(CNBC) - On a foggy morning in Napa Valley, California, Tim Cheng is walking through a vineyard surrounded by California's classic golden hills. He bends down to look at one vine. 

"Is the webbing from the mealy bug?" he asks Debby Zygielbaum. She manages soil health for Robert Sinskey Vineyards, an organic winery on Napa's Silverado Trail. Zygielbaum has been fighting the mealy bug with a blend of pheromone traps and beneficial predators. "When we first got infested, we lost 25 percent of the crop," she said. "Now you can hardly find them."

Cheng is an accredited organic certifier, but he doesn't work directly for the USDA. He is a third-party contractor for California Certified Organic Farmers (CCOF). There are an estimated 24,000 certified organic farms and other operations in U.S. that need annual audits, and even more outside the country. Meanwhile, there are only about 80 accredited agencies — some for profit, others nonprofit — that are doing the work for the USDA to certify that products from Whole Foods, Walmart, farmers' markets and elsewhere deserve the government's organic seal of approval. 

It's big business. The Organic Trade Organization claims total food and non-food organic sales last year topped $39 billion. Organic food now accounts for nearly 5 percent of all food sold in the United States, an amazing milestone considering that it's more expensive than food that isn't certified as organic.

Everyone wants in, with many conventional farmers now setting aside a portion of their property to go organic and cash in. "There's so much competition," said Garrett Nishimori of San Miguel Produce, which grows conventional and organic greens like kale and chard. "With the recent growing challenges with the drought, we've seen costs rise and prices kind of come down."

Some are worried about how fast the industry is growing, and whether there's enough oversight. "Sadly, we can't trust the organic label right now," said Mischa Popoff, who used to certify organic farms.

What does "organic" mean?

To become certified, a farmer must start with land that hasn't grown anything conventionally for three years, a transition that can be costly. Farmers are not supposed to use synthetic pesticides or herbicides (though the USDA still allows a few), no genetically modified seeds, and they must meet other standards set out by the government's National Organic Program.