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Soybean Downy Mildew (07/30/15)

We have received dozens of calls about soybean downy mildew in the last week. This is a disease that is often seen in soybeans, but generally occurs at very low levels and rarely causes economic yield loss.

Soybean Downy Mildew

We have received dozens of calls about soybean downy mildew in the last week.  This is a disease that is often seen in soybeans, but generally occurs at very low levels and rarely causes economic yield loss.  However, wet weather and temperatures have been favorable for disease development and we are seeing more downy mildew than normal. We are writing this article to explain what we know and what we do not know about soybean downy mildew management.

Cause.  Importantly, soybean downy mildew is not caused by a fungal pathogen, but rather an oomycete pathogen, Peronospora manshurica.  This is very important for management, which we explain at the bottom of the article.  The pathogen is in the same group of pathogens that cause sunflower downy mildew, Phytophthora root rot of soybean and late blight of potato.  Like those other pathogens, the pathogen that causes soybean downy mildew has swimming spores and needs lots of humidity/water to cause problems. 

Symptoms and signs.  Downy mildew initially shows up as small, pale yellow spots on the upper leaf surface (Figures 1 and 2).  As the downy mildew lesions develop they may become light gray or brown and sometimes have a yellow margin.  These are easily confused with other leaf spots, especially Bacterial blight and Septoria brown spot (Please refer to the previous article to help distinguish among diseases).

The key to distinguishing downy mildew is the formation of gray fungal-like tufts that develop on the underside of the leafs, directly opposite the lesions on the top side of the leaf (Figure 3). 

Disease development.  The downy mildew pathogen survives as thick-walled spores; these can occur on soybean debris or on seed.  Once infection begins, which likely started very early this year, spores are produced on the underside of the leaves and will blow from plant to plant and from field to field.  Wind driven rain will facilitate dispersal and infection.  High humidity, heavy dews and temperatures from approximately 50o to 80o are favorable for disease development. 

Yield loss.  Downy mildew is considered a minor disease and few instances of yield loss are documented; however, yield losses of up to about 15% have been reported in rare cases.  Similarly, some seed quality issues have been reported; the pathogen can cause seeds to have a slightly dull white appearance (again, rare).  Unfortunately, the level at which yield and quality loss occurs is very poorly documented. 

Fungicides.  Here’s the tricky part… 

Other crops where downy mildew pathogens are a problem (i.e. hops, grapes, cucumbers, lettuce, etc.), the disease can be managed with fungicides.  However, the fungicides used target oomycete pathogens and are not generally available for soybeans; such as Revus, Zampro, Forum, Reason and Presidio.   Growers who have experience with potatoes may recognize them for late blight (Phytophthora) management, and will know that these are relatively specialized products and often applied in a fungicide program approach (multiple applications).

The fungicides available in soybeans generally target the fungal pathogens.  While we would not expect any of the labeled products to have excellent efficacy on soybean downy mildew, the fungicide class available that may have some efficacy are the strobilurins (Aproach, Evito, Headline, Quadris, Priaxor, etc.). Strobilurins can reduce the oomycete severity on other crops, but are often not as effective as the products we listed above. Unfortunately, virtually no data on soybean downy mildew exists and downy mildew is not listed on the label for all strobilurins. The lack of the soybean downy mildew control listed on some fungicide labels may be a result of the rarity of the disease.

Prognosis.  The development of disease will depend largely on climate.  If the humidity drops the downy mildew pathogen will suffer and future disease spread will likely be reduced to a dull roar. 

If the humidity stays high and rainfall is consistent, downy mildew will likely continue to develop.  Even in this situation though, history would suggest that that yield loss will not occur or it will be quite limited; we are fairly comfortable saying that this is the likely to be the case for the majority of soybeans in the state.  However, there are some fields where severity in the mid-canopy is fairly high and if the disease spreads to the upper canopy with the same severity, yield loss may occur. However, with virtually no data about what severity level yield loss occurs at, a fungicide threshold, or efficacy data, it is very hard to provide fungicide management recommendations with any degree of confidence. We will definitely be looking to obtain some data this year and try to bridge the gap in future years.

 ppth.markell.5.soybean downy mildew 1

ppth.markell.5.soybean downy mildew 2

ppth.markell.5.soybean downy mildew 3

 

Sam Markell

Extension Plant Pathologist, Broad-leaf Crops

Shana Pederson

Area Extension Specialist/Cropping Systems

 

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