Studies on the Downy Mildew of Onions 



17 



of seed transmission, has already been mentioned in the discussion of 

 infested seed. Such distribution may also indicate infected seed. 

 Another reason for thinking that infected seed may have a part in the 

 spread of onion mildew is that seed plants are frequently attacked by 

 the mildew. This was observed at Elba, New York, and onion mildew 

 has been reported nearly every year as causing heavy losses in the 

 onion-seed-producing sections of California. 



In order to determine whether or not seed infection actually occurs, 

 close attention was given to the plantings of seed onions. Although 

 seed are not produced commercially at Elba, a few are grown each year 

 by some of the growers for their own use. 



The seed plots were closely observed and many flowers and seed were 

 sectioned, but it was not until 1928 that proof of floral infection was 

 obtained. At that time a single seed-stalk having an infected inflo- 

 rescence was found. This stalk had a mildew lesion just below the flower 

 umbel, and the fungus was fruiting sparsely on some of the flower 

 pedicels. Microscopic examination of the conidiophores showed the 

 fungus to be Peronospora destructor. Sections of the seed-stalk, the 



Figure 2. cross section of a pedicel of an infected onion flower, showing the 

 intercellular hyphae and the intracellular haustoria of the fungus 



