 PRINCIPLES OF SOIL MICROBIOLOGY 



temperature (25-27°) also favors Sclerotium rolfsii and certain other 

 plant pathogenic fungi. 



On the other hand, not only the onion smut as noted earlier, but 

 certain grain smuts are favored by low soil temperatures, e.g., the stink- 

 ing smut of wheat, Tilletia tritici, thrives at 9-12° according to some, 47 

 and even at 5° to 10°, according to others. 45 Rhizoctonia solani, which 

 may attack potato and many other plants, is aggressive only at relatively 

 low temperatures. 49 Of the two fungi capable of causing tomato wilt 

 or "sleepy disease," V erticillium albo-atrum operates only at a low soil 

 temperature (21-23°) whereas Fusarium wilt as already noted is a high 

 temperature disease (28-29°) . 60 



Soil moisture also exerts a potent influence on many plant parasites. 

 As previously noted, Sanford found dry soils favorable and wet soils 

 inhibitory to potato scab. More often soil fungi, especially of the 

 "damping off" types, are favored by high moisture; wet soils, even to 

 the saturation point, favor the club root parasite of cabbage, Plasmo- 

 diophora brassicae. 51 Spongospora subterranea (powdery scab) develops 

 best in periods of damp, rainy, and cloudy weather and is favored by 

 poor drainage. 52 



Hungerford recorded that there is a definite relation between the 

 amount of moisture in the soil at seeding time and the amount of 

 bunt or stinking smut which occurs in the resulting crop of wheat; the 

 drier the soil at seeding time the less will be the amount of infection. 

 When the soil is moist and cultivated frequently, the spores of Tille- 

 tia tritici rapidly lose their power of infection. 



Of course in all such cases more than one variable factor is concerned. 

 In the case of both cabbage club root and potato scab, it has long been 

 known that soil reaction influences their occurrence. The above cited 

 investigations, as well as the fact that high soil temperature, 22° or 



47 Hungerford, C. W. The relation of soil moisture and soil temperature to 

 bunt infection in wheat. Phytopathol., 12: 337-352. 1922. 



48 Faris, J. A. Factors influencing the infection of wheat by Tilletia tritici 

 and Tilletia laevis. Mycologia, 16: 259-282. 1924. 



49 Richards, B. L. Pathogenicity of Corticium vagum on the potato as affected 

 by soil temperature. Jour. Agr. Res., 21: 459-482. 1921. 



50 Bewley, W. F. Sleepy disease of the tomato. Ann. Appl. Biol., 9: 116- 

 134. 1922; Jour. Ministry Agr., Great Britain, 30: 430-457. 1923. 



61 Monteith, J. Relation of soil temperature and soil moisture to infection by 

 Plasmodiophora brassicae. Jour. Agr. Res., 28. : 549-561. 1924. 



62 Melhus, J. E., Rosenbaum, J., and Schultz, E. S. Sponjospora subterranea 

 and Phorna tuberosa on the Irish potato. Jour. Agr. Res., 7: 213-254. 1916. 



