It’s Up in the Air: Concern about the Impacts of Smoke from Wildfire on Horticultural Crops

Written By: Kaelyn Lupyrypa, Horticulture Research Assistant, OMAFRA Kathryn Carter, Fruit Specialist, OMAFRA and Erika DeBrouwer, Tree Fruit Specialist, OMAFRA

The current wildfires have been top of mind, not only raising concern about your own health and safety, but also if the smoke from the wildfires can impact horticultural crops. Unfortunately, there hasn’t been a lot of research done to evaluate the impact of smoke on horticultural crops. However, the research that is available suggests that whether you are growing apples, berries, grapes, tender fruit, or a specialty crop, the smoke from wildfires may have an impact on crops.

The effect of the smoke on the crops varies considerably depending on the crop, the proximity of the crop to the fire, how thick the smoke is, the duration of the smoke, the stage of development of the crop and how much light is affected.

Table of Contents

Photosynthesis

In general, the smokey haze that is in the atmosphere is essentially creating shade coverage for fields/orchards/vineyards and cuts the direct sun resulting in diffusion of light, and reduced photosynthesis. If there is enough particulate matter from smoke, it can land on and clog the plants stomata (tiny openings on the surface of the leaf and other parts of the plant that open and close to bring in the carbon dioxide and release oxygen) which can have a negative impact on photosynthesis and plant growth. However, interesting enough some research from the Central Valley in 2018 showed that when smoky skies only blocked about 4% of the sunlight, the smoke scattered the sunlight allowing the light to reach further down into the foliage of more dense canopies resulting in increased photosynthesis efficiency and leading to productivity increases.

Ozone on Plant Growth

Ozone is the main pollutant in smog and ozone injury to vegetation has been reported and documented in many areas throughout North America, including the southwestern and central regions of Ontario.

Ozone symptoms characteristically occur on the upper surface of affected leaves and appear as a flecking, bronzing or bleaching of the leaf tissues (Figure 1). Although yield reductions are usually with visible foliar injury, crop loss can also occur without any sign of pollutant stress. Conversely, some crops can sustain visible foliar injury without any adverse effect on yield. Injury levels vary annually and white bean, which are particularly sensitive, are often used as an indicator of damage.

Figure 1. Photo depicting ozone damage on soybean leaves.

Smoke contains high levels of ozone (O3) which leaves will absorb as they try to photosynthesize, requiring energy to combat, and decreasing photosynthesis overall. This takes away from the plants’ ability to put energy towards fruit production and other metabolic processes. If plants uptake enough ozone it can lead to an overall decrease in the size and quality of fruit that is produced. This is caused by the plant dedicating resources to fighting and repairing the oxidizing effects of ozone, while also having its sugar production decreased by ozone damage.

For reference, The maximum Ontario Ambient Air Quality Criteria (AAQC) for 1-hour average ozone concentrations is 80 parts per billion (ppb), which has been incorporated into Ontario’s Air Quality Health Index. For reference, on June 6, 2023, at 10 a.m., concentrations averaged 32 parts ppb (Figure 2) where last year, on June 6, 2022 at 10 a.m. concentrations averaged 30 (Figure 3), meaning there hasn’t been a significant increase in ozone. For more information on Ontario’s Air Quality Health Index, please refer to this link: http://www.airqualityontario.com/aqhi/index.php.

Figure 2. Map illustrating ozone concentrations (ppb) across Ontario on June 6, 2023 at 10 a.m.

Figure 3. Map illustrating ozone concentrations (ppb) across Ontario on June 6, 2022 at 10 a.m.

Ethylene on Plant Growth

Bushfire smoke contains ethylene which, depending on the crop stage, can contribute to fruit maturity. In short doses there is little to be concerned about, but extended exposure of several weeks may speed fruit maturity by 5 to 7 days towards the end of the season.

Smoked Apples?

At our current crop stage, fruitlet sizing, the impact of smoke is an ambiguous one. A reduction in photosynthesis caused by the smoke could affect carbohydrate balance – leading to less reserves later in the season, but also the potential to affect apple thinning. Normally, cloudy days are ideal to increase the efficacy of chemical thinners, but a delay in chemical thinner response could occur due to the smoke stress. This is an area that isn’t well researched, but growers have been noting delays in response to various product applications – which could be caused by the severely dry weather, low humidity and potentially the smokey skies.

The impact of the smoke from wildfires on apples has been noted to help apples as they develop, although this comes from reports in different climates, such as British Columbia, California, Washington and Minnesota. The smoke aids apples when the sun gets diffused by aerosols in the atmosphere, allowing for softer light to hit more of the tree canopy. The haze and its shade effect will also contribute to lowering overall temperatures in an orchard, creating more steady average daily temperatures for apples. Lastly, if there continues to be fires or the fires come back closer to harvest, there are reports of the smoke preventing sunburn on apples from harsh direct sunlight.

Plant Protection

There isn’t much that growers can do besides trying to keep plants cool and keeping plants clean in the response to wildfires. Lowering the temperatures and washing the plants off is the best mitigation strategy, whether that be via irrigation or rain (we hope soon!)

Smoke Health & Safety

Protecting yourself, your family and your employee’s health and safety are of top priority. Wear proper PPE to protect your heart and lungs from the smoke in the air. Keep tabs on the air quality in your area and respond accordingly, this website monitors air quality across Ontario. Ensure that everyone on your team also has access to proper PPE such as N95 masks, air purifiers if working inside, and try to limit the amount of time spent inhaling the smoke and ash from the fires. Additionally, follow all guidelines set out by local authorities, follow all fire bans and take extra precaution surrounding anything that may be flammable. Finally, have a plan in place should you need to respond to a fire emergency. Additional information on wildfire smoke and worker safety can be found at Wildfire Smoke: Frequently Asked Questions (gov.bc.ca)

Avoid burning trees and rubbish that will help contribute to the smoke in the air.

Monitor your crop for signs of heat stress and irrigate where needed and for symptoms of ozone damage.

Conclusion

While the impact of wildfires on your crop(s) are important to monitor, it should have little to no impact for the time being. To see a large impact from the wildfires, plants must be exposed for an extended amount of time and under severe conditions.

It is important to note that there is significant variability in the contents of wildfire smoke, proximity of the crop to the fire, how thick the smoke is, the stage of development of the crop and how much light is affected and therefore it is difficult to make accurate statements about the impacts on specific crop species.