Feb. i, 1921 



Onion Smudge 707 



inserted in the soil; and the jars, each covered with a glass plate, were 

 placed in incubators running at temperatures ranging from 5 to 32 . 



In the first experiment 10 onions were placed in each of four jars 

 which were placed in incubators held at 5 , 13 to 14 , 23 , and 28 to 31 

 C, respectively. The extent of the disease on the various lots at this 

 time is shown in Plate 82. It was apparent that infection took place 

 very slowly at 13 to 14 , while that at 28 to 31 ° was slightly less ad- 

 vanced than at 23 . 



In the second experiment jars containing 10 onions each were held at 

 5 to 6°, 9 to io°, 14 to 15°, 17 to 18 , 20 to 21. 5 , 22 to 23 , 26 to 

 27 , and 30 to 32 C. They were allowed to remain for 17 days before 

 examination. At the end of this period, no infection had taken place 

 at 5 to 6°, a very slight infection at 9 to io°, and as the temperature 

 rose the amount of disease increased up to 2 6° to 27 , at which point it 

 was greater than in any of the other jars. At 31 to 32 it was slightly 

 less than at 26 to 27 . A third experiment confirmed the results of the 

 first two. 



Infection takes place and the disease progresses, then, at or above io° 

 C, but it is quite evident that for very rapid development a temperature 

 of 20 or above is needed. Since the fungus develops in the soil prior to 

 infection, the range of soil temperature during the growing season is 

 undoubtedly an important factor in determining the severity of the 

 disease. 



PRODUCTION AND DISSEMINATION OF CONIDIA 



After the appearance of the first stromata on the bulbs, subsequent 

 spread of the disease is effected to a considerable extent by conidia 



Sporulation does not take place except under fairly humid conditions- 

 In order to determine the range of temperature at which fructification 

 may occur, infected scales were placed in Petri dishes lined with moist- 

 ened filter paper and exposed in incubators running at a range of temper- 

 atures from 2 to 28 C. Abundant sporulation occurred within 36 hours 

 at 20 to 28 . The process was much retarded at lower temperatures, 

 though a few spores were formed at 2 to 3 after several days. 



Under optimum conditions for spore production the conidia accumu- 

 late on top of the acervuli, forming gelatinous masses which remain in- 

 tact among the setae. Exposure of portions of scales bearing fresh spore 

 masses over sterile agar plates has yielded no indication of spore dis- 

 charge. The mucilaginous material surrounding the spores appears to 

 dissolve partly when a spore mass is placed in water, and the conidia 

 thus become separated. 



It is thus to be expected from the nature of the fungus that warm, 

 rainy weather is especially favorable for the development of smudge, 

 since high humidity promotes the production of spores, and meteoric 

 water, especially in the form of spattering rain drops, is important for 

 their dispersion and dissemination. 



