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Thin early and effectively: Practices can manage apple crop load, explains U.S. researcher

Practices can manage apple crop load explains researcher

March 4, 2008  By Peter Mitham

Thin early, thin effectively.  That’s the message Tory Schmidt – a horticultural research associate with the Washington Tree Fruit Research Commission in Wenatchee, Wash. – has for British Columbia apple growers

Thin early, thin effectively.  That’s the message Tory Schmidt – a horticultural research associate with the Washington Tree Fruit Research Commission in Wenatchee, Wash. – has for British Columbia apple growers.

“It’s really important to adjust your crop load as early as possible if you want to give yourself the best chance of having good-sized fruit at harvest as well as having solid return bloom,” Schmidt told growers during the B.C. Fruit Growers’ Association’s Horticultural Forum, held last November in Penticton, B.C.


Schmidt is conducting 240 chemical thinning trials on major apple varieties and rootstocks at various locations in Washington state. Thinning trials are also being undertaken on pears, cherries and other soft fruit.

The trials aim to find thinning regimes that reduce production costs, yield high-quality fruit and promote return bloom, which ensures consistent annual cropping and therefore more reliable returns. Consistent annual cropping is an issue often overlooked by growers, Schmidt said.

Thinning materials of particular interest to Schmidt for the trials include ammonium thiosulfate, NC99 (a calcium-magnesium blend), lime sulphur and Crocker’s fish oil, among other natural oils known to be effective thinning agents. The materials were used by themselves as well as in various combinations.

The trials are randomized, replicated and analyzed using appropriate statistical methods, Schmidt explained. Approximately half the field trials of chemical thinning agents employed a tower sprayer operated by the research commission, and the other half by participating growers who used their own equipment.

Trials on a Honeycrisp block near Yakima, Wash., in 2003 were typical in yielding results indicating that tandem applications of Crocker’s fish oil and lime sulphate could be good options. The thinning regime yielded a reduction of between 25 per cent and 35 per cent in blossoms versus the control, as well as a four-fold increase in return bloom the following year.

“The gold standard in our experience has been this Crocker’s fish oil and lime sulphur program,” Schmidt said.

Another trial with promising results saw an application of Crocker’s fish oil reducing blossoms 63 per cent. Even better, fruit diameter increased 30 per cent and return bloom was 52 per cent – a three-fold increase over the control.

The trial indicated that effective thinning practices can also enhance fruit development, or at least have an effect that should be taken into consideration when developing a thinning regime.

Schmidt pointed out as an example a 2005 trial in a Golden Delicious block near Manson, Wash., where the trees were grafted on M26 rootstock.  A blend of Crocker’s fish oil and lime sulphur were applied at the beginning of May, when fruitlets were about five millimetres in diameter. Within 10 days, when fruitlets were about 100 millimetres, the number of set fruit was down by about a third. By mid-May – two weeks after the initial application of thinner – fruit set was down to a quarter of the original bloom count.

A couple of things were going on here, Schmidt explained.  First, applying a bloom thinner earlier rather than later reduces competition among the fruitlets for nutrients.  “There’s a two- to three-week window in here where trees treated with bloom thinner are only carrying about half the crop load of trees you’ve waited to spray with post-bloom thinners,” he said.

The more intense the competition, the smaller the fruit will likely be.

Meanwhile, gibberellins – plant hormones that regulate growth – are being produced during the two-week interval. These send a message to the tree not to flower in the following season. The earlier the fruitlets are removed, removing the source of gibberellins that issue the signal to not bloom, the better the chances of return bloom are.

“You have got a much lower return bloom the longer you leave those fruitlets on the tree,” Schmidt said.

The results are pretty good evidence that growers have more to fear from inaction than action.

“Be early and be aggressive,” Schmidt told growers. “Your ability to manage your crop load effectively diminishes each day you wait.”

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