Special downy mildew update - August 26, 2005

Downy mildew marches across the state
Mary Hausbeck
Plant Pathology

Since downy mildew (see images as pdf) was confirmed in the state on August 5, the disease has developed in several regions including major vegetable production regions (see map as pdf ).  Thousands of acres have been affected or are threatened.  Cucumbers represent the majority of the acres that have become diseased.  Downy mildew has also been confirmed on yellow squash and zucchini.  Nearby states report this disease on pumpkins, gourds and melons.

Downy mildew is a new disease for Michigan.  It has not been confirmed in the 15 years that I’ve been in my job, and there is no documentation of it ever occurring in the state.  Downy mildew is different from powdery mildew.  Powdery mildew occurs each year in Michigan on cucurbits.  The tell-tale symptom of downy mildew is the dark, purplish/gray fuzz on the underside of the leaf giving a somewhat “dirty” or “velvet” appearance.  Powdery mildew is white.

Downy mildew is well-known for causing catastrophic losses in a brief period of time.  When the conditions are favorable, unprotected foliage can become completely infected and appear to be frosted within 10 days of initial infection.  The worst case scenario is that once the foliage dies, the fruit stop developing, quality decreases, sun scald occurs and secondary rots develop.  In a best case scenario, if the fruit are mature enough when the foliage becomes infected with downy mildew, they can be harvested with minimal yield loss even though the foliage has completely collapsed.

Chemical control must be focused on using the most effective products, alternating the products, and applying the fungicides at short intervals.  Early results from our downy mildew spray program indicate that the most effective spray programs include the following:

Previcur Flex + Bravo alternated with Tanos 50DF + Mancozeb (or Tanos + Bravo)

Note:  This study is being conducted in a young pickling cucumber field that was already showing early downy mildew symptoms and sporulation.  This is a standout program in the Michigan State study among the 18 programs tested.  Previcur Flex is a critical component of this program.  It appears to be especially effective and offers a different mode of action from that of Tanos 50DF.  It is my understanding that this product is in relatively short supply in the state.  For growers battling downy mildew, Previcur Flex is a must-have tool.

Tanos 50DF has a three-day PHI and Previcur Flex has a two-day PHI.  The addition of mancozeb increases the PHI to five days.  Bravo has a zero-day PHI.  Please note: Mancozeb is not labeled for use on pumpkins.  The Curzate fungicide can also be used and is one of the active ingredients in Tanos.  If Curzate is used it should be mixed with either Bravo or Mancozeb.  Curzate should not be used in alternation with Tanos 50DF because they have an important active ingredient in common.  Growers who want to protect their plantings need to use the program outlined above and spray every five to seven days.

Based on the ongoing Michigan State downy mildew trial and results of trials from North Carolina, I cannot recommend the strobilurin fungicides or the Ridomil-based products for downy mildew control.  The strobilurin fungicides include Quadris, Amistar, Cabrio, Flint and Pristine.  The disease has affected too many acres in the state and the threat of resistance is too high.  The downy mildew fungus can mutate and become resistant to these fungicides.  These products are not as effective as either Previcur Flex or Tanos. Similarly, Acrobat is not controlling downy mildew in the Michigan State trial.

The widespread cucumber mosaic virus has made matters worse.  Many growers are trying to determine whether they should invest in downy mildew sprays given the virus showing up in the foliage and the fruit.  For growers trying to coax another week or two from their virus-infected fields, applications of Previcur Flex + Bravo alternated with Tanos + mancozeb will be needed.

As soon as harvest is complete, the remaining vines should be killed with herbicide or plowed under immediately so that they do not become infected and serve as a source of downy mildew for nearby crops.  The downy mildew is primarily windborne.  This pathogen overwinters in southern states that do not experience a hard frost.  This pathogen should not overwinter in Michigan.  The early hurricane season is likely responsible for the movement of downy mildew into the Michigan growing regions.  The 2005 hurricane season has been noted to be unusually active compared with other years. 

My lab and the Diagnostic Services on campus are working together to confirm downy mildew.  The diagnosis takes just a few minutes.  It is important to track this outbreak.  I can be reached via cell phone at 517-927-4532.