Oct. 29, 1921 Relation of Soil Temperature to Onion Smut 251 



EFFECT OF HIGH AIR TEMPERATURE UPON THE DEVEI^OPMENT OF SMUT 



The experiments reported above in which the soil temperature was 

 varied were carried on at an air temperature of 15° to 20° C. The latter 

 is considerably lower than the air temperature which prevails in southern 

 onion sections at the time when young seedlings are starting off. This 

 is shown for one section (Laredo, Tex.) in Table XIII, where the mean 

 air temperature is about 30° during most of the onion-planting time. 

 The question arose as to what efifect these high air temperatures might 

 have upon the development of smut in the aerial portions of the plant. 

 Five clay pots of onions in naturally infested soil were started off in a 

 greenhouse running at about 25°. Previous observations had shown 

 that the pustules become evident in the cotyledons on about the twefth 

 day under these conditions. Accordingly the plants were allowed to 

 grow at this temperature for nine days, at which time a few lesions were 

 barely visible. In order to prove that good infection had aheady taken 

 place, 10 plants were removed from infested soil and washed thoroughly 

 in running water to remove any external inoculum, after which they were 

 transplanted to clean soil. Within two days lesions were distinctly 

 visible in these plants, and smut developed in 8 out of the 10. 



As a control on this method of removing the external inoculum 11 

 plants grown in clean soil were moistened and covered thoroughly with 

 infested soil. They were then washed in running water and trans- 

 planted to clean soil. No smut developed. On the ninth day after sow- 

 ing, 4 of the 5 pots were removed to a greenhouse running at 30° to 33° C, 

 one being allowed to remain at 25°. One pot was then transferred from 

 the higher temperature back to 25° at the end of each the second, fourth, 

 ninth, and fourteenth days. This exposure to the higher temperature 

 resulted in a stimulation of host plant growth for a few days. When 

 the plants were allowed to remain at this high temperature, however, 

 for three weeks distinct stunting became evident, while more prolonged 

 exposure resulted in death. The plants so transferred were allowed to 

 continue growth at 25° for three weeks or more, when they were examined 

 for the presence of smut. The final results are given in Table X. It 

 was evident that the gradual elimination of smut which took place was 

 proportional to the length of exposure to the higher temperature (29°). 

 After 14 days of exposure only small lesions developed on 16 per cent 

 of the onion plants, although presumably 80 per cent or more of these 

 plants were originally infected while they were growing at the lower 

 temperature. This experiment was repeated several times with prac- 

 tically the same results, namely, that exposure of plants bearing in- 

 cipient infections to a temperature of 30° to 33° for 12 to 15 days almost 

 entirely checked further development of the parasite. 

 65768°— 21 2 



