Studies on the Downy Mildew of Onions 11 



Onion mildew causes losses to the crop in storage as well as in the field. 

 Murphy and M'Kay (1926) found that bulbs containing- perennial 

 mycelium of the onion-mildew fungus produced green shoots prema- 

 turely and then turned soft and rotted. These writers found that 

 such losses were common in the very susceptible varieties. The present 

 writer has noticed that in years when mildew was severe, many thick- 

 necked onions were produced. Such onions do not cure properly, and 

 rot in storage. 



Seed plants also may be attacked. In such cases the stalks are either 

 killed outright, causing a total loss of the seed, or they are weakened 

 to such an extent that the seed which is formed is of inferior quality. 

 Also, there are strong indications that the seed obtained from diseased 

 plants may be infected or infested and thereby introduce the disease 

 into the next crop. 



FIELD OBSERVATIONS ON THE OCCURRENCE OF ONION MILDEW 

 SIGNS AND SYMPTOMS 



The first signs of onion mildew on early-infected plants is the produc- 

 tion of conidiophores a .short distance back from the tips of the older 

 leaves. Murphy and M'Kay (1926) considered this a sign of systemic 

 infection. The conidiophores have a purplish tinge when fresh, and 

 give the lesions a downy appearance. Soon the affected part of the leaf 

 turns yellow, withers, and breaks over. Afterward the fungus may 

 fruit at any point on the leaves with the same ultimate result, the death 

 of the affected tissues. The writer's observations do not confirm 

 Murphy and M'Kay 's report, that the formation of the conidiophores 

 is preceded by a pale yellowing of the affected parts of the leaves. 

 Instead the yellowing was found to follow closely after the formation of 

 the fungus fruiting structures. 



The fungus may fruit on any part of the seed plants, but the fruiting 

 structures usually appear first on the leaves and then on the seed-stalks. 

 They may occur at any point on the seed-stalk from the base to the top, 

 and occasionally are found on the infloresence. Shortly after the 

 fungus has fruited, the epidermal and palisade cells of the affected parts 

 collapse and the resulting lesion becomes white, slightly sunken, and 

 roughened. The lesions are usually circular or elliptical in shape, and 

 involve only one side of the stalk. In some cases, however, they are 

 large and girdle the stalk. Lesions very similar in appearance often 

 result from mechanical injury, such as is caused by two stalks rubbing 

 together. Such lesions have been called the "white spot disease" 

 by Edgerton (1921), who, finding that the tissues were sterile 

 at first, suggested that they were probably physiological in nature. In 



