Gartman: This commodity has a serious problem

In 2014, there was only one commodity that Dennis Gartman wanted to own: corn.

But after a torrid run following his recommendation, corn futures have cooled off in 2015, falling 7 percent year to date. And according to the "commodities king," it's about to get a lot worse.

Read MoreDennis Gartman: If you buy one thing, buy this

"I think that corn has a problem at this point," said the CNBC contributor and editor of The Gartman Letter on Thursday's "Futures Now."

The first issue being oversupply. "American farmers are going to plant a lot of corn this year," said Gartman. "Famers still haven't sold much of last year's crop. That has to get done first. So you have a large portion of last year's crop to be sold, and a big crop this year," he added. "This could put corn prices under some very real pressure."

But the biggest threat to corn, Gartman said, is the recent avian flu outbreak. "What's really interesting is the psychology in avian flu."

The influenza, also known as bird flu, has recently been detected in chickens at farms in Wisconsin and Iowa. "If the avian flu continues to break out it means there are migratory birds moving avian flu, not one flock to another in a confined area," he added. "That will ultimately make the consumer very hesitant to buy poultry, poultry is a very big user of corn. And that's terribly bearish for corn."

Read MoreUS poultry exports plunge with avian flu scare

So, how low does Gartman expect corn to go?

"I think we could see it 50 to 60 cents per bushel lower in the not-so-distant future." That would put corn at its lowest level since October 2014.

"This is a real concern," said Gartman.

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