Resistance management is of utmost importance for cucurbit downy mildew

Changes in fungicide regulations have reduced the available treatments

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Management of cucurbit downy mildew (CDM) requires more diligence with limited fungicide options.

Recent changes in fungicide regulations leaves fewer options for farmers to control CDM, economically the most important disease for cucumbers.

Why it matters: Re-evaluation of chlorothalonil has reduced the number of applications for cucurbit growers from seven to two. CDM has become resistant to other single-site fungicides meaning the available fungicides must be used appropriately.

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CDM is a disease caused by oomycete pseudoperonospora cubensis.

“Clade II (a way of characterizing downy mildew) is where we think a lot of the resistance concerns are happening,” says Katie Goldenhar, horticulture pathologist with OMAFRA who presented on the topic at the 2020 Ontario Fruit and Vegetable Convention.

In cucumbers the common symptom of CDM is angular lesions that are primarily identified in water soaking in their early stages. It’s easily identifiable during a period of leaf wetness.

“If there is a heavy dew and you go out and you see a heavy darkening around an angular lesion that is typically this pathogen at its very early stage,” says Goldenhar.

In Ontario, cucurbit downy mildew has two potential sources for outdoor growers as it does not overwinter.

It can migrate from frost free areas to Michigan. The disease then moves by wind into Ontario as it can survive up to two weeks host-free and travel up to 1,000 kilometres on the wind.

As well, greenhouse production has been known to provide initial inoculum.

“If there is year-round greenhouse production surrounding us there is the capability of this pathogen to infect the crops and survive over the winter months and then infect in the field.”

There are no resistant cucumber varieties but there are some with tolerance, which can limit the effects of downy mildew.

For cultural management, it is best to increase the airflow within the canopy to decrease crop wetness.

“It’s a water mould. It loves moisture. It needs at least 45 minutes of leaf wetness when that spore lands on that crop in order to infect,” says Goldenhar.

Producers can’t prevent the pathogen from entering the province but by monitoring its location farmers can observe when the risk of infection is high.

“When we have the host, pathogen and environment for infection, we have to rely heavily on fungicides for management.”

This pathogen, P. cubensis, is known to have a high risk for developing fungicide resistance because it has many generations each season.

“That life cycle can go through in five to seven days and with any of that reproduction you are potentially having a chance of a mutation that could create resistance to a specific fungicide or a group of fungicides.”

Since it’s re-emergence within Ontario in 2005 there is already resistance to mefenoxam and strobilurin active ingredients. As well, there is some sensitivity to the single site fungicides.

With the high risk of resistance of this pathogen and only three highly active products that are single site, it’s imperative farmers follow resistant management practices.

  • It’s best to rotate and tank mix with multiple modes of action on the same pest and do so with low-risk fungicide.
  • Limit the number of applications per season.
  • Ensure the product has proven activity on the pest.
  • Apply preventively.
  • Always follow the label rates.
  • Ensure proper coverage for each application.

A study completed at Michigan State University in 2019 shows when farmers are in a high risk year for downy mildew it’s best to use Orondis Ultra, Torrent/Ranman and Zampro fungicides.

Both Zampro and Orondis Ultra only have one effective active ingredient for the downy mildew, and Torrent/Ranman only has one active ingredient.

“Luckily they are all in different groups which is good for resistance management. However, with the label restrictions, you need to have a rotation with at least three different products. That’s what we have and that’s all we have that is effective,” says Goldenhar.

Research completed at the University of Guelph and Michigan State University shows that there are two pickling cucumber varieties, Peacemaker and Citadel, that have tolerance.

They found that Peacemaker consistently resulted in less disease than Citadel under high disease pressure.

“That is a cultivar that seems to be more suited for our downy mildew populations. But both cultivars did show a reduction in disease when compared to some of the susceptible standards.”

Farmers should monitor when CDM is within the area, scout and when risk is high and use fungicides effectively.

About the author

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Jennifer Glenney

Jennifer lives on a farm in Cayuga, Ontario and has a lot of experience in the many aspects of agriculture.

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