Studies on the Downy Mildew of Onions 



21 



flowers, from diseased shoots contained abundant hyphae. He did not 

 report the presence of the fungus in the ovules, however. 



Katterfeld (1926), Murphy and M'Kay (1926), and Hiura (1930a) 

 obtained negative results in experiments on seed transmission. In the 

 two latter investigations, seed from infected plants were planted but 

 none of the seedlings developed the disease; while Katterfeld made a 

 microscopic examination of the seed from diseased plants and was un- 



FlGURE 6. 



LONGITUDINAL SECTION OF AN OVULE OF AN INFECTED ONION FLOWER, 

 SHOWING A MAT OF MYCELIUM IN THE TISSUES AT THE BASE 



able to find the fungus. The results obtained by these investigators 

 are not surprising, since it is probable that only a small pro- 

 portion of seed from diseased plants is infected. Although many 

 of the seedstalks may bear several mildew lesions, it is only rarely 

 that the fungus is found on the flower pedicels. Even when it is found 

 fruiting on some of the flower pedicels, usually others in the same umbel 

 are not invaded. It is probable also that only a small number of the 

 invaded flowers mature viable seed. 



