Oct. 29, 1921 Relation of Soil Temperature to Onion Smut 241 



An experiment was therefore carried out in which onion seedlings 

 were grown in smut-infested soil in pots which were held at different 

 degrees of soil moisture. Galvanized iron pots 5 inches in diameter and 

 4 inches deep were used for these trials. Greenhouse sandy loam soil 

 was used and its water-holding capacity was determined in advance by 

 the two standard methods recommended by soil physicists ^ — that is, 

 by means of the lo-inch cylinder and the i-cm. cup. The soils were 

 brought to the desired low and medium water contents before they were 

 placed in the culture pots ; and in those cases where the desired moisture 

 approached the water-retaining capacity, the water content was finally 

 adjusted after the soil was potted. Although these methods failed to 

 secure exact uniformity in the physical compactness of the soils in the 

 several series, they were considered satisfactory as to initial moisture 

 conditions. The pots were weighed daily during the progress of the 

 experiment, and water was added to replace the losses. Since it was 

 realized that the surface layer of soil would change in moisture content 

 through evaporation more rapidly than the lower layers, an effort was 

 made to reduce this surface evaporation so far as practicable. To this 

 end, tar paper covers were used until the seedlings came above ground, 

 when glass covers were substituted for a few days, and finally mineral 

 wool was packed on the surface about the seedlings to reduce evaporation. 

 Absolutely uniform moisture throughout the pot could not be maintained 

 even by this method, and the upper layers of soil unavoidably assumed 

 a somewhat lower water content than the average for the pot. There- 

 fore, at the end of the experiment, moisture determinations were made 

 of the upper inch of soil, since this was the important part from the 

 standpoint of smut infection. 



The soil was inoculated at the outset by the introduction of spores 

 from diseased leaves and scales. The data from this experiment are 

 given in Table III. Good germination took place within the range of 

 10 to 15 per cent moisture content (45 to 70 per cent of the moisture- 

 holding capacity). A high percentage of infection also occurred within 

 this range. Above 15 per cent there was some decrease in germination 

 together with a gradual reduction in infection. At the extreme, how- 

 ever, where germination of seed was practically eliminated (pot i), one 

 of the two plants surviving became infected. It is evident from these 

 data that a good percentage of infection may be expected at a soil mois- 

 ture content up to the limit where good germination and growth of the 

 host plant occur. There was a gradual reduction of infection below 5 

 per cent (23 per cent of the moisture-holding capacity), but this was not 

 sufficient to insure good germination and support good growth. It 

 may be concluded, therefore, that soil moisture does not function as a 

 factor limiting infection with onion smut within the limits at either 

 extreme where good germination and growth, of the host occur. 



1 We are especially indebted to Prof. E. Truog, of the Department of Soils of the University of Wiscon- 

 sin for advice in connection with this work. 



