REED: PHYTOPATHOLOGY 161 



nection with the rusts that the aecial host might be a meeting place for different 

 races, resulting in a changed capacity for infection in the uredial stage. It is 

 now known that on the aecial host hybridization of the races of the pathogen 

 may take place, and thus new races arising might differ greatly in their capacity 

 for infection. 



Reddick and Mills (1938), in connection with the potato blight organism, 

 have suggested that when it is grown on partially resistant hosts, it may ac- 

 quire an ability to infect a wider range of varieties, thus bringing to the fore 

 again the question of bridging hosts. 



3. Environal factors. Before 1867 the view was that environal factors 

 were the principal cause of plant diseases and, as a corollary of this, the fruit- 

 ing bodies of the fungi which appeared upon the plant followed the disease. 

 Epiphytotics, such as the potato blight in the 1840's were largely attributed to 

 the weather. 



We now recognize the very great importance of environal factors as pre- 

 disposing the appearance of a diseased condition ; in fact, three different things 

 are necessary: (1) a susceptible host, (2) a causal agent such as a fungus, 

 bacterium, or virus, and (3) environal factors that are favorable for the es- 

 tablishment of the relation between the two. We must emphasize the interrela- 

 tions of environal factors, including soil temperature, moisture, and reaction, 

 since it is impossible to find a fixed optimum for any one, regardless of the 

 possible associated variables. 



While we know that the real cause of many diseases is due to specific or- 

 ganisms, we also know that particularly disastrous epiphytotics occur only 

 under peculiar environal relatiqns. Jones, Giddings, and Lutman (1912) 

 worked out the relation of weather conditions to the development of potato 

 blight. The prevalence of wheat bunt depends upon low soil temperature at the 

 time of seeding. Oat smuts are not as destructive, ordinarily, in the Eastern 

 United States as in the Western. 



Intensive studies on the relation of environal factors to plant diseases have 

 been made. The relation of temperature and moisture to the infection of wheat 

 by the two species of Tilletia was made by Hecke (1909), Heuser (1922), 

 Munerati (1922), Hungerford (1922), and Faris (1924). Paris (1924) stud- 

 ied the temperature and moisture relations for infection of barley by the 

 covered smut, Bartholomew and Seymour (1923) for the loose smut of oats, 

 and Reed and Faris (1924) for the covered smut of oats and the loose and 

 covered smuts of sorghum. 



On the establishment of the Department of Plant Pathology at the Univer- 

 sity of Wisconsin, Professor L. R. Jones and his students conducted exten- 

 sive studies over a period of years, with elaborate equipment, on the influence 

 of environal factors on the development of many plant diseases. While empha- 



