
Sunburn browning
With the recent bout of cooler weather, you’re forgiven to ask, “where has our summer gone?” The good news is, NIWA is 90% sure that temperatures for our region, during the Jan-Mar period, will be either near (40% chance) or above (50% chance) average. Great apple growing weather yes, but with it comes an increased the risk of Sunburn damage (Figure 1).
The most common damage is called Sunburn browning. This requires both heat & light. Sunburn browning occurs when the fruit surface temperature (FST) climbs to 46 – 49oC, combined with the high UVB rays that we are blessed with here in New Zealand. Photooxidative damage can also occur when FST climbs above 30oC. This is expressed as a bleaching of the skin. Both result in rejected fruit.
The FST can be 11oC higher than the air temperature but on stressed apple trees this can be up to 17oC hotter. For example, on a 30oC air temperature day, the FTS on a healthy apple tree could be as high as 41oC and thus avoids sunburn browning but on a stressed apple tree the FTS could be as high as 47oC, resulting in sunburn browning. Managing stress in the orchard is therefore the most effective way to mitigate Sunburn browning.
Some strategies to consider:
1. Avoid water stress
a. Avoid water stress but equally avoid over irrigating.
b. Base irrigation scheduling on accurate soil moisture monitoring.
2. Balanced nutrition
a. Avoid Potassium deficiency. Potassium is involved with stomal opening & closing so critical for efficient water movement in the tree.
b. Ensure adequate Fruit Calcium status. Calcium protects cells from toxins.
3. Appropriate crop load – do not over crop.
a. Over cropped trees are stressed trees and much more prone to Sunburn.
4. Balanced/calm canopy
a. Optimum leaf to fruit ratio
5. Good management of pests & diseases
a. Minimise disease pressure and lower tree stress.
6. Avoid sudden exposure of fruit to direct sunlight.
a. Take care with late summer pruning.
i. Note: heavy summer pruning is symptomatic of more serious underlying problem. Seek expert advice.
b. Take care with late fruit thinning and breaking up bunches exposed to the light.
c. Take care/avoid leaf plucking without over-head net protection
7. Overhead netting reduces fruit surface temperature. Light intensity is reduced under net because the light is diffused or scattered when it enters the net.
8. Keeping sward cover at a longer length will keep the temps lower in bottom of the canopy.
9. Sun protection products such as white particulate coatings and clear sun blockers, can help reduce FTS and solar radiation on exposed fruit. Note: Care needs to be taken with the use of these products on certain varieties. Seek expert advice before application.
10. Don’t leave bins of Apples baking the sun for too long after harvest. Move bins to the shade sheds a.s.a.p. or cover them.
Figure 1
Dean Rainham
Agfirst Horticultural Consultant
027 839 9613