Oct 29, 1921 Relation of Soil Temperature to Onion Smut 247 



EFFECT OF soil. TEMPERATURE UPON INFECTION 



At present the chief interest in these data focuses upon the question of 

 any possible bearing of the rate of development of the host plant at 

 different temperattires upon predisposition to, or escape from, smut 

 infection, recalling that such infection is practically limited to the seed- 

 Hng stage before the maturity of the cotyledon. It has just been noted 

 that the promptest seed germination and most rapid growth of tops 

 during this early seedling stage occur at fairly high temperature, 20° to 

 25° C, with a rather pronounced drop in rate of aerial growth at tempera- 

 tures below 20°. It is to be expected, therefore, that in the northern 

 onion-growing sections where the seed is planted in early spring there 

 will be a rather tardy germination and slow early development of tops, 

 the growth energies during the seedling stage being directed under this 

 climatic environment to a relatively stronger development of the root 

 system. In the South where the seed is planted in the comparatively 

 warm period of early autumn, we should expect a more rapid top growth 

 at the outset, with correlated strength of root development coming later 

 in the autumn as the soil becomes gradually cooler. 



The naturally infested soil was secured near Racine, Wis., from a badly 

 diseased field of sandy loam rich in organic matter. The soil which was 

 artifically inoculated consisted of a greenhouse mixture of loam and sand 

 to which were added fresh spores from smutty onion leaves. In order to 

 test the efficacy of this method of soil inoculation a preliminary planting 

 of onion seed was made in advance of the final experiments. This gave 

 a high percentage of smut infection, showing that the method of introduc- 

 ing the inoculum was satisfactory. Several early trials indicated that 

 below 25° C. soil temperature variations have little effect on the relative 

 amount of infection. The results of two such experiments, nearly cover- 

 ing the range of onion seed germination, are given in Table VI (experi- 

 ments I and 2). It is evident from these figures that abundant infection 

 occurred between 10° and 25°, both with naturally infested and with 

 artificially inoculated soil. The number of pustules per plant as shown 

 in Plate 25 proves that the fungus was very active even at low tempera- 

 tures. Above 25° infection is reduced very rapidly, as indicated by 

 both the percentage of infected plants and the number of pustules per 



plant. 



In order to determine more closely the point at which infection is 

 inhibited four more serial experiments were conducted (experiments 3 

 to 6) in which the temperature was kept as constant as possible at 2- 

 degree intervals between 25° and 31° C. The results given in Tables 

 VII and VIII, and illustrated in Plates 26 and 27, show that abundant 

 infection took place at 25° to 26°, while it was greatly reduced at 27° to 

 28° and completely inhibited at 29° or above. The infected seedlings 

 from experiment 5 show the great reduction in the amount of disease per 



