Oct. 29, 1921 Relation of Soil Temperature to Onion Smut 259 



varying somewhat with environment. Cotyledons remaining free from 

 infection during this period become resistant and serve as a barrier to 

 subsequent invasion of the embryonic region of the true leaves. Conse- 

 quently, if infection is prevented by environing conditions during this 

 period of susceptibility, the plant will remain free during the remainder 

 of its growth. 



Experiments were conducted in which seedlings were grown on smut- 

 infested soil held at various soil moisture contents. A high percentage 

 of infected plants resulted over the entire range in which good germina- 

 tion and growth of the host occurred. At either extreme, very high or 

 very low moisture, there was some reduction in amount of infection, but 

 with it occurred a corresponding decrease in seed germination and rate 

 of growth of the plants. Soil moisture, therefore, does not appear as a 

 serious limiting factor in onion smut infection. .^rj v/ot; 



The relation of soil temperature to the development of the host and 

 the parasite was studied by growing plants in pots held experimentally at 

 a series of constant soil temperatures in the special apparatus known as 

 the "Wisconsin soil temperature tank." 



Seed germination and growth took place over a range of soil tempera- 

 ture from 10° to 31° C. Most rapid seed germination and development 

 of tops occurred at soil temperatures of 20° to 25°, while as a rule the 

 best development of roots occurred below 20°. 



A high percentage of plants grown on smutted soil were infected at 

 soil temperatures ranging from 10° to 25° C. A decided reduction in 

 infection was noted at about 27°, and complete freedom from the disease 

 resulted at 29°. In these experiments all plants were under uniform 

 conditions of air temperature, which ranged from 15° to 20°. 



The relation of variations in air temperature to the development of 

 the disease was then studied. 



Exposure of plants bearing incipient infections of the fungus in the 

 aerial parts to an air and soil temperature of 30° to 33° C. so disturbed 

 the relations between parasite and host as to preclude any further develop- 

 ment of the disease. This was shown by growing plants at a tempera- 

 ture favorable for infection (15° to 20°). Then, just as the pustules of 

 the disease were beginning to appear (tenth to twelfth day), the plants 

 were removed to a room held at 30° to 33°. This stimulated top growth 

 for a few days, which was followed by a decided checking of the plants 

 and death after three or four weeks. However, if after 12 to 15 days 

 at the high temperature the plants were returned to the original tem- 

 perature (15° to 20°), they grew normally, but the fungus in nearly all 

 cases failed to produce spores, and the plants remained free from further 

 invasion. 



Experiments were then performed in which seedlings were grown on 

 infested soil held at 20°, 25°, and 30° C. with a uniform air temperature 

 of 30° to 33°. A high percentage of infection resulted at soil temperatures 



