Vine Crop Update
by Elaine Roddy, Vegetable Crop Specialist, OMAFRA
(elaine.roddy@ontario.ca)

Downy Mildew Status
Frequently Asked Questions

Downy Mildew Status

There has been considerable downy mildew activity in the Southern States lately.  Over the past week, new infections have been identified in Florida, Georgia, Texas, California and North Carolina.

Much of the continent has been experiencing unsettled weather recently.  This trend is expected to continue up to the weekend.  The risk of long distance transport of the spores is currently low.  The prevailing winds are generally moving west to east, with very little south-to-north activity.  However, it is well worth keeping an eye on the radar to watch for a change in the wind patterns.  Be mindful of weather patterns that move quickly from infected areas into the Great Lakes region.

Downy mildew forecast predictions are posted every Monday, Wednesday and Friday on the Cucurbit ipmPIPE website at: cdm.ipmpipe.org

If these unsettled weather patterns continue, it may be prudent to start a preventative fungicide program before the plants begin to vine.

Frequently Asked Questions

Q  If I start spraying early, will I reach the maximum number of fungicide applications before I have finished harvest?

A Tattoo C has a maximum of 5 applications per year and Ranman has a maximum of 6 applications per year.  Based on a 7-day spray interval, the combined 11 applications will be more than sufficient to protect the field for the full harvest season.  It is better to start early and keep the crop clean, than to miss an early infection and spend the rest of the season trying to keep ahead of the disease.

Q How rainfast are the downy mildew fungicides?

A Most fungicides available today are rainfast when dry.  This will usually take approximately one hour.  However under humid conditions or a dense canopy it could take longer.  Exceptionally heavy rainfalls may result in a need to shorten the spray interval.

Q  Is it better to apply a downy mildew fungicide before or after a rainfall?

A  All downy mildew fungicides should be used preventatively.  Wherever possible apply them before a rainfall or irrigation event.  While some products do offer a limited amount of kick-back, they are not curative.  If the leaf is not adequately covered for the period of leaf wetness during and immediately following the rain, there is a high risk that an infection will occur if spores are present in the air.

Q High water volumes add a lot to the application costs.  How low can I go?

A All downy mildew fungicides rely on thorough leaf coverage to fully protect the plant.  One of the most effective ways to penetrate a dense cucumber canopy is with a high water volume.  Even air assist nozzles work better with increased water volumes.  Aim for at least 30 gal of water per acre.

Q If I turn up the pressure on the sprayer, will it force the droplets deeper into the canopy?

A Not necessarily.  The pressure is used to create the droplet size.  There is no downward force associated with the spray pressure.  Depending on the nozzle used, a very high pressure will create a fine mist of droplets.  These fine droplets may actually drift away or evaporate before they hit the leaf surface, offering very little protection for the crop.  Select the pressure required to create a medium-sized droplet based on your nozzle type.  These medium droplets will hit the target and offer a good uniform coverage of the crop.

Q Is a banded application really worth the effort?

A When the plants are small, a banded application covers only the leaf surface and saves valuable spray from being sprayed on bare ground.  This can cut fungicide costs by 60 to 75%.  Banding does not change the concentration of the spray mixture.  It does allow you to cover more area with the same amount of product.

When banding, use the following formula to calculate the amount of product required:

Q The downy mildew targeted fungicides are expensive.  Is there a less costly option?

A Tattoo C and Ranman have been proven effective in downy mildew trials across North America.  Broad spectrum fungicides such as Bravo and mancozeb may offer a limited amount of downy mildew suppression.  However, under moderate or high disease pressure these products will not fully control or limit the spread of cucurbit downy mildew.  Cucumber growers should only use broad spectrum fungicides under low risk conditions, before downy mildew is identified in the Great Lakes region.

Questions?  Comments?
Give me a call at 519-674-1616 or email elaine.roddy@ontario.ca