Weather Risks: Strategies to Mitigate the Risk of Excessively Hot Temperatures
Table of Contents
This in the 7th in a series to help apple and tender fruit growers in Ontario assess the weather risks that can damage their trees and crops. It is important to recognize the weather risks at each location, and develop strategies to reduce or eliminate the impact on your business operation.
What it is
Summer temperatures have been increasing in recent years, both with record-setting temperatures, as well as the duration of heat waves. Records have been set for both day-time and night-time high temperatures. Hot temperatures cause stresses to fruit trees, as well as workers, and make it challenging to complete orchard work on a timely basis. Hot temperatures often occur in conjunction with dry periods, so soil moisture levels are depleted more quickly under intense sun and high temperatures.
When it occurs
High temperature injury may occur anytime that temperatures exceed critical levels for each species (often above 30°C) and incoming solar radiation is high. Fruit injury can occur any time after fruit set in the spring until postharvest, although most of the damage occurs during July, August and early September. Peak injury is from noon until late afternoon each day, especially when skies are clear, although solar radiation can be high under cloudy conditions as well.
Where it occurs
High temperatures cause stress on the tree, increasing evapotranspiration and limiting photosynthesis and accumulation of stored carbohydrates and fruit sugars. Solar radiation may cause sunburn on the skin of the fruit and on leaves. In extreme cases, the tissue below the sun cheek of the fruit may actually cook. Such heat can cause fruit to mature unevenly with flesh near the skin being mature, and internal flesh is under ripe. Harvested fruit that is not removed from the heat will continue to respire at a high rate, reducing weight and quality, and shrivelling from water loss. If extremely high temperatures occur during bloom, pollen may lose viability, reducing fruit set, although this is not common. Applying pesticides in excessively hot temperatures can also result in phytotoxicity to the crop.
What can you do
Some of these potential mitigation strategies may help reduce or eliminate damage due to winter injury:
- Production insurance (PI): Production insurance can cover yield loss from reduced fruit size from extreme heat. Your premiums will depend on the coverage you choose, your claim history, and the yield potentials of your orchards. Over time, your premiums can be reduced if you are lucky enough not to have claims. However, some growers struggle with the premium costs (especially in their start-up years or if they have claims) and the fact that PI is not intended to fully cover your loss, either in yield or price. There is also the problem of reduced coverage levels in the years after your crop is reduced, due to the effect of the loss on your long-term average yields. Also, spot loss insurance is not available, so growers with multiple orchard sites may be penalized when good yields occur on the non-damaged sites.
- Surround Crop Protectant: This kaolin-clay product will protect the fruit from sunburn, and reduce the temperature in the leaves, which improves photosynthesis and subsequent yields. It has the added benefit of discouraging several insect pests. However, Surround can be irritating to workers as well. Because Surround is not rain-fast nor does it re-distribute as the fruit sizes, re-application is frequently needed, especially after each rain. It may be difficult to remove Surround from the fruit after harvest, although some consumers prefer stem and calyx end residue, once they understand its origin. There may be potential export issues with Surround.
- Bagging fruit: Tucking each individual fruit into specially-designed bags will prevent sunburn, and is a common practice for elite markets when labour is less expensive. These bags also prevent some insect injury. The main disadvantage is the cost of bags, labour to apply and labour to remove. Environmental issues can be a problem, since growers must dispose of the bags. Care must be taken to time the removal when sunlight is less intense to prevent last-minute sun damage. When properly timed, use of fruit bags can produce a desirable blush colour on fruit.
- Evaporative cooling: Use of overhead irrigation at low rates will cool the trees and fruit when used properly, which increases photosynthesis and yield and improves fruit quality. This technique has been used successfully by berry growers in hot harvest seasons. There is a cost for solid set equipment and labour to move, and it may be a challenge to source water during the traditional dry season. However, care needs to be taken to prevent disease infections, and to avoid droplet accumulations that may cause sunscald on the fruit. Late weed germination may increase.
- Orchard floor cooling: Under-tree micro-sprinklers may be useful to cool the orchard environment, but the investment in the system and its maintenance is large. Again, care needs to be taken to reduce the risk of disease, and late weed germination may result.
- Harvest management: Temperature and humidity can have a significant negative impact on fruit quality. After harvest, fruit are often left in the heat of the sun for several hours before being shipped to the packer or storage. Harvested fruit needs to be moved quickly out of the sun. It is preferable to move to refrigerated storage as soon as possible, but at minimum, fruit needs to be moved into the shade (either trees or with a sun cover). Research in some fruit crops has shown that lowering fruit temperatures immediately after harvest can result in firmer fruit with reduced decay. Reflective tarps constructed of Mylar material sandwiched between a white painted surface and a shiny silver metallic surface, have been shown to improve fruit quality in cherries and blueberries.
- Increased storage capacity: When high temperatures continue through harvest, your storage needs to have enough capacity to quickly remove field heat from the harvested fruit. The investment cost in refrigeration and/or smaller rooms is needed to preserve fruit quality and extend storage life. Additional labour is needed to ensure that fruit is quickly but gently moved to storage and to ensure record-keeping for easier packing.
- Avoid spraying during warm temperatures: To reduce issues with phytotoxicity, avoid applying pesticide sprays when temperatures exceed 30°C. Instead apply sprays in the early morning or overnight when temperatures are cooler.
Avoiding or reducing heat damage is an ongoing challenge for many fruit growers. The extensive damage from a short period of intense sunlight, or the continuing stresses caused by longer heat waves can be debilitating to your crop and workers. As weather patterns produce more hot weather and intense sunshine, growers should be planning to reduce the problem. The investment in intensive orchards and high-valued varieties may encourage more investment in heat strategies in the future.
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