• White downy growth appears on the surface of the leaves.
• Finally the infected leaves are dried up.

• The sporangiophores are non septate, long and swollen at the base.Sporangia are pyriform to fusiform, attached to the sterigmata by their pointed end. These sporangia germinate by one or two germ tubes. The coenocytic mycelium is intercellular with filamentous haustoria. Oogonia are formed in the intercellular spaces.

Management:

• Three spraying with Mancozeb 0.2 % is effective.
• Spraying should be started 20 days after transplanting and repeated at 10-12 days interval.

Source:
Image source: www.seminis.com/sitecollectiondocuments/onion-disease-guide.pdf
Microscopic view: http://web2.mendelu.cz/af_291_projekty2/vseo/stranka.php?kod=125