Tackling the mixed maturity problem
New Zealand apple production is focused at the premium market and needs to be there to sustain the viability of our industry in our high-cost economy.
This means that we need to deliver the consumer crisp, juicy, flavourful fruit capable of generating repeat sales from loyal customers prepared to pay the premium our fruit needs to cover costs.
Judging by the comments coming back from the market regarding the consistency of our product, we are in danger of losing the premium position if we are not careful. Mixed maturity leading to poor out-turn of some fruit after long term storage appears to be part of the problem.
I also suspect that sun-tinting and mild sunburn may be issues. With good fruit colour, mild sunburn symptoms tend to be masked out by high skin colour. So there’s a tendency for harvest of this fruit to be delayed until the sun tinting is no longer obvious. By this stage, the fruit is likely to be over-mature and not at all suitable for medium or long term storage.
Post-harvest scanning technology for internal defects is advancing rapidly so it is probably only a matter of time before problem fruit can be segregated from the line and kept out of fruit destined for premium markets. When this happens we might see visually good fruit headed straight for the reject bin.
Packhouse operators may need experience in psychology when this happens to be able to explain to irate suppliers why their packouts are so low.
To maintain packouts and avoid this problem, mixed maturity and sunburn disorders will need to be managed in the orchard before and during harvest.
Warm regions with daily temperatures exceeding 28 to 30°C may have a sunburn risk to exposed fruit. As fruit size increases, sunburn risk rises. At about 45mm fruit diameter, fruit becomes vulnerable to injury. The level of sunburn injury is determined by fruit surface temperatures (FST). Mild
sunburn injury, referred to as sun-tinting or skin browning, usually occurs when FST is in the range of 45 to 49°C, with more severe necrosis injury occurring at FST above this level. FST on calm, sunny days is usually in the range of 14 to 17°C above ambient air temperatures. A slight breeze will reduce FST by about 3°C. This probably explains why there is more sunburn injury in the lower tree fruit than the more exposed upper tree fruit.
In addition to these two levels of sunburn injury there is a third form which can occur at lower temperatures. This sunburn symptom initially shows up as a white patch on the fruit and is known as photo oxidative bleaching. It occurs when there is a sudden change in light exposure. This is the typical sunburn you see from late thinning, selective picking, excessive summer pruning or branch movement as crop weight comes on to branches. Studies into sunburn incidence shows that it’s the midafternoon sun that causes most injury. This is the hottest and driest part of the day so fruit exposed to sun at that time gets the harshest sun and is also likely to be under moisture stress as well, so transpirational cooling is minimal.
As already mentioned, mild sunburn browning will often mask out as fruit colour intensifies towards harvest. While such fruit may now appear unaffected, its internal quality has been changed and the fruit is less suited to long term storage. The main internal changes include more starch degradation, probably because the fruit is more mature by the time there is enough colour to disguise the sunburn, increased flesh firmness and dry matter, higher sugar levels but lower fruit acidity and lower flesh moisture content. Affected fruit therefore, has poorer flavour and drier fruit texture.
FACTORS ASSOCIATED WITH INCREASED SUNBURN INJURY
Sunburn is made worse by;
• Over cropping. This leads to higher fruit to leaf ratios and lower levels of annual extension growth necessary to give adequate leaf cover protection for the fruit.
• Water stress. This has a major influence on sunburn incidence, particularly for varieties ripening in the first half of the harvest season when temperatures are high. Water stress as harvest approaches can easily drop packouts by 20 percent due to sun tinting injury.
• Nutrient deficiency. Anything which reduces leaf quality may increase sunburn injury. The literature reports nitrogen deficiency to increase sunburn and here magnesium deficiency is certainly a factor.
• Spray and spray injury. Leaf injury from calcium chloride or sulphur sprays during periods of high temperature.
• Fruit size. Larger fruit have lower surface area to volume so will develop higher flesh temperatures than smaller fruit.
• Late thinning and selective picking of bunchy crops.
MANAGING THE SUNBURN PROBLEM
Thin the crop early and well. Spaced singles will minimise sunburn injury during later crop grooming and harvest. Try to avoid water stress during the heat of the summer. There is good evidence to show that mulching will conserve soil moisture, lower root temperatures and reduce moisture stress in the tree. Mulching reduces sunburn injury and with