Siini\al of Plant Pathogens in the Soil 289 



The soil fusaria have been divided into true soil inhabitants and 

 soil invaders. The latter are dependent on the host for their con- 

 tinued existence in the soil; once the host plant is removed, the 

 fungus gradually dies out. 



Cabbage- and tomato-sick soils may show as many as 40,000 colo- 

 nies (on plate) of the parasitic organisms per gram of soil. Tricho- 

 dcrma koniniii and T. U<i,nonn)i, two common saprophytic soil fungi, 

 are known to cause rots of stored sweet potato, as well as "ring 



Fig. 116. Sweet-orange seedlings in nonsterilized soil: A, control; B, RJiizoctonia 



inoculated into soil layer in bottom of jar; C, RJiizoctonia as in B, plus Tricho- 



dcrma in top layer of peat (from Weindling and Fawcett). 



rots." The common soil and dust fungus Rhizoptis nigricans is known 

 to be the cause of soft rot of sweet potatoes. 



The plant-pathogenic actinomycetes are represented in the soil 

 by several species, the most important of which is Streptomyces 

 scabies, the causative agent of potato scab. The limiting reactions 

 for growth of these organisms are pH 5.5-6.0 and 9.0, with an opti- 

 mum at pH 7.0-7.5. A greater acidity, pH 5.0 or less, either con- 

 trolled or reduced the disease but did not eliminate the organism. 

 At pH 4.8 or lower, the potato plant as well was injured. An exam- 

 ination of some 100 fields for the presence of scab organisms in 

 Nebraska gave no field free from scab, 24-30 per cent causing de- 

 cided infection of the tubers. Previous cultivation of the soil, fre- 

 quent growth of potatoes, heavier soil, and larger numbers and 

 percentages of actinomycetes in soil corresponded with a higher 

 percentage of scab. 



