706 Journal of Agricultural Research voi.xx. No. 9 



on the same field for many years, results in a heavy infection of a large 

 part of the white set crop annually. Examination of a large number of 

 fields in Wisconsin and Illinois has revealed the fact that "clean" white 

 sets are secured as a rule only from land growing its first crop of onions. 

 In a majority of cases the second crop of white sets is badly infected. 



In all fields examined where the first crop of onions was being grown, 

 an occasional bulb infected with smudge was found. A satisfactory 

 explanation of these original infections has never been reached. Many 

 possible means of introduction of the fungus from neighboring infected 

 fields immediately suggest themselves, such as manure, farm imple- 

 ments, man and farm animals, drainage water, and wind, and undoubtedly 

 some of these often do play a part in the distribution of the disease. 

 The possibility of seed as a carrier is also to be considered in this con- 

 nection. Although smudge has never been found attacking the floral 

 parts of the plant, it is conceivable that those seed umbels which fall over 

 and come in contact with the soil before harvest might become infected 

 or be the means of introducing bits of infected scales to the seed. It 

 should be noted in this regard that the spores of onion smut, a disease 

 which is also confined to the bulb and leaves of the plant and in fact does 

 not attack onion seed plants, have previously been found on onion seed 

 samples (9, 18). 



One experiment was performed on the relation of seed to the dis- 

 semination of the fungus. Samples of six varieties of seed were sown 

 in pots of sterilized soil in the greenhouse on December 5, 191 6. On 

 January 16, 191 7, all the seedlings were examined. Fruiting bodies of 

 Colletotrichum circinans were found on the outer scales of two seedlings 

 of the White Globe variety and of one seedling of the Queen variety. 

 No other signs of the disease were found. The identity of the fungus 

 was confirmed by isolation of pure cultures and comparison with authentic 

 strains. Two subsequent plantings of the same sample of White Globe 

 seed were made, but no further sign of the disease was found. The small 

 amount of the fungus occurring in this experiment is not surprising, 

 since only a very limited amount of infectious material can be expected 

 to be seed-borne. However, although the evidence at hand indicates 

 that the fungus is carried on seed to some extent, further data are 

 necessary before a final conclusion on this point can be made. 



RELATION OF TEMPERATURE TO INFECTION AND TO DEVELOPMENT OF 



THE DISEASE 



Studies of the relation of temperature to the germination of conidia 

 and to their subsequent growth have shown the optimum to be about 

 20 C. for the former and 26 for the latter. The range in each case, 

 however, is wide. Accordingly a set of experiments was started for the 

 purpose of determining the range and optimum temperature for infection. 



Sterilized loam soil in glass or glazed crock jars was inoculated with 

 a water suspension of spores. Healthy white onion sets were then 



