Oct. 29, 1931 Relation of Soil Temperature to Onion Smut 253 



^FFBCT OF MODERATEI/Y HIGH TEMPERATURES UPON SYSTEMIC INVASION 



OF THE PI^ANT 



Thaxter (jo, p. 134) observed that in some instances the smut fungus 

 may infect and develop in the cotyledon without invading the first leaf, 

 with the result that the plant eventually outgrows the disease. Obser- 

 vations lead us to believe that this may vary with different tempera- 

 tures. It has been pointed out that at a temperature of about 25° C. 

 the most rapid top growth of the onion seedling occurs, while at tem- 

 peratures below 20° the top growth is much retarded. Two pots of 

 infested soil were sown with onion seed and placed in greenhouses, 

 one at 24° to 28°, with a maximum of about 36°, for one or two hours 

 on sunny days, the other at 15° to 20°. A high percentage of cotyledon 

 infection occurred in both pots. After 31 days 24 out of 29 plants at 

 the high temperature were infected, but the pustules were all confined 

 to the cotyledons and no infection of first leaves had developed, although 

 the plants were now in the second leaf stage. At the low temperature, 

 on the other hand, of approximately the same number of plants, only 

 9 had survived on the thirty-seventh day, and 8 of these showed infec- 

 tion in the second leaves. It appears, then, that rapid growth of tops 

 at about 25° may result in a large percentage of plants outgrowing the 

 disease after the cotyledons become infected. The results of successive 

 field plantings, discussed in the next paragraph, seem to confirm this 

 judgment. The importance of the practical bearings of this matter are 

 such as to justify further critical attention. 



EFFECT OF SUCCESSIVE PLANTINGS THROUGHOUT THE GROWING SEASON 



UPON INFECTION 



The laboratory experiments described early in this paper have shown 

 that onion smut infection is greatly reduced where a constant soil tem- 

 perature of 27.5° C. is maintained during the susceptible period of the 

 plant's growth, while a temperature of 29° thus applied completely 

 inhibits infection. Moreover, as explained in the last paragraph, when 

 plants are growing in infested soil with temperature of air and soil 

 held at about 25°, although a high percentage of cotyledon infection 

 may occur, there is a greater tendency than at lower temperatures for 

 the plants to outgrow the disease, owing to the rapid growth of tops. 

 These results combined to justify the expectation that successive field 

 plantings of onion seed throughout the growing season might show 

 considerable variations in the percentage of smut infection. In the 

 onion field the soil temperature usually varies widely during 24 hours, 

 often reaching a maximum considerably above 29° during the day and 

 descending to a minimum much below this at night. Under Wisconsin 

 conditions the daily mean temperature gradually rises during the spring 

 and early summer months and falls during the latter part of the growing 



