242 



journal of Agricultural Research voi. xxii, ^fo. s 



Table III. — Relation of soil -moisture to infection by Urocystis cepulae 



Pot No. 



2. 

 3- 

 4- 

 S- 

 6. 



7- 

 8. 



9- 

 10 

 II 

 12 



13 

 14 



Original 

 moisture 

 content 



(percentage 

 of dry 



weight)." 



Moisture 

 content of 

 surface lay- 

 er at end of 

 experiment. 



19.6 

 17. 1 

 16.3 

 18.2 

 20.0 

 15.0 

 15.0 

 10. o 

 10. o 

 10.7 



5-0 

 8. I 



7-5 

 6.6 



27. 1 

 24.7 

 23-7 

 23-4 

 14.0 

 12.4 

 II. 6 



9-3 

 9.0 

 2.4 



2-5 



1.4 

 3-4 

 1-3 



Number of 



seeds 



planted. 



100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 



Total num- 

 ber of 

 plants. 



52 

 48 

 50 

 60 

 66 

 66 



71 

 66 



32 

 66 



63 



37 



Percentage 

 of plants 

 smutted. 



50 



O 



42 



71 

 90 



97 



85 

 94 

 59 

 50 

 19 

 35 



« Water-holding capacity, as determined by lo-inch cylinders, was 22.3 per cent; as determined by the 

 i-cm. cup, it was 27.8 per cent. 

 The calculated wilting coefficient of the plants was 2.3 per cent. 



TEMPERATURE RELATIONS 



In calling attention to the importance of soil temperature as a factor 

 in the development of certain plant diseases, Jones (5) points out that 

 several investigators have stressed its bearing upon infection in the case 

 of stinking smut of wheat, Tilletia triiici (Beij.) Wint., and of the oat 

 smuts, Ustilago avenae (Pers.) Jens, and Ustilago Levis (K. and S.) Mag. 

 Heald and Woolman (j) showed that the amount of infection with the 

 stinking smut of wheat was reduced as the mean soil temperature rose 

 above 65° F. (18.3° C.) or fell below 40° F. (4.4° C). Humphrey (4) 

 states, for the same disease, that soil temperatues of 0° to 5° C. and 

 above 22° C. are decidedly unfavorable to infection. 



In studying the relations of soil temperature to the developemnt of a 

 parasitic disease, consideration must be given to the possible influence 

 of such temperature upon the host and the parasite independently. This 

 may enable one to analyze with more confidence the effects when host 

 and parasite are subjected simultaneously to the experimental condition. 

 This has been done as far as practicable in connection with the present 

 work. 



Unfortunately, germination of the fungus spores under artificial con- 

 ditions has been so scanty that the effect of temperature upon the fungus 

 has been necessarily limited to inoculation experiments with infested 

 soil. However, the disease is produced so readily and consistently by 

 the latter method that a very accurate index to the development of the 

 fungus can be secured by varying the condition of the infested soil dur- 

 ing the germination and early growth of the onion seedling. 



The soil-temperature experiments were all carried out in the green- 

 house at Madison, Wis., during the winter months. The apparatus in use 



