But judges at these competitions say the more important quality is precision and cleanliness, which can mean the difference between a vine stocked with healthy bunches of grapes and one rife with bunch rot. In general, each of the vines is expected to carry 11 “spurs”  essentially offshoots of the main vine  which are expected to produce two bunches each. But too many spurs can cause overcrowding on the vine, increasing the possibility of potentially devastating diseases spreading from grape to grape. Too few spurs, meanwhile, means not enough product.

“If you screw it up here, you’re going to be paying for it all summer,” said Nick Frey, the president of the Sonoma County Winegrape Commission, which has sponsored the competition for the last seven years.

Mr. Frey said the level of interest and number of competitors had been growing ever since the first competition in Sonoma in 1999. “This is the biggest pruning event we have ever had,” he said. “The quality and speed of the pruning was exceptional.”

Image The contest highlights the importance of pruning to the wine industry. Credit... Jim Wilson/The New York Times

Spectators say the events are oddly riveting, especially for anyone with a passing interest in how wine gets made. Like at most big games, the crowd in Sonoma was predominately male and occasionally rowdy, clapping and cheering as the pruners snipped, hacked and yanked a series of rugged chardonnay vines into shape. Unlike most athletic crowds, however, the spectators here could talk you into the ground about plant biology, soil acidity and the relative merits of cane versus spur pruning. (Don’t even ask.)

One spectator, Rosa Brown, an office manager at the local Kendall Jackson winery, said she had tried pruning once and was impressed by the men’s skills and endurance. (For now, pruning seems to be a male-only sport.)

“It’s amazing what they do,” Ms. Brown said. “I went out there for an hour, and I came back and my arms were sore, my back was sore, my shoulder blades were sore. And they do it eight hours a day.”