Orchardists who have not covered their apple trees with netting, and can already see considerable sunburnt fruit, may now be considering other methods to protect their future crops.
For instance, is overhead cooling with sprinklers a good alternative?
In a tree, water helps to regulate the temperature of fruit and leaves. During the normal process of water loss through the fruit and leaves (transpiration), the fruit and leaves are cooled.
The cooling process depends on the temperature and humidity gradients surrounding the fruit and leaves.
Overhead cooling is effective when the water is evaporated before it hits the ground. It is not the temperature of the water that cools down the orchard, but it is the evaporative action that cools the fruit. In a similar way, we feel cold at the beach as our skin dries.
US research
Much research on overhead cooling was done in the USA, and aimed to enhance colour and prevent fading of the skin of Red Delicious in hot weather.
Cooling with overhead sprinklers was widely used in Washington State in the 1990s, and proved effective with Red Delicious apples.
However, with the introduction of redder varieties and strains, overhead cooling was no longer needed to improve colour.
Overhead cooling was also used to reduce sunburn on other varieties, including Fuji.
When the temperature in an orchard is 39C, uncooled fruit can have a temperature of 46C to 49C. An effective overhead cooling system can reduce this fruit temperature to 32C.
Fruit sprayed with a clay-base sunscreen can reduce the fruit temperature to 37C.
Causes of sunburn
Sunburn is not strictly due to heat alone, but to an interaction between heat and exposure to ultraviolet light. This is why apples can still get sunburnt even when you use overhead cooling.
Fruit covered by leaves at some part of the day, particularly in the afternoon, does not get burnt.
With a heavy crop load, trees often have not enough leaves to cover the apples, which are then more exposed to the sun and become more susceptible to sunburn.
Experience with overhead cooling
A few orchardists in the Goulburn Valley have tried overhead cooling, but soon gave up because of the problems associated with overhead cooling, and because netting systems—with advantages over overhead cooling—became popular.
American experience also showed that overhead cooling often created more problems than it solved. Here are some of their experiences:
- The greatest challenge was ensuring that the orchard was cooled at critical times. It was a real hassle all summer to decide when to turn the water on and off at the correct times.
- It only took one missed day (because of a faulty pump, or no power for the day) during the height of summer, and the apples ended up burnt.
- Overhead cooling increased incidence of pests and diseases. Codling moth was particularly troublesome because any insecticide was washed off trees and lost by overhead cooling.
- Mating disruption was used with automatically dispensed pheromones from ‘puffers’. The puffers had timers that dispensed the pheromone in the evening when moth activity was greatest.
- There was also an increase in mildew pressure from overhead cooling, and fruit was often downgraded because of diseases such as black spot (scab), associated with high humidity.
- Overhead cooling supplied the water that such diseases needed to become established.
- Overhead cooling interfered with irrigation scheduling.
- Mineral deposits built up on fruit and leaves and were difficult to remove, and have decreased leaf function.
- Without clean water, overhead cooling became a bigger problem than no cooling at all.
- Sodium built up on leaves that damaged the leaves and sometimes defoliated the trees.
- Overhead cooling caused fruit on varieties (such as Fuji) to crack.
- A lot of water was needed for cooling to be effective. In one example, the cooling cycles began in June (December in Australia) with cycles of, ten minutes on and ten minutes off, until the temperature dropped to 30C. Cooling ran for up to eight hours a day and applied 730 litres of water per minute per hectare. As harvest drew nearer, the temperature with overhead cooling was adjusted to 32C and never reached above 34C.
- Overhead cooling reduced the amount of water the trees needed to take up from the soil. As the trees cooled, they did not use as much water internally to cool themselves down.
New research in the US
At the Michigan State University in the USA, a two-year project funded by the Specialty Crops Research Initiative is developing a Solid Set Canopy Delivery System which is suspended over an orchard.
This system is fixed in place and can do all the things that an airblast sprayer can do—and more— such as cooling the orchard or controlling frost.
This cutting-edge research costs millions of dollars, but has enormous potential to produce fruit more efficiently and with high packouts.
This kind of R&D work should also be done in Australia to keep our fruit growers competitive in the global world markets. Unfortunately, our state governments have opted to close research establishments and extension offices.
Netting
There is no question that netting prevents sunburn to a large extent.
The major drawback is the tremendous cost. Only a high-density orchard that maximises surface area of the canopy per hectare, and is well managed, has a chance to re-coup the investment quickly, especially with repeated heat waves.
Using better management to prevent sunburn
Instead of tackling sunburn with expensive solutions such as overhead cooling or netting, you could protect your apples from getting burnt by using an orchard system with good training, pruning and canopy management, combined with a semi-dwarfing rootstock. This involves:
- using an orchard system such as the Open Tatura, with its inclined thin canopies
- growing leafy trees with short stiff fruiting wood
- using the MM106 rootstock which has a good proven track record of imparting both vigour and precocity on the scion.
Rootstocks and sunburn
The dwarfing M9 and M26 rootstocks have successfully been used in countries with cool climates to produce large crops from dense orchards of small trees.
But in countries with long hot summers, orchardists that use these rootstocks in such orchards and not cover trees with netting, will produce a high percentage of sunburnt apples—and quickly lose the profits that dense orchards with small trees can generate.
See all the images in the February 2013 Tree Fruit
