Sun Burn/Sun Scald
Identification
Pear Fruit
- Occurs on fruit exposed to the sun
- Bleached yellow areas on exposed fruit surface
- Injured areas may turn dark while fruit is still on tree, easily graded out at harvest
- On occasion, small, yellow spots develop that are easily overlooked at harvest
- In storage, injured area turns dark brown, referred to as sunscald
Peach and Plum Fruit
- Double and triple fruit fused together
- Sometimes with doubles (twins), one fruit may abort and the resultant peach will have a malformed suture at harvest also making it unmarketable
- Cracking or brown discolouration on exposed side of fruit
Larger branches or scaffold limbs
- Affects scaffolds not shaded by in-canopy foliage that are directly exposed to the sun
- Bark becomes pinkish or reddish
- Rough, dark patches of outer bark as dry and crack
Biology
Sun burn is a result of heat damage to fruit with UV radiation reducing the temperature at which damage occurs. It can be a significant problem following extreme heat spikes - particularly pre-harvest when fruit begin to soften. Direct sunlight on unprotected fruits, such as those previously in shade, is common. A tree’s water status allows it to cool itself through transpiration. This means that water stressed trees are more prone to sunburn damage.
Flower buds for next year’s crop are initiated this summer. Sustained periods of high temperature during the flower initiation phase can cause the production of 1, 2, or 3 pistils. If pollinated, this may cause single, double, or triple fruit.
Period of Activity
Fruit sun burn will be worse if a week of cool, windy, overcast weather is followed by a hot (over 30°C) calm day with full sunshine. If the weather forecast is for a dry, windy day with low UV radiation then the air temperature for the appearance of damage is increased to 35°C.
While sunburn can occur at any stage of fruit development, conditions are most suitable for sun burn in the three months prior to harvest and particularly in the afternoon, between 2.30 and 4.45 p.m.
The symptoms will first appear on fruit that has been recently exposed to the sun (within the last 24 hours).
Sun burning of fruit can occur from an exposure as short as 10 minutes to suitable conditions. This means that sun burning can potentially occur in the picking bins during the harvesting process where shade grown fruit or portion of fruit could be exposed to the sun.
Management Notes
Perform summer pruning on cool days when the forecast is for cool weather in the following three to four days.
Maintain adequate moisture levels in the orchard.
When harvesting in temperatures over 30°C provide shade for fruit in the field bins ensuring they do not overheat.
Apply light reflecting materials, such as kaolin clay to pear trees to reduce the absorption of radiation by the fruit. Applied to the tree canopy, kaolin forms a microscopic particle film that protects leaves and fruit from high UV light and heat, which can slow the photosynthetic rate in the tree canopy. For more information on kaolin clay refer to Kaolin Clay (Surround WP) in OMAFRA Publication 360, Fruit Crop Protection Guide.
Apply white latex paint to the trunks of young trees to reflect sunlight and reduce injury from sun burn.
Information included above has been excerpted from:
