By Geraldine Warner
Crop load could be a key factor in the Braeburn browning disorder, and it may be advisable not to harvest lightly cropped trees, said Dr. Eric Curry, plant physiologist with the U.S. Department of Agriculture's research laboratory in Wenatchee, Washington.
Braeburn browning is an internal disorder that can appear in fruit either on the tree or during storage.
Curry began studying maturity and storage parameters of Braeburn five years ago. During the first two years, he did not see the browning disorder. But during the four seasons since, it has occurred in some fruit in some orchards.
In March, he spent a week in New Zealand to share his knowledge and learn more about the disorder. Braeburn is New Zealand's top variety and has been grown commercially there for at least 15 years.
Curry said he categorizes the Braeburn browning disorder into three types:
--The first develops on the tree. It is a dense, watery kind of browning that does not form a pattern in the fruit. It has appeared as early as August.
--The second type has been found in fruit that is packed within days of being picked and stored for three or four weeks in the cartons. This browning usually forms a symmetrical pattern around the core. This appears to be the same as a condition called brown heart that was reported on some varieties, such as Worcester Pearmain and Australia Sturmer Pippin, in England in the 1920s.
--The third type is found in fruit that has been held in controlled atmosphere (CA) storage and subjected to long-term low oxygen levels. Brown areas develop throughout the fruit tissue.
Fruit can be affected by any combination of the three types.
Curry said the type of browning that appears while the apples are on the tree is the most difficult to understand, and no single cause has been identified. It may be a combination of several factors, including the weather at bloom when the fruit cells are dividing, extreme changes in temperature during the season, water stress, calcium uptake by the fruit, or crop load.
"All those things have been implicated," he said.
Curry has set weather monitors out in the Washington orchards from which he has been taking samples, to try to figure out some kind of pattern associated with the disorder. It has been seen more in orchards with sandy soils and after severe winters.
Dr. Gene Kupferman, Washington State University postharvest specialist, who recently spent six months in New Zealand, said the disorder seems not to be a problem in some of New Zealand's growing districts, and is most severe in the Central Otago district in the southern part of the South Island, a desert area whose climate is the most similar to that of eastern Washington.
Kupferman has brought back weather records, showing the maximum and minimum temperatures for the past three years in two districts--one where the disorder is not a problem and the other where it is. He will compare the data for the two districts with weather records from Washington State to try to pinpoint any climatic causes.
Curry said trees in New Zealand grow very differently from trees in Washington. Braeburn was discovered as a chance seedling in an orchard near Nelson, on the north coast of New Zealand's South Island, and seems well adapted to conditions there. The trees grow large and lush, and crop heavily. Typically, a tree will produce between 700 and 900 apples, about three times as many as a tree in Washington. Trees are about twice the size of those in Washington, and they are heavily cropped to prevent them growing larger still.
The crop load affects the calcium level in the fruit. Curry said Dr. Richard Volz, a researcher in New Zealand, has established that apples from trees that carry a light crop generally have lower calcium levels and are more prone to the browning disorder. This is because lightly cropped trees have a high leaf-to-fruit ratio, and when the leaves' demand for water is higher than the roots can supply, water containing calcium is drawn away from the fruit, Curry said.
In New Zealand, growers are discouraged from harvesting fruit from lightly cropped trees because of potential calcium-related problems, such as the Braeburn browning disorder and bitter pit. This is despite the fact that they apply about ten calcium sprays per season.
He acknowledged that the strategy of not harvesting fruit from lightly cropped trees may not work as well in Washington, as growers typically avoid overcropping Braeburn trees in the early years for fear of stunting the tree.
Curry has been testing fruit from a couple of Washington orchards that have consistently had problems with the disorder. Because of the evidence that crop load plays a part, he plans to compare fruit from lightly and heavily cropped trees this season.
The reason the second type of Braeburn browning disorder appears in fruit packed soon after it is picked is believed to be a combination of temperature and poor gas exchange within the fruit. Dr. Nigel Banks, at Massey University, Palmerston North, New Zealand, heads a research group that is studying this disorder and is looking at the factors influencing the permeability of the fruit.
Curry said fruit that has not had all the field heat taken out of it continues to respire, using up oxygen and generating carbon dioxide in the process, and if it is packed in a box, the air cannot circulate.
"Packing them with any kind of field heat, and putting them in storage, and allowing them to sit for weeks is not a good idea," he said.
Kupferman said the New Zealand industry has learned to isolate late-harvested fruit from the Central Otago district, where the disorder is most commonly found, and hold it in storage in bins for at least 14 days before it is packed and shipped.
His advice to Washington shippers is to avoid waxing the fruit or keeping it in storage with high carbon dioxide levels. The oxygen in the CA storage should be brought down slowly.
The New Zealand industry does not store fruit in long-term CA storage, and so has not had problems with the third type of Braeburn browning disorder that Curry described. Most fruit is packed and shipped soon after harvest and would be held for only a short time in regular storage.
Curry's trip was sponsored by the Washington Tree Fruit Research Commission. Curry said he was impressed by how willing the New Zealand researchers were to share their knowledge and experiences.
Copyright © 1996, Good Fruit Grower