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.

 

Creating a Balance - Vegetative vs Generative Growth

Part 3 - Tampering with Temperature

So far, we have explored the differences between vegetative and generative growth, and the role that water availability and EC play in steering the crop in either direction. Now it is time to explore the impact that temperature and humidity have on these two types of growth.

• Water availability
• EC
• Day/Night temperatures
• Vapour Pressure Deficit (VPD)
•  Pruning
•  Fruit load

Day/Night temps

While we are accustomed to focusing on the temperatures through the day, it is also important to consider the greenhouse temperature at night. With a high night temperature, you are going to see higher rates of respiration, which results in a decreased fruit weight. Allowing the space to cool down over the night is a good strategy to maximize yield. Be careful here though, because too low of a night temperature will result in poor fruit quality.

Not only does a cooler night temperature (compared to the daytime temperature) reduce respiration in your fruit, but it also encourages generative tendencies in the plant. Holding your temperature steady over 24 hours puts the plant into a more vegetative growth pattern.

A good strategy for transitioning from you night temperature to your day temperature, is to allow the space to heat up 1-1.5 hours before sunrise. This is important in maintaining the shelf life of your crop once it has been harvested. Without this warm-up period, you will likely end up with condensation in the production space, which can lead to a variety of issues.

Like irrigation, these highs and lows should be modified on cloudy days – reducing that temperature gap when there is less sunlight will prevent the crop from becoming overly stressed.

Vapour Pressure Deficit (VPD)

Vapour pressure deficit (measured in kPa) compares the amount of moisture in the ambient air, to the amount of moisture in the air surrounding the plant. It is a driving force for transpiration and determines how quickly or slowly the moisture moves out of the plant.

High VPD

•      low moisture content in the ambient air
•       big difference in moisture content between the air and the plant
•      water is rapidly pulled from the plant leaves, putting pressure on the roots to transport more water

Low VPD

•        high moisture content in the ambient air
•         small difference in moisture content between the air and the plant
•         water is not pulled from the plant as quickly
•        more vegetative plant

How do I know where I stand?

The chart posted below can be used to help visualize the energy relationships of moist air. Based on your VPD reading and the temperature of the grow space, you can determine what the ideal humidity is (and vice versa). By maintaining a VPD in the target zone (shown in green in the chart below), you can rest assured that the air-water relations have been optimized within the plant and prevent encouragement towards vegetative production. Once you start steering towards the red zones on either side, an adjustment should be made to get back into the target zone. Fluctuation in temperature or humidity is okay if the appropriate adjustments are made to keep the system in balance.

Figure 1.  The above chart serves as a guideline for creating an ideal greenhouse environment. Taking regular VPD measurements, in addition to temperature and humidity, will help to keep the system in check. Table was sourced from https://scienceinhydroponics.com/2017/04/vapor-pressure-deficit-vpd-in-hydroponics.html

Posted by: Talia Plaskett