Oct. 29, 1921 Relation of Soil Temperature to Onion Smut 237 



the main field. Therefore, if the organism were present and environing 

 factors were favorable, it does not seem probable that this method of 

 culture would completely inhibit the disease. Indeed, judging from our 

 experience with cabbage transplantation in relation to clubroot and 

 other soil- or seed-borne diseases, this method, instead of reducing the 

 trouble, is likely to serve as a ready means of distributing the parasites 

 with diseased seedlings from localized centers to wider areas. We are 

 thus forced to turn for explanation of the absence of smut in the South to 

 the third suggestion, that relating to climatic differences, bearing in mind 

 the respective cultural seasons. The most evident environmental dif- 

 ferences associated with the two types of culture relate to soil tempera- 

 ture and moisture during the time of seed germination and early seedling 

 development, which constitute the smut infection period. In the north- 

 em type, the spring-sown seed develops in a soil which is comparatively 

 cool and which has in general a relatively high and constant surface 

 moisture content. In the southern type, the summer-sown seed must 

 germinate and pass the early developmental stages in a soil of relatively 

 high temperature and subject to superficial desiccation. Our problem 

 has, therefore, necessitated an attempt to analyze and evaluate the pos- 

 sible factors associated with variations in soil moisture and soil tempera- 

 ture during the seedling stage. 



INFECTION PERIOD 



It has been of obvious importance in this study to know quite defi- 

 nitely the period in the development of the host at which infection actu- 

 ally occurs. Thaxter {10) gave critical attention to the time and manner 

 of infection, concluding that the fungus always invaded the young seed- 

 lings below the surface of the soil and that, by subsequent growth of the 

 host, the infected cells were commonly carried above the ground before 

 visible signs of the disease appeared. He also noted that onion sets and 

 onion bulbs replanted for seed growing were not attacked and suggested 

 that the seedling was probably subject to attack in only the early stages 

 of its development. Sturgis (9) later found that seedlings half as thick 

 as a lead pencil and about 5 inches high, transplanted into smutty soil 

 did not contract the disease. Sirrine and Stewart {8) , in an experiment 

 started at Jamaica, N. Y., on May 2, sowed eight rows of onion seed, 

 each ID feet in length, in soil free from smut. Alternate rows were left 

 as controls. Soil from a smut-infected field was introduced in three 

 ways: (i) in the furrow with the seed in two rows; (2) on the surface of 

 the soil after the furrow was closed in one row; and (3) in a fourth row, 

 on the surface of the soil 1 1 days after planting, this being shortly after 

 the seedlings appeared above ground. The control rows remained 

 healthy. Heavy infection occurred with the first treatment, slight 

 infection with the second, no infection with the third. It seems possible 



