VEGETABLE DISEASES ROUND TABLE 



195 



"The smut of onions first appears to the naked eye as a dark area 

 in the leaf, which finally bursts, letting out the black, powdery mass 

 of spores" (Thaxter). 



It may be readily recognized by the dwarfed and sickly condition 

 of the plant, and particularly by the presence of black smutted 

 masses in the leaves and on the bulb itself. In extreme cases, the 

 foliage is entirely killed and most of the bulb is converted into a 

 smutty mass. 



The disease is caused by a fungous parasite, Urocystis cepuloB 

 Frost. The smutty portion of the bulb is composed almost entirely 

 of spore balls of the fungus, a single smutted onion often containing 

 several million spores. The spore balls are exceedingly small, measur- 

 ing only eighteen to twenty microns (about ¥1200 of an inch) in diame- 

 ter. Although seed onions are not affected, the spore balls are very 

 frequently found adhering to the seeds of the onion, according to 

 Chapman ('10) of Massachusetts, and are doubtless commonly 

 carried from place to place in this way. The spores are very resistant 

 bodies, enduring extreme weather conditions and retaining their 

 vitality for many years. Professor Thaxter notes seemingly well 

 authenticated cases where the spores have lived in the soil for twenty- 

 five years. When a smut spore is placed under favorable conditions 

 of heat and moisture, conditions, for example, which are favorable for 

 the sprouting of onion seeds, it germinates by a hair-like tube. 

 This germ tube or thread may possibly penetrate the tender leaf 

 directly. It usually' branches and bears at the tips secondary spores, 

 which are short lived, but which germinate readily and thus increase 

 the possibilities of infection. The exact method of infection is a 

 matter now being studied by Mr. Jagger. It is at least certain, as in 

 the case of oats, that infection of the onion occurs while the plant is in 

 the seedling stage. This usually takes place before the first leaf is 

 out of the ground and rarely occurs after the plant is three or four 

 inches high. 



Just why all the spores do not germinate at the same time is not 

 quite clear, but it is certain that some lie for several years in the soil 

 and eventually produce infection. It might be thought that nearness 

 to a germinating onion seed would exert an influence on them, but 

 often they may be germinated in a drop of water on a glass slide. 



('10) Chapman, Geo. H. Notes on the occurrence of fungous spores on onion, 

 seeds. Mass. Agr. Exp. Sta. Rpt. 22: 1910. 



