Downy Mildew of Onion (Chris Tyson, Tattnall County Extension Agent)

Downy Mildew of Vidalia Onion (Peronospora destructor) was confirmed in the Vidalia Onion growing region on March 19, 2018.  This can be a highly destructive disease, causing serious losses in yield and bulb quality.  Growers need to be aware of its presence in the area so they can make decisions regarding protection of their crop.  This disease does not survive year round in our region, but instead moves in the area from wind currents from warmer areas.  When it does make its way to our area, growers need to be prepared to protect their crop with the proper fungicide sprays.  Here are some pictures that I took of the disease on onions this week:

Visible Sporulation of Downy Mildew on Vidalia Onion Leaf. Photo Credit: Chris Tyson

Visible Sporulation of Downy Mildew on Vidalia Onion Leaf. Photo Credit: Chris Tyson.

400X view of sporangiophores and sporangia of Downy Mildew of onion, Peronospora destructor. The branching sporangiophores resemble “antlers” are a key diagnostic feature in determining if the disease is present. Photo credit: Chris Tyson.


Treatment Options

(Provided by Cliff Riner, UGA Area Onion Agent)  As far as products that can be used,  know that there are numerous products with a label stating control of downy mildew.  However, I want to provide some options and how I rank them in terms of potential.  I use these as a TIER system of better products to somewhat beneficial.  Below are the products and a brief justification.  Hopefully this information can be utilized to help you make a decision, if the time comes that you want to spray for prevention of downy mildew.


Tier 1 (Most Reliable, based on Data)

Reason `             Reason was the most effective at slowing Downy Mildew in a local trial that was conducted the year that the disease was so strong.

Zampro               Zampro was statistically as effective as Reason in that same trial.

**Orondis             Orondis is a new product from Syngenta.  **Other trials have suggested that this product has very good efficacy on Downy and some residual activity, however we have no local UGA data to clarify this either way.



Tier 2 (Moderate)

Omega 500        Omega has shown benefit in the UGA trials, however it was slightly less effective than the two above.  However, Omega is also very effective at preventing other fungal pathogens.

Ridomil Bravo   Ridomil Bravo has multiple states with documentation of some control of downy.  It has never been excellent, but it also is a cheaper option than most above.


Tier 3 (Some benefit)

Phosphoric Acid              Phosphoric acids such as Phostrol, ProPhyt, and others have shown to be slightly effective at preventing downy in some studies. They are a cheaper option, and have some benefit to the overall fungicide program.

Maneb                              Dithane, Pencozeb, etc… the maneb products are listed as somewhat effective.  I wouldn’t count on much help if we get to the point of seeing downy, but they may be offering some prevention.

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