Status report on Cucurbit Downy Mildew

As of Wednesday, September 1st, cucurbit downy mildew has been found in Annapolis, Kings, and Hants Counties.  While all Cucurbits are susceptible to downy mildew, there are different Clades which affect different subsets of the Cucurbit Family.  Clade I predominantly infects watermelon, pumpkin, squash, and zucchini.  Clade II predominantly infects cucumbers and cantaloupes.  So far, we have only found downy mildew in cucumber fields, suggesting that we have Clade II. 

There is a great Cucurbit Downy Mildew Diagnostic Key, which might help you determine if you have downy in your field. 

Downy mildew spores blow in on storm systems from the United States, so it is possible that Thursday's storm will bring in Clade I, so growers should be aware.  Downy mildew cannot overwinter in Nova Scotia unless the infection spreads to greenhouses.  Please reach out to Rosalie Gillis-Madden, Perennia’s Vegetable Specialist by email or cell (902-670-9655) if you want support around managing this disease. This late in the season, growers of squash and pumpkins might choose not to put on a protectant spray.  

• Growers who do not yet have downy in their fields and are hoping to continue picking their cucumbers and cantaloupes are advised to put on a protectant spray (i.e. Zampro or Torrent for conventional growers, Serenade Opti or copper for organic growers).   
• If you have downy mildew in your cucumber or melon field, and you wish to keep picking, an application of Orondis Ultra is advised (copper for organic growers).  Conventional growers should rotate to a different FRAC group and spray again 7 days after the first spray as advised by the pesticide label (suggested Torrent as a second application for conventional growers, and another copper spray for organic growers.)  Please also call Rosalie so we can document the spread of this disease. 
• If you are hoping to keep picking zucchini and watermelons for a few more weeks, a protectant spray is advised as downy mildew spores from Clade I might blow in on Thursday’s storm (i.e. Zampro or Torrent for conventional growers, Serenade Opti or copper for organic growers).  You should rotate to a different FRAC group and spray again 7 days after the first spray as advised by the pesticide label.
• If you find downy in your squash, pumpkins, watermelons, or zucchini, please contact Rosalie right away.   

Please note that powdery mildew sprays ARE NOT effective against downy mildew.  Downy mildew can very quickly breed fungicide resistance so rotating FRAC groups and following the label closely is key to management.  

If you are currently picking crop, please pay close attention to the PHI listed on the fungicide label.  

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Hailstorms and crop impact

The hail storm over the weekend caused scattered damage in the Annapolis Valley, as well as in Hants, Colchester, and Cumberland Counties.  The degree of damage depends a lot on location and crop type, as well as on wind velocity, the size and shape of the hailstones, and how long the hail lasted.  The warm, wet weather that followed the storm can exacerbate disease problems, especially bacterial infections.

Fig. 1. Healing hail damage on butternut squash,
two weeks post-storm.

Crops with large leaves such as Cucurbits will typically look pretty destroyed, but, depending on the severity of the damage to the leaves, and how well the fruit were protected, might pull out of it with minimal yield loss.  If the Cucurbits are on a drip system, a little extra nitrogen will encourage the regeneration of leafy growth.  Damaged zucchinis and summer squash should be removed.  Leaving damaged fruits on the plant, which are unlikely to be marketable, will be a drain on the plant's resources and could result in more prolonged yield losses.  Depending on the severity of damage, many winter squash will scar-over (Figure 1, Figure 2).  Healed scars should not pose a disease risk, but may render the fruit undesirable to some buyers.

Fig 2. Healed hail damage on butternut squash,
seven weeks post-storm.

Other crops that are susceptible to bacterial rots (Figure 3) such as onions will have long-lasting damage - crop injury (Figure 4) opens the door for bacterial infection, which quickly spreads down the leaves into the bulbs.  Dead tissue is prime for stemphylium infection.  Growers are encouraged to continue with preventative stemphylium sprays.  Copper may help with bacterial infection, but results have been poor on onions in the past.

Leaf loss in crops close to harvest such as sweet corn or root crops should have minimal effects on yield.  However, direct impact on sweet corn ears can render them unmarketable, depending on the degree of damage.

Fruiting vegetables such as peppers will be more prone to sunburn if protective foliage has been badly damaged.  Fruits that have taken a direct hit, depending on the size of the hail stone and the velocity of the strike, can be unmarketable.

Fig. 3. Bacterial infection spreading down an
onion leaf

Losses of flowers and small fruit will reduce yield and delay harvest in crops such as green beans.

Wounds in plant tissue open the door for diseases to come in.  It is recommended that a protectant such as copper or Bravo be applied (if labeled) for your damaged crop, which will help guard against infection while the plant heals.  Always read and follow the label, the most up to date labels can be found on the Health Canada website.

If you have Crop Insurance, it is recommended that you reach out so that they can document the damage.  Should a claim be necessary down the road, they will have a record of how extensive the damage was, which will make filing a claim easier.  The days just after a hail storm are often the worse.  Apply a protectant, maybe give your crop a bit of extra fertility, if appropriate, and then come back in a week to see how things look.  If the crop pulls out of the damage, then no insurance claim needs to be filed.  Registered farms in Nova Scotia shouldn't hesitate to contact Perennia if you are unsure of best management practices in crops that have been affected by adverse weather.

Fig 4. Hail damage on onions

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