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Getting a Hand on Downy Mildew

Tuesday, February 15, 2005
filed under: Disease

Downy mildew has been a consistent problem for growers in much of the northern region for the last several years due to excessively wet and cool spring growing conditions.

However, producers concerned about downy mildew again this spring have clear choices to eliminate or greatly reduce this seedling disease.

Downy mildew had been kept in check for years with an inexpensive, seed-applied fungicide called Apron-XL® (mefenoxam) or Allegiance® (metalaxyl). However, in 1998 that situation came to an abrupt end when the mildew fungus developed resistance to the seed treatment. Unfortunately, there was no other fungicide available that provided the same level of downy mildew control. So the challenge faced by the sunflower industry and researchers was to develop genetic resistance and identify another seed treatment fungicide.

The USDA-ARS Sunflower Research Unit in Fargo had already identified resistant genes to all races of downy mildew, and released resistant inbreds. However, it has taken the seed industry several years to cross that gene into their elite material and to produce finished hybrids.

This season, farmers have a number of hybrid choices that are immune to all races of downy mildew. Check the seed company ads in this issue that are promoting their downy mildew resistant hybrids, or call your seed company for details. Remember to ask for their hybrids that are immune to ALL known races of downy mildew.

Another alternative is a seed treatment called Cruiser DM (the ‘DM’ stands for downy mildew control). Syngenta has put together an insecticide-fungicide combination that is placed on the seed for protection against early season insects, such as wire worm, and a host of seedling diseases including downy mildew.

Dynasty® (azoxystrobin) is the fungicide used in the Cruiser DM treatment that provides protection against downy mildew, and was recently registered by the EPA. Dynasty will not provide immunity to the disease, but will provide up to 70% or more suppression.

Here’s how that measure of control would play out. Generally, a 25% downy mildew infection level would be at the high end of disease incidence. With a population of 20,000 plants, an infection rate of 25% would impact 5,000 plants. The seed treatment at 70% control would result in 3,500 plants protected against the disease and 1,500 plants with the disease. That would be a plant stand loss of about 7%, which is certainly tolerable given sunflower’s ability to compensate for early season plant loss.

It is important to keep in mind that the investment of the Cruiser DM seed treatment combination provides protection against both a host of early season insects AND diseases. Work done by Mycogen in nine locations using Cruiser across the sunflower-producing region in 2004 resulted in an average increase in plant stand of about 15% and an average yield increase of 225 pounds. That is a good payback considering an initial investment of approximately $6.00 an acre.

In previous National Sunflower Association surveys, the downy mildew fungus was found in most of the soils tested from the Canadian border to Kansas. Does that mean that a producer should not plant sunflower in those fields? USDA-ARS sunflower pathologist Tom Gulya says that is not the case.

The disease needs the right kind of weather conditions, specifically nearly flooded soils right after planting. If there is enough soil moisture for seeds to germinate, but not excessive rain, seedlings will escape infection even if the DM fungus is present. The good news is that growers have two options to combat DM next spring - resistant hybrids and a new seed treatment. – Larry Kleingartner

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