Downy mildew disease of Onion
Downy mildew of onion is caused by fungus called Peronospora sp.
Main symptom is that White growth appears on the lower surface of the leaves.
The downy mildew fungus produces spores called sporangiophores
These sporangiospores are are non septate, long and swollen at the base
The main sources of perennial survival of the fungus are the diseased bulbs used for propagating the crop in many areas and oospores present in diseased crop residues.
If infected bulbs are planted, the fungus grows up with the foliage produces sporangia and these spread the disease to other plants.
It is suspected that spores of the fungus that persist in the soil may directly infect the roots of young onion plants.
The downy mildew fungus over winters as mycelium
When favorable environmental conditions occur, the over wintering fungal mycelium in systemically infected plants produces spores.
After dissemination through the air, these spores infect the leaves of onion plants in commercial fields.
Spores are formed at night when high humidity and temperatures of 4–25˚C occur, with an optimal temperature of 13˚C.
The spores mature early in the morning and are disseminated during the day.
Spores remain viable for about 4 days.
Germination occurs in free water from 1–28˚C with an optimal range of 7–16˚C.
Rain is not needed for infection if heavy dews occur continuously during the night and morning hours.
To control downy mildew, three spraying with Mancozeb 0.2 % is effective. Spraying should be started 20 days after transplanting and repeated at 10-12 days interval.