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The beer giant took action in 2008 against craft brewer Green Man, saying its use of the name was a trademark breach.
DB trademarked Radler in 2004, two years after its 5% Monteith's Radler hit the market.
Radler is a term commonly used in Europe and elsewhere to describe a shandy-like beverage with an alcohol content of around 1% to 2% by volume.
DB's Monteith's Radler has an alcohol of 5% by volume.
Mr Seaman said Green Man started producing its version because it wanted to make a true radler beer.
That radler was a registered trademark only became widely known when Green Man was notified it had infringed DB's trademark and was required to cease producing the beer, or rename it.
Unable to afford a protracted legal battle, Green Man relabelled its beer "Green Man Cyclist".
In 2009, the Society of Beer Advocates (Soba) stepped in and, supported by Hamilton intellectual property lawyer Ceri Wells, who offered his services free of charge, applied to have DB's trademark revoked, arguing radler was a generic term for a style of beer, and could therefore not be trademarked.
Yesterday, the Intellectual Property Office (Iponz) decided DB's trademark should not be cancelled on the basis the term radler was not known as a descriptive term in New Zealand when the trademark was granted in 2004.
Mr Seaman believed the decision was a "no-brainer", but it appeared Iponz held a different opinion.
"Basically, this means we can go out anywhere in the world and find a style of beer, and if it is not well known here we can trademark it," Mr Seaman said.
The situation highlighted the "completely" uncompetitive nature of the beer market in New Zealand, he said.
In a statement DB marketing general manager Clare Morgan said DB did not do it to prevent competition or restrict consumers' access to different types of beer products.
"Our trademark has never stopped shandy or lemon or lime-flavoured beverages being made by local brewers under their own brand name." While costs were awarded to DB, the brewer would not be asking Soba to pay up, she said.
Soba said the finding was "majorly disappointing".
"This whole thing is eroding any confidence people might have in trademarking," Soba spokesman Martin Bulmer said.
He paid credit to Mr Wells, who he said had a challenge going up against DB and its financial might.
The radler story had garnered international interest, and he was sure there would be many disappointed people all over the world.
No thought had been given yet to an appeal, he said.
Soba's position had always been it did not think any brewery should own the name of a beer style.
Marketing professor Phil Gendall, from Massey University, who made a submission to Iponz in support of Soba, said he was surprised by the decision.
While he would not have thought a style of beer could be trademarked, it appeared this decision was made on the basis of the level of the population's knowledge of the radler beer style at the time the term radler was trademarked, he said.
It seemed a term that was well known in other parts of the world, but not yet in New Zealand, could be trademarked without any consideration it might become well known in New Zealand in the future.
It was interesting to note DB had also trademarked another European style of beer - saison - but deregistered the trademark last year, Prof Gendall said.
"I wonder why they thought that was less defensible. Perhaps they weren't going to make any more saison."