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Vol. XXII Washington, D. C, October 29, i923e^ew YtpNo. 5 



RELATION OF SOIL TEMPERATURE AND OTHER FAC- 

 TORS TO ONION SMUT INFECTION 



By J. C. Walker, Pathologist, Office of Cotton, Truck, and Forage Crop Disease Inves- 

 tigations, Bureau of Plant Industry, United States Department of Agriculture, and 

 Assistant Professor of Plant Pathology, University of Wisconsin, and L. R. JONES, 

 Professor of Plant Pathology, University of Wisconsin 



OCCURRENCE OF ONION SMUT IN RELATION TO CLIMATE AND 

 CULTURAL PRACTICES 



The onion smut fungus, Urocystis cepulae Frost, was first reported by 

 Ware {11) ^ in the Connecticut River Valley in 1869. At this early date 

 it was causing some injury to the onion crop, and in 1888 it was reported 

 by Thaxter (jo) to be of much importance in the old onion soils of south- 

 em New England. During the years which have since elapsed it has 

 successively appeared and become an economic factor in nearly all the 

 more westerly regions of intensive onion culture of the northern States, 

 from New York to Oregon. It is possible that this fairly rapid distri- 

 bution of the parasite has been occasioned to some extent by smut spores 

 carried with the seed, as already noted by Chapman {2) and Munn (7, 

 p. 412). It has, however, more probably been brought about by the 

 increasingly widespread distribution of onion sets. Many of these sets 

 are grown in the northern States on smut-infested soils, and since they 

 are shipped in quantity to all parts of this country, and even exported, 

 their role in the wide dissemination of smut spores is obvious. 



Chance introduction of the smut fungus in this way in the northern 

 commercial onion-growing sections seems almost certain to lead to its 

 permanent establishment. This evidently results from the fact that the 

 common intensive practice of continuous cropping with onions for an 

 indefinite term of years tends, when once the inoculum is introduced in 

 the soil, to favor its increase and wider distribution season by season until 

 it becomes a factor limiting further success with this crop. While this 

 holds true for the northern States, it does not seem to be so in the 

 southern sections. This is the more noteworthy since northern sets 

 grown on smutty soil are each year shipped into the southern onion 

 districts for propagation. This regional limitation of the smut fungus 

 was impressed upon one of the authors (Walker) in connection with a 



1 Reference is made by number (italic) to *'I,iterature dted," p. 261, 



Journal of Agricultural Research, Vol. XXII, No. s 



Washington, D. C. , Oct. 29, 1921 



aaf Key No. 0-250 



