opinion among mycologists as to the constancy of the sterile layer of covering cells, 

 we prefer to retain for the onion smut organism the name which has been in contin- 

 uous use for the last fifty years. 



The true onion smut fungus U. cepulae Frost, does occur, however, on other 

 species of Allium. Chnton (8:451) reported it on specimens of A. Nevadense from 

 Nevada. In England the disease is said to be more severe on leeks (A . Porrum) than 

 on onions (13:170) and was reported from France on this host as early as 1881 

 (29:277). Quite recently (55:57) ZiUig has reported that he was able to infect A. 

 fistulosum L., A. glohosum Red., A. odorum L. and A. Porrum L. 



One of the writers, before he was aware that Zillig was working on the same 

 problem in Germany, began a series of inoculations by planting seed of all obtain- 

 able species in soil which was very heavily infested with U. cepulae. Records taken 

 every week after the plants came up gave the total percentage and severity of smut 

 infection for each species. A full description of these experiments and the results 

 are published elsewhere (4) . Briefly, it was found that out of 39 species which were 

 tested, 8 proved to be immune to smut, wliile 31 showed varjdng degrees of suscepti- 

 bihty. Thirteen species were just as susceptible as the common onion in that the 

 disease affected them in just the same way throughout the season. In five others it 

 caused heavy mortahty in the cotyledon stage but did not persist tliroughout the 

 season. In 13 others the cotyledons were sometimes affected, but the plants rarely 

 if ever died from the effects of smut. These results lead us to beMeve that if the 

 numerous other species — there are some 250 species in the genus — were tested in the 

 same way, a large proportion of those which reproduce by seeds would be found to 

 be susceptible. (Many species escape the disease by reproducing only by bulblets.) 



Within the species Allium Cepa, the cultivated onion, there are numerous varie- 

 ties, but up to the present there are no data or observations to indicate that any of 

 them show any degree of resistance to smut. It has been stated at various times, 

 however, that the white varieties are somewhat more susceptible than the red or 

 yellow ones. Walker and Jones (50:236) tested several varieties, and Whitehead 

 (52:449) tested 21 varieties of onions and 11 varieties of leeks (A. Porrum) but none 

 was foimd which showed resistance. Anderson (4) tested 54 varieties of cultivated 

 onions but found no significant indication of resistance among them. 



Morphology and Life-History of the Smut Fungus* 



If one examines microscopically the dusty black mass from the leaves or bulbs, 

 mentioned above, he ■will find that each particle of dust is a spore (chlamydospore) 

 of the fungus which causes smut. It is a compound spore composed of one large 

 central brown cell, to the surface of which are attached 15-40 smaller hemispherical 

 cells (see Fig. 3A). The dark central cell is the fertile cell, while the numerous 

 smaller surrounding cells, which are transparent, but with a brownish tint, are the 

 sterile cells (pseudospores or accessory cells) . 



The spores get into the soil either by the rupturing of the sorus (pustule) or by 

 decay of the infected part of the onion which has fallen to the ground. In the soil 

 they may germinate at once or after a few weeks or months, or they may remain for 

 years before germination occurs. In either case they grow into long slender branch- 

 ing tubes (the myceUum) and in a favorable soil this growth may be sustained for 

 many years even though onions are not planted again on the same field. The 

 mycelium does not produce other spores (sporidia or conidia) but it may break up 

 into short pieces wluch are able to lie dormant for long periods and then germinate 

 on the return of favorable conditions. The soil becomes so infested with spores, 

 mycehum and these detached myceUal segments that without preventive measures 

 onions can no longer be raised on it. 



Method of germination of the spores. In a previous publication Anderson (1 :108) has 

 described the process of germination as it occurs during the first few months after ma- 

 turity of the spores. Briefly it is this : A globose hyaline body, at first no larger than one 

 of the sterile cells but later becoming as large as the fertile cell, develops on the surface 

 of the fertile cell (Fig. 3B). From this body one to eight germ tubes grow out (Fig. 



*In a previous publication from this station (1) the reader will find more detailed information on the full 

 life history of Vrocystis cepulae. In the present publication the seasonal life-cycle is outlined very briefly. 



