The maximum temperature of the sun-exposed fruit surface of apples is often 10 to 18°C higher than the maximum shaded air temperature. Sunburn can occur when the shaded air temperatures are above 30°C and fruit surface temperatures are above 45°C.

From 30°C to 35°C, damage is variable, depending on wind, sunlight intensity (cloud cover), humidity, and level of fruit acclimatisation to sunlight. Above 35°C, there is a high risk of browning damage. Above 40°C, there is a high risk of a necrotic patch.

Smaller fruit, an overcast sky, a light breeze, and a well-watered orchard reduce the risk. Vigorous trees with more vegetative growth and more internal shading also at a reduced risk of sunburn.

“This year’s a struggle,” said Michael Crisera, Grower Services Manager at Fruit Growers Victoria. “As fruit gets close to harvest, heat stress can be an issue. Once it gets in excess of 36-37 degrees, the stress makes trees stop, which impacts size.”

Michael indicated that at this time of year, Galas are most affected by the heat, and depending on maturity the heat can have a detrimental impact on marketing yield.

“Growers should be encouraged to put nets on their high-value crops.”

Management practices
In addition to using the appropriate tools, growers can undertake a variety of management practices to protect their crop:

Schedule irrigations to avoid tree water stress. Healthy, fully irrigated trees receive the maximum cooling benefit from transpiration. Irrigation scheduling techniques based on weather forecasts and soil moisture measurements should be used to ensure irrigation is well-matched with the crop’s water requirements.
 

Train fruit trees to develop an appropriate canopy. Apple and pear fruit sunburn is often associated with thin exposed canopies or with the sudden exposure of fruit to sunlight when branches move under the weight of the developing crop.
 

Avoid excessive summer pruning or leaf stripping. This is often done to allow light into shaded parts of trees to enhance apple colour development, but this should be done carefully to avoid sudden exposure of fruit to direct sunlight.
 

Protect picked fruit in the bin from direct sunlight. Even a relatively short period of exposure to intense direct sunlight could cause significant damage.
 

Establish suitable cover cropping in the inter-row space. Light coloured bare earth and dead vegetation in the inter-row space is likely to reflect more sunlight into the orchard canopy than green vegetation. This can be an issue in the hottest parts of summer.
 

Improve air movement through the fruit block. The temperature of fruit skin in direct sunlight is higher than the temperature of the surrounding air, so air movement around the fruit helps to equalise the temperature.