SOIL TEMPERATURES IN PHYTOPATHOLOGY 231 



Orton, discussing the potato plant in its relation to disease, 

 has pointed out (6, 7) that the Fusarium \\dlt of the potato has a 

 southern range as compared with the similar Verticillium mlt 

 of the northern regions. Haskell of Cornell University (in 

 correspondence) states that his studies lead him to the conclu.sion 

 that ''soil temperature is the most important limiting factor in 

 the development of Fusarium wilt of potato in New York State." 



Link (8) in connection with his physiological studies of two 

 strains of Fusarium in their causal relation to tuber rot and 

 vine wilt of potato finds that F. oxysporum has a higher opti- 

 mum temperature than F. trichothecoiodes and concludes that 

 this may explain the fact that the first is the cause of field wilt 

 under warm soil conditions whereas the latter develops as a 

 tuber rot under cooler storage temperatures. 



Humphrey (9) working in the state of Washington concluded 

 that the occurrence of the Fusarium tomato blight of the Pacific 

 Northwest (Fusarium orthoceros and F. oxysporum) is condi- 

 tioned upon high temperatures. \\Tiile he recogTiizes as pos- 

 sible factors au- temperature and winds in relation to trans- 

 piration, he concludes that it is when the soil temperature rises 

 too near the optimum for the parasite that the disease results. 

 He finds that these tomato parasites show infective powers at 

 18°C. (65° F.) and that their virulence increases with rise of 

 temperature to their optimum 30°C. (86° F.). The writer (10) 

 and Gilford (11) have recorded the association in Vermont of the 

 Fusarium damping-off in coniferous seedlings with high soil 

 temperatures. 



These things, together with the prevalence of Fusarium root 

 diseases in the southern states, seem to justify Wollenweber's 

 generahzation (12) that the root-invading Fusariums are warm 

 soil organisms. 



The smuts. The grain smuts constitute another group of 

 parasites where facts of similar significance are available. With 

 these the period of possible infection is limited to a brief stage 

 in the early development of the host seedling. The possible 

 influences of soil temperature upon host as well as parasite are, 

 therefore, involved. 



