Ugly duckling's unique quality

NSW DPI research horticulturist Trevor Olesen
NSW DPI research horticulturist Trevor Olesen says the growth habit of custard apple trees is unlike that of most other trees.

Like any "ugly duckling", there’s often something unique and interesting under the surface. So it is with custard apples, a sub-tropical deciduous fruit that has developed into a small but thriving commercial industry on the NSW north coast.

The fruit surface may be a bit rough but the flesh is sweet and creamy and increasingly popular with consumers.

What’s more, the tree has a very unusual growth habit that may hold the key to significant improvements in tree management, fruit quality and harvest timing.

NSW DPI research horticulturist Trevor Olesen says the growth habit of custard apple trees is unlike that of most other trees.

"Normally, when you prune a tree, it releases a new shoot from an axillary bud further back along the branch," said Dr Olesen, who is based at Alstonville.

"But with custard apples, a leaf has to be removed for a new shoot to develop," he said.

"Axillary buds won’t release until the leaves are broken off, or drop off naturally."

Dr Olesen said this opens up all sorts of options for manipulating the way custard apples are pruned to improve tree management, fruit quality and harvest timing.

Custard apple
The custard apple’s surface may look a bit rough but the flesh is sweet and creamy and increasingly popular with consumers.

"In a small trial, I found that I could limit the size of custard apple trees by pruning in January and not removing any leaves, yet still produce the same number of fruit as trees that weren’t pruned," he said.

"Custard apple trees are very vigorous. By pruning in this way I found I could reduce the current season’s growth by up to 1.5 metres.

"Less regrowth means less rubbing of the fruit - which can be a cause of fruit damage - and less material to prune off the tree next year.

"The reduced canopy can also slow fruit development which is good for the NSW crop because it means fruit come onto the market later in the season, when prices are higher."

Dr Olesen thinks there may be further benefits in new pruning techniques and the time of pruning.

"We might be able to improve the shape of the tree by pruning just the top in summer and leaving a crown of foliage in the middle."

He said this is likely to encourage lower branches to grow more horizontally.

"The fruit on lateral branches are generally more plentiful, less exposed and not as subject to cold damage."

The timing of pruning is also important with the aim to have the fruit hard green going into winter, to reduce the threat of splitting.