July Newsletter

Sunday, July 7, 2024

 Upcoming Events/Perennia Updates

Below is a summary of some of the industry events that are coming up in the next few weeks! The events listed below are primarily hosted by Perennia, and more information on each of them can be found here.

  • TunnelTalk (virtual Series)
    • July 10 - Implementing Commercially Available Bio-Control Agents
    • August 14 - Expert Panel Discusses Greenhouse Structures
  • Understanding Alternative Nutrient Amendments (Virtual Series)
    • July 2 @ 6:30 pm - Management and Considerations with Compost
    • July 9 @ 6:30 pm - Nutrient Amendments and Soil Interactions 
    • July 16 @ 6:30 pm - Conversion and Creation of Raw Material to Nutrient Amendments
    • July 23 @ 6:30 pm - Biostimulants in our Systems
  • Nova Scotia Young Farmers Summer Event
    • July 27 @ 2 pm
  • ACORN: Gathering at Abundant Acres
    • July 28 @10 am 

New Service Launched!

The Plant Health Lab is now offering plant parasitic nematode analysis, following a two year study of nematode presence and distribution across Nova Scotia! 

Check out our website for more information about nematodes, sampling, and general submission guidelines.

Weather Updates

Growing degree days (GDD) are important for anticipating key dates for crop growth stages, as well as pest emergence and flights. Consider some of the most persistent critters on your farm, and when they typically start to show up. Even if we are still early in the season for your biggest headache, its a good reminder to revisit our target GDDs, and think about what needs to happen to prepare for them on your farm. We are still tracking ahead of the 5 and 10 year averages, with comparable recent accumulations recorded in 2017 and 2021.

Based on reports out of the North Eastern United States...tarnished plant bug is alive and well! Be sure to familiarize yourself with this pest, and expect it'll be coming around the corner in no time!

Figure 1. Degree day accumulation as of June 24, 2024. All data are taken from the Environment and Climate Change Canada weather station located at the Kentville Research and Development Centre, provided by Jeff Franklin. 

Table 1. Degree day accumulations as of June 24, 2024. All data are taken from the Environment and Climate Change Canada weather station located at the Kentville Research and Development Centre. Calculations are based on a start date of March 1, and calculated using the single-sine method. Provided by Jeff Franklin.

Seasonal Considerations: Humidity and Vapour Pressure Deficit

When it comes to managing humidity in protected spaces, it is (unfortunately) more complicated than setting a target value (ex. 60%), and walking away. The target humidity for any cash crop is dependent on the temperature of the air, meaning that number is going to change depending on what is happening with the air temperature. For anyone who has monitored greenhouse/tunnel temperatures throughout the span of a spring, summer, or fall day, you know that number changes a lot from sun up to sun down. 

Vapour pressure deficit (measured in kPa) is a metric which takes the relationship between temperature and humidity into account. It is the driving force for transpiration and water movement through the plant, starting at the roots and ending at the stomata. The plant can be likened to a straw: the more pull you exert at the top of the straw, the more liquid is pulled from the cup (in our case, the growing media), through the straw (the plant) and out (the surrounding air).

High VPD is defined by:

  •      low moisture content in the ambient air (aka a dry day)
  •      high moisture content surrounding the stomata (assuming 100% humidity)

This results in a large difference
 in moisture content between the air and the plant cells, and therefore we see the mass movement of water from areas of high concentration (the stomata) to areas of low concentration (the surrounding air). Water is rapidly pulled from the plant leaves, and due to the 'stickiness' of water molecules in the plant, causes a chain reaction all the way down to the roots. 

When this happens too quickly, there are going to be some sacrifices to water, and water-transported nutrients, to the 'extremities'. That is usually leaf/tip margins, but we can also see an impact of that on the tips of fruits and vegetables as well. The plant is unable to cycle water to where it needs to go, at the rate it is being extracted into the surrounding air. 

Figure 2. A bean crop showing leaf margins that have lost their turgor pressure. 

Figure 3. A tomato crop with leaf tips/margins that were subject to high VPD a few days prior. The soft tissue will then turn brown and die off after being subject to these conditions. 

Figure 4. A lettuce plant with leaf margins that were subject to a low humidity environment on a high temperature day. 

Low VPD is defined by:

  •        high moisture content in the ambient air (aka a very muggy day)
  •        high moisture content surrounding the stomata (assuming 100% humidity)
This smaller difference in moisture content between the air and the plant significantly slows down the movement of water out of the soil, up the plant, and into the surrounding air. With less water movement through the plant, the rate of photosynthesis (which converts carbon dioxide and water into sugars) slows down. This one is a bit harder to notice, but is typically characterized by much slower growth compared to a plant growing in a more balanced growing environment. 

Figure 5. These young radishes had been planted many weeks prior to the photo being taken. Their development was significantly delayed compared to what you would expect this many days post germination.

How Do I Measure VPD?

There are sensors which will measure VPD directly, and then it is just a matter of staying in the ideal range. The optimal range of values is 7.5-10 mb, or 0.75 - 1.0 kPa

For those who don't have the specific sensor, the use of temperature and humidity information collected inside the tunnel will do just fine. The following has been re-designed by our team, but pulled from a chart provided from Delphy UK. It showcases the ideal VPD values for long cane raspberry, though in general, these numbers are going to be good ones to aim for. We can see that the 'danger' zone exists on either side of the green band, either for being too low, or too high for that temperature. Yellow is mellow, but ideally we are able to exist in the green range of values as much as possible. 

Figure 6. A vapour pressure deficit guide for deciphering where we stand in terms of temperature and humidity relationships! Green represents a good dynamic between temperature and humidity, and red shows that there is an imbalance happening (either too much is moving out of the plant, or not enough depending on which side of the scale you are on). 

A great place to start when thinking about managing the relationship between humidity and temperature is to purchase sensors for your greenhouse. Keep an eye on those values over time, and what region your readings are putting you in for the VPD chart in Figure 6. Seeing a pattern you don't like? Let's chat!

How can I change my VPD?

This can be done by either altering the temperature or humidity in the greenhouse. Typically this time of year, we are going to be talking about changing the humidity. Humidity can be added a few ways depending on the setup: spray bottles, buckets/containers of water left to sit out, or under-trough sprinklers can be used to add moisture into the system. For a more targeted infrastructure option, high pressure fogging systems can be installed to help regulate our humidity. 


This will result in increased disease pressure/spread in the greenhouse. We don't want the plants to be wet, its more about introducing moisture to the space than giving it to the above-ground tissue. This is definitely a slow practice to get acquainted with to make sure we aren't tipping the scales to far in either direction. 

If our humidity is too high, that's when we talk about air movement. 'Wind' does a great job of drying up excessive moisture in a tunnel, and the use of fans or ventilation can help to pull some of that water through the plant when the humidity is not allowing for that process to happen on its own.

If you have any questions about this, please don't hesitate to reach out to Talia, Perennia's Protected Crop Specialist. We can come up with a plan of action that makes sense for you and your setup. 

Production Tidbits: Diagnosing Deficiencies vs Disease

Distinguishing between plant disease and nutrient deficiency is no easy task, especially once the season goes on and we could be looking at multiple factors overlaying each other, making that diagnosis even harder to put our fingers on. 

Here is some helpful information to help get to the bottom of it:

Are we observing signs or symptoms?

With a fungal or bacterial disease, there will often be signs of the actual organism on the impacted plant tissue. 

  • angular leaf spot typically presents with a wet spot/cloudy ooze emerging from lesions on the underside of the leaf
  • bacterial wilt will show a stream of bacterial ooze if a piece of tissue is submitted in water, or if you make a cut in the stem and slowly pull those two pieces apart from one another
  • powdery mildew and botrytis are examples of fungal infections with very obvious growth, where the white and grey masses are the physical forms of the infection
Figure 7. Late blight of tomato photographed by Dr. Lina Quesada (https://content.ces.ncsu.edu/tomato-late-blight). Notice the white growth on the underside of the leaf

  • fungal fruiting bodies may also be present on the plant tissue
  • A point of infection can help to differentiate between a disease and a nutrient deficiency as well. Concentric circles around a singular point can be an indication that a pathogen is the culprit for the symptoms observed. This is disease dependent, so lacking concentric circles or obvious 'targets' does not mean that you are free of disease. 

Figure 8. Late blight of tomato photographed by Inga Meadows (https://content.ces.ncsu.edu/tomato-late-blight). Notice the obvious concentric circles through the leaf lesion, which indicates the presence of an organism as the cause of the symptom. 

The exception to this rule is viruses, which don't present as obviously as some of the above indicators (ex fruiting bodies, ooze, physical fungal growth). If you suspect you have a viral infection in the crop, reach out to your extension specialist immediately to verify. 

What are some other things we should pay attention to when the crop starts to look off?

1) Where on the plant are the symptoms located? 

Are you seeing it primarily on the new growth of the plant, or the older leaves? Have the symptoms spread to the stem? Are flowers and fruits impacted? 

2) What is the distribution of the observed plant symptoms? 

Are you seeing it throughout the greenhouse or just in one patch? Is it concentrated on one side versus the other? Down the middle or along the edge? Concentrated in low patches in the production space? 

3) How quickly did it show up? 

Are you seeing quick spread or is it staying contained within the first few impacted plants?

4) Are the symptoms uniform across the leaf or irregularly scattered?

Typically a highly regular/uniform distribution of discolouration/necrosis can be attributed to abiotic factors (sprays gone wrong, growing environment stress, nutrient deficiency) as opposed to biotic factors (disease and/or pest pressure). 

The earlier we can identify and address the issue, the better! 

Additional Resources:

Feature Pest/Disease: Fungus Gnats

The following information is a summary of session 5 of the BugBites Series: Fungus Gnats and Shorefly Suppression with Beneficial Organisms. To watch the full session, click here

Adult fungus gnats are mosquito-like insects with long legs, and a characteristic y-shaped wing vein. Adults can be found hanging around or near the soil, mostly under plant canopy. The fungus gnat larvae are white with a shiny black head. Larvae spend most of their time underground and will feed on the roots. 

Figure 9. A photo of an adult fungus gnat stuck to a yellow sticky card. Below it is an image of the fungus gnat larvae. If you look closely, you can see the characteristic black head at the end of the larva. Image taken from the BugBites! Session 5 presentation. 

Monitoring and Scouting: 

Adults are not strong fliers, meaning that sticky cards will only be effective when placed close to the soil line. If placed correctly, traditional sticky cards are a great tool to monitor population sizes.

As an attractant, some growers will use half a potato, or a potato slice, placed in a pot to draw larvae up from the soil. As the larvae are accustomed to feeding on roots, the use of a tuber is a big draw.

For the adults there are various traps available. These traps typically involve the use of an attractant (ex. UV bulb) and a way to capture the insects (sticky cards, soapy water solution etc.). This is useful for general scouting purposes as well as mass trapping.

Biological Control Options: 

As with any biological control program, prevention is the first line of defense in controlling population sizes. By conducting regular scouting checks of the crop, and maintaining populations of biological control agents, you can limit the degree of pest infestation.

Dalotia coriaria; Rove beetle

  •  Predator to both shoreflies and fungus gnats
  •  Both adults and larvae are predators
    • Orange larvae will be noticeable in your soil
    • Adults readily move around via flight
  • Establish readily in greenhouses and work well in hydroponic systems

Figure 10. A photo of a rove beetle in the adult (left) and larval (right) stages. stuck to a yellow sticky card.  Image taken from the BugBites! Session 5 presentation

The use of rearing boxes allows for a slow-release approach and will help maintain beetle populations through the season. Rearing boxes can be as basic as media (ex. peat, vermiculite) set inside a Tupperware container, and supplemented with food (ex dog or fish food, artemia). It is crucial to keep this moist, but not wet. These can be placed under benches.

Bacillus thuringeniensis israeleinsis 

  •  Functions as a stomach toxin for fungus gnats
  •  Best applied via soil drench or chemigation. Irrigation is not ideal as the soil needs to get very wet to allow for good establishment of the biopesticide

Stratiolaelaps scimitus; predatory mite

  • Target fungus gnat larvae
  •  Well suited for greenhouse environments: optimum development temperature of 25oC (77oF)
    • Ideal range is 15-30oC (59-86oF)
  • Live in the top half inch of the soil: best applied to the substrate surface after transplant
    • Want to avoid burying too deep in the growing media
  • Does not establish well in rockwool
  • Persistent control agents, and can provide season-long protection

Figure 11. An image of an adult predatory mite. Image taken from the BugBites! Session 5 presentation

S. scimitus can be mixed with Dalotia, but it is not recommended to introduce them before adding into the crop. They will feed on each other, so make sure they are spread evenly throughout the canopy when distributed.

Steinernema feltiae; entemopathogenic nematode

  • Target fungus gnats, shoreflies, thrips pupae and many others
  • Forage in the top inch or two of the soil
    • Water into your growing media lightly- if they get too far in, they will not work
  • Not highly mobile
  • Well suited for greenhouse environments with an ideal temperature range of 10-25oC (50-78oF)
  • Susceptible to being washed away in rockwool
    • When growing in rockwool, apply after the last irrigation of the day to give them a chance to establish

Recent Uploads


That's all for now! If you have any questions, feel free to reach out to your extension specialist.

Happy growing everyone!