808 PRINCIPLES OF SOIL MICROBIOLOGY 



This organism may cause a potato wilt disease. F. lycopersicum, can also live as a 

 saprophyte in the soil, upon the dead stems of the wilted tomato plants and on the 

 soil organic matter; it can live in the soil several years retaining its virulence, even 

 without the host plant. 25 The same is true of F. conglutinans and F. lini. F. 

 hyperoxysporwn and F. batatatis, causing the stem-rot of sweet potato, may 

 also be generally disseminated in the soil; also other pathogenic Fusaria. 26 



Among the other plant pathogenic fungi which can find their habitat in the 

 soil, we may include different species of Rhizoctonia. 27 Rh. solani, for example, 

 is abundant in cultivated land, where it lives on dead organic matter in the soil. 

 When a proper host is introduced, the organisms may become active parasites, as in 

 the damping-off of carnation cuttings, stem-rot and potato diseases; they can also 

 attack a variety of weeds. Rh. solani attacks as many as 165 species of plants. 

 Spongospora subterranea causes powdery scab of potatoes. Thielaina basicola 

 causes root rot of tobacco, legumes and many other plants. Synchitrium 

 endobiolicum produces the wart disease of the potato and its spores may re- 

 main in the soil for two to eight years. 28 Asterocystis radicis attacks flax. 

 Urophlyctis alfalfae produces swellings on the roots of alfalfa. Aphanomyces 

 laevis causes "Wurzelbrand" on beets. Pythiacystis citrophthora causes the brown 

 rot of the lemon. Ozonium omnivorum produces root rot on cotton and alfalfa. 

 Sclerotium rolfsii can propagate itself by mycelium in the soil, forming sclerotia 

 under unfavorable conditions. Rosellinia necatrix and Melanospora 29 must also be 

 mentioned. Cercospora personata is capable of multiplying in the soil saprophyti- 

 cally, preserving its virulence for eleven years. 30 Various smuts are often found 

 in the soil and may persist there for long periods of time ; however, the extent to 

 which rusts may persist in the soil has not been established yet. 31 



Cabbage and tomato sick soils may show as many as forty thousand 

 colonies (on plate) of the parasitic organisms per gram of soil. 32 On 

 land showing much root rot of corn, Fusarium moniliforme and a Cepha- 

 losporium have frequently been found. Trichoderma koningi and Tr. 

 lignorum, two of the most common saprophytic soil fungi, are the causes 

 of storage rots of sweet potato; the former is also associated with the 



25 Scott, 1924 (p. 811). 



26 Harter, L. L., and Field, E. C. The stem-rot of sweet potato (Ipomea 

 batatus). Phytopathol., 4: 279-304. 1914. 



27 Peltier, G. L. Parasitic Rhizoctonias in America. 111. Agr. Exp. Sta. Bui. 

 189. 1916. 



28 Schander and Richter. tJber den Nachweis von Dauersporen von Chryso- 

 phlyciis endobiotica Schill. (Kartoffekrebs) in der den Kartoffeln anhaftenden 

 Erde. Centrbl. Bakt., II, 58: 454-461. 1923. 



29 Delacroix and Maublanc. Maladies parasitaires des plantes cultivces. 

 Bailliere Ed. 



30 Miege, E. Le disinfection du sol. Paris. 1918. 



31 Klebahn, cited by Waget, P. Sterilization et disinfection du sol. Rev. 

 Prod. Chim., No. 22. 1920, 655; No. 4. 1921, 115; No. 6, 183. 



32 Manns, T. F. Soil bacteriology. Del. Agr. Exp. Sta. Bui. 133, 35-36. 1922. 



