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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Updates on Cucurbit downy mildew

Monday, August 30th, 2021 downy mildew was found in King's County.  Please read the original blog post here.  Downy mildew that infects Cucurbits is specific to Cucurbits - it won't infect onions, grapes, hops, lettuce, spinach, basil, etc.

There is a great Cucurbit Downy Mildew Diagnostic Key, which might help you determine if you have downy in your field.  Please do not hesitate to reach out if you need support in identification and you are a registered farm in Nova Scotia.

So far, downy mildew has only been confirmed here on cucumbers.  The Nova Scotia Vegetable Blog will be updated if that changes.  There are two different host-specific clades of downy mildew.  Clade I mostly infects watermelon, pumpkin, squash, and zucchini.  Clade II mostly infects cucumbers and cantaloupe.  It is possible that we currently only have Clade I in Nova Scotia.  However, there is a storm system blowing in on Thursday/Friday that will certainly bring downy conducive conditions (i.e. prolonged leaf wetness), and perhaps more infections spores of either Clade I or Clade II.  

Cucumber and melon growers should apply protective fungicides as outlined in yesterday's blog post.  Pumpkin, squash, zucchini, and watermelon growers can take a gamble - either apply a protectant as previously mentioned; do nothing and hope there isn't downy that will infect the pumpkin/squash subset; or harvest early.  For growers that do not have established drive rows in their winter squash and pumpkins - especially if they are close to harvest - it may be worth the risk to wait see and thereby avoiding driving over crop to make a fungicide application.  Harvesting early is a good option if the crop is mature and you have curing facilities, especially in light of the forecasted precipitation coming our way on Thursday/Friday.  Downy mildew does not affect fruit, however in the case of cantaloupes, the fruit will be less sweet.  

Cucurbit downy mildew!!!

Fig 1. Heavily infected cucumbers.

ALERT!  Downy mildew has been found in Cucurbit fields throughout Kings County!  

We are lucky here in Nova Scotia, we do not usually get downy mildew in our Cucurbit fields.  This disease is catastrophic - it can take out a field in a matter of days (Fig. 1).  It affects all Cucurbits: melons, watermelons, cucumbers, pumpkins, and squash.  It can very quickly breed fungicide resistance so rotating FRAC groups and following the label closely is key to management.  

Fig. 2. Downy mildew
sporulating on the
underside of a cucumber
leaf.  Sporangia can vary in
colour from light grey to
deep purple with a downy
appearance.

Monday's cool, drizzly weather has been perfect downy breeding conditions.  Abundant sporangia are produced if humidity is at 100% for 6 hours with temperature between 15-20˚C (Fig. 2). These sporangia are wind blown or splashed onto susceptible host tissue and germinate if leaves are wet for at least 2 hours at 20-25˚C.  Spores can also be carried on your tools, clothes and hands so DO NOT move from infected fields to healthy fields. Spores do not last indefinitely on non-host surfaces, for example exposing spores to sunlight for 4.5 hours kills 63% of the spores.  Spores can survive where there is moisture, for example on wet clothes or farm equipment, but needs living plant tissue to survive for any length of time.  It does not overwinter in our region.  This infection likely blew in from the Southern US.  

Fig. 3a. Early downy infection on
cucumber, leaf surface.

Pale green or yellow spots can be seen on the upper leaf surface as an early sign of infection.  These spots later turn brown and spread.  The spots are typically bound by leaf veins giving it a bit of an angular appearance, especially in cucumbers.  As the lesions spread, the whole leaf will turn brown and look like it's been zapped by frost.  

If downy mildew develops in fields nearby, but not in yours, use weekly rotations of Manzate + Zampro (FRAC group 40, 45) or Manzate +Torrent 400 SC (FRAC Group 21).  If downy mildew is present in your field, apply Phostrol (FRAC Group 33) or Orondis Ultra (FRAC Group 49, 40).  

For organic growers, if you have downy, use copper.  If you don’t have it, Serenade Opti (44), Serifel (BM02– cucumber only) with a liberal dose of prayer.

Fig 3b.  Underside of cucumber leaf.

For all pesticides, be sure to read the label carefully, and follow exactly.  If the label differs from what you read here, always follow the label.  Up-to-date labels can be found here.  

Do you know about Perennia's Pest Management Guides?  Every year we update pest management options for the major crops in Nova Scotia.  A complete listing can be found here (click on Vegetable Crops for the drop down menu).

Fig. 4a. Older lesions of downy
mildew on cucumber leaf surface

Fig 4b. Advanced downy mildew
infection, underside of cucumber leaf.

Fig 5. For contrast, this is powdery
mildew on squash. Powdery mildew
sprays are NOT effective against
downy mildew.

 

Romance in the pumpkin patch

We are well ahead of the average with our growing degree days, so pests are showing up earlier than you would typically expect them.  

Degree day accumulations as of July 5, 2021.  All data are taken from the Kentville weather station, based on a start date of March 1, and calculated using the single sine method.

If you haven't already, start checking your Cucurbits for squash bug!  The recent rains have speckled the adults with soil, making them extra hard to see, so be vigilant.  A female squash bug can lay up to 250 eggs in a season, so controlling this pest early is key.  The adults aren't affected by pesticides, and the youngest instars are the most susceptible, so it's best to flag a few egg clusters and check back regularly to time your sprays.  Be careful of pollinators, only spray when bees aren't active. 

Consenting adult squash bugs having fun in the sun.

These crafty fellows like to hide.

The bronze coloured eggs can usually (but not always), be found in the crux of leaf veins, and will typically be in groups of 10-20, although I've seen as few as four in a cluster.  With smaller plantings, you can remove leaves with eggs on them, or wrap a piece of duct tape around your fingers and blot the eggs off.
.

This female laid two sets of eggs on this leaf.

If you have multiple types of squash in a field, squash bugs seem to prefer pumpkins, blue hubbard, buttercup and kabocha types, so be sure to scout those extra carefully.  I have yet to see squash bug in watermelons or cucumbers, but it's always good to be thorough.  Have you noticed varietal preferences in your field?  If so, I'd love to hear about it.  

Did you catch Perennia's Virtual Field Days last summer?  Acting Vegetable Specialist Caitlin Congdon and Suzanne Blatt from AAFC discussed using blue hubbard squash as a trap crop and nasturtiums as a push crop in butternut and buttercup plantings with interesting results.  Skip to 10:20 for details on the trial, and stay tuned until 45:30 for the question period.  

For more information about squash bug life cycle and control measures, check out Perennia's Squash Bug Fact Sheet.

Do you know about Perennia's Pest Management Guides?  Every year we update pest management options for the major crops in Nova Scotia.  A complete listing can be found here (click on Vegetable Crops for the drop down menu).

Late-season foliage loss in winter squash

Hurricane Dorian caused widespread damage in Nova Scotia.  Many cucurbit plants lost significant amounts of foliage.  Late-season foliage loss in winter squash and pumpkin leave the fruit exposed to sunscald.  If your crop has lost enough foliage so that there is no longer sufficient canopy protection to prevent sunscald, it is recommended that you harvest your squash and pumpkins.

Fig. 1. Sunscald on butternut squash, three
weeks post-storm.

Sunscald injury (Figure 1) occurs when cucurbit fruit are exposed to direct sunlight during the later stages of fruit ripening - while this year this is predominantly due to Hurricane Dorian, this can also occur as a result of powdery mildew, bacterial wilt, Phytophthora blight, or other pests.  Sunscald injury will result in rind tissue collapsing in the exposed area.  Initially the rind may appear water-soaked - on pumpkins this can be hard to observe at first and the area might just appear slightly sunken.  Darker skinned squash (particularly green acorn) will show sunburn fairly quickly.  Affected tissues can look bleached (Figure 2) and the rind may slough off when rubbed.  Over time, the fruit tissue may become tan, brown, or black (Figure 3), a result of secondary pathogens invading, and will eventually collapse.

Fig. 2. Sunscald on Sweet Mama, three weeks post-storm.

Typically, it is recommended that pumpkins and winter squash are harvested when they are fully mature for optimum quality and storability.  Fully mature pumpkins and winter squash will typically have a more hardened skin that is resistant to puncture with a thumbnail.  Fully mature
fruits will often have lost rind surface gloss and may have ground-spot yellowing.  Mature fruit that are left attached to vines are prone to infection in the stems and fruit.  As long as pumpkins have started to turn colour, they will continue to ripen off the vine if held under proper conditions.  When harvesting, avoid bruising the fruit or damaging the skin as this will result in storage rots setting in.

Proper curing and storing will greatly enhance the storability of your crop.  Note that chilling injury will occur if stored below 10°C (50°F).

Fig. 3. Sunscald on Sweet Mama with secondary infection,
three weeks post-storm.

Pumpkins
The best conditions for ripening pumpkins are in a well-ventilated area at 27- 30°C (80-85°F) with relative humidity of 80-85%.  The curing period should be about 10 days.  This will help the fruit to ripen, the skin to harden, and any wounds to heal.  After curing, pumpkins should be stored in a cool, dry place (10 - 15°C or 50 - 60°F) with a relative humidity of 50-70%.

Squash
The best conditions for ripening squash are in a well-ventilated area at 21- 27°C (70-80°F).  How long squash will typically keep under optimal storage conditions depends on squash type.  Cornell has a chart outlining optimal storage temperatures and relative humidity based on squash type, which can be found here.  Note that if Acorn squash is stored below 13°C, it will become stringy.

The goal is to maintain enough humidity so that the fruit doesn't desiccate but not so much that condensation occurs which can promote bacterial and fungal growth.  Storing at higher temperatures results in weight loss, and at lower temperatures chilling injury can occur.  All fruit that is placed in storage should be free from disease and in good condition to promote the longevity of the crop's storage life.  Pressure bruises from packing fruit too tightly or too high can also result in reduce marketability of your squash and pumpkins.

Cucumber beetles are here

Stripped cucumber beetle, illustration by Jessica MacDonald

Striped cucumber beetles have made their annual debut in the Annapolis Valley.  These pests are problematic in many ways.  Feeding damage can stunt plants, and when flowers are predated, it can reduce fruit set and yield.  Cucumber beetles also vector bacterial wilt.  Scarring on fruit by adult beetles reduce the marketability of the crop. 

Newly emerging cucurbit plants are particularly susceptible to stunting and bacterial wilt, while older plants can withstand up to 25% defoliation.  Early season control is essential with this pest, and it is important to scout your fields regularly. 

For more information on cucumber beetle biology, beneficial insects, and organic management, check out Managing Cucumber Beetles in Organic Farming Systems on eXtension.
 
Please click here for Perennia's Pest Management Guides for Cucumbers and for Melons.  For Perennia's Pest Management Guides for Pumpkin and Squash, please click here.  Insect netting can also be an effective control option for this pest, but netting must be removed at flowering for pollination to occur.

Integrated Weed Management in Cucurbits

Plywood separators used to partition a grain drill seed box.

In the Fall of 2016, Perennia, in conjunction with local growers and Agriculture and Agri-Food Canada, undertook a new on-farm trial that demonstrates the use of mulched cover crops to suppress weeds and promote soil health in cucurbit production.  

Partitions expertly secured with a suitable application of duct tape.

In a previous project (Cover crops and zone tillage for reduced risk weed management in field vegetables in Eastern Canada), Agriculture and Agri-Food Canada scientists evaluated weed control methods in cucurbits using cover crops and generated recommendations for using certain techniques to manage weeds in squash production. The technique that we are trying in Nova Scotia integrates the use of fall-seeded rye and banded fall-seeded oats.  

Two rows of oats seeded for eight rows of rye, September 30, 2016.

Oat and rye establishment on October 19, 2016.

The oats will winter-kill, leaving a clear narrow band into which squash can be planted.  The surviving rye is left to grow until late spring at which point it is rolled down to serve as a weed-controlling mulch. This technique has been shown to reduce weed pressure and the amount of herbicides needed, as well as increase marketable squash yield. Squash that is grown on the rolled down rye mulch should be cleaner at harvest, have less disease symptoms, and require less cleaning prior to heading to market.

 

Another added benefit of this system is the organic matter that the cover crops will add to the soil.  This, along with the season-long soil cover provided by the mulch, will contribute to improvements in soil health.  The mulch will protect the soil from erosion and will also preserve soil moisture should we have a repeat of last year's growing conditions!

We intend to hold a field day this summer to demonstrate this technique, stay tuned for more details!!  

Winter-killed oats alternating with fall rye, January 31, 2017.

Time to crimper roll winter rye for subsequent cucurbits production

Even though winter rye is much shorter than in previous years, it is fully headed and ready to be crimper rolled for subsequent pumpkin and squash crop planting.

 

Winter rye provides an excellent cover crop for pumpkin and squash.  Not only does it help to supress weeds, but it also contributes to lower disease pressure and better storage life.  In the Annapolis Valley winter rye is fully headed and it is time to begin to roll it now.  In the conventional system it works better if it is sprayed with glyphosate either before, or after, crimper rolling.  In an organic system, timing of rolling is important - if it is done too early the winter rye will come back.  If it is done too late seeds might become viable and volunteer winter rye might be a problem later on during the growing season.

 

It is recommended to plant the pumpkin or squash crop within 7-10 days of crimper rolling/ spraying.  The residue of the winter rye will immobilize some of the nitrogen, therefore 15-25 percent extra nitrogen might be necessary for proper crop growth and development.

Winter rye crimped and treated with Glyphosate.

Winter rye crimped only.