Studies on the Downy Mildew of Onions 15 



systemically infected plants 



Murphy (1921), Murphy and M'Kay (1926), and Katterfeld (1926) 

 reported that a large number (approximately 45 per cent) of the onion 

 bulbs from fields in which the disease was severe were systemically 

 infected with perennial mycelium of the mildew fungus. They pre- 

 sented evidence that this perennial mycelium is one of the principal 

 sources of primary inoculum where onions are sown in the fall and 

 allowed to overwinter in the field. Since this cultural practice is not 

 followed at Elba, fall-grown onions could not be a source of primary 

 inoculum in this section. 



Perennial mycelium in onion sets, seed plants, and volunteer onions 

 may be a source of a part of the primary inoculum in the area where 

 these investigations were made. This is suggested by the fact that 

 the fungus fruits on sets and seed plants several weeks in advance of 

 its appearance on market onions. Mildewed sets and seed onions can- 

 not be considered the main source, however, in the Elba section because 

 there are comparatively few plantings of these on the muck, and the 

 seedlings become diseased as early in the fields at a distance from the 

 sets and the seed onions as in those near by. Although no volunteer 

 onions have been observed to be infected, it is conceivable that some 

 of them are diseased and furnish a part of the primary inoculum. 



Tubeuf (1895) stated that the onion-mildew fungus occurs on wild 

 species of Allium, but did not mention the specific names of the sus- 

 cepts. The only wild species of this genus at Elba are wild leeks ; and, 

 since these were never observed to have mildew, it is very doubtful 

 whether they are of any importance as a source of primary inoculum. 



INFESTED SOIL 



Oospores in the soil are probably one of the sources of primary in- 

 oculum. Although Dudley (1889), Murphy and M'Kay (1926), and 

 many other writers on onion mildew, stated that oospores are rarely 

 formed, Shipley (1887), L. R. Jones (1896, 1897), Whetzel (1904), 

 Katterfeld (1926), and Hiura (1930 a) reported having found them 

 in relatively large numbers. Jones (1897) determined experimentally 

 that plants grown on oospore-infested soil which had overwintered be- 

 came diseased, while those on clean soil did not. In spite of the lack 

 of agreement on the importance of oospores, they have been observed 

 in large quantities by a sufficient number of capable workers to estab- 

 lish their importance as a means of propagating the fungus. 



Conidia are found in the soil, but, since they are very short-lived, 

 it is unlikely that they are of importance in producing infection. 

 Under the most favorable conditions conidia remain viable for only a 



