It's a secret as dark as the Brandywine Valley houses where 40 percent of America's 844 million pounds of mushrooms are grown — in part because mushrooms are grown inside low, nondescript buildings; in part because of how mushrooms' growing medium literally stinks up Andrew Wyeth's bucolic rural landscapes; and in part because of the fungi's vaguely illicit bulbs and folds. It's a well-kept secret until the second weekend in September, that is, when local mushroom growers open their doors and offer tours as part of the annual Mushroom Festival.

SHROOM BOOM: Workers sort through thousands of mushrooms grown indoors on specially designed rack beds.

Pennsylvania's largest cash crop started more than 100 years ago when a frugal Quaker farmer decided to make profitable use of the dank space beneath his carnation beds. Other farmers followed suit, employing Italian immigrant quarry workers to help with the harvest.

Today, most of the 70 local mushroom farms are owned and operated by the grandchildren of those Italians, who now employ Mexican immigrants, at least four of which have already gone on to start their own mushroom farms, says Christopher Alonzo, a board member of the American Mushroom Institute and president of Kennett Square's Pietro mushroom farm. A Mexican bakery and ice cream shop add spice to Kennett Square's WASPy main street.

Likewise, almost everyone milling about the breezeway of the Kaolin Mushroom Farms headquarters building on the outskirts of Kennett Square one recent weekday was speaking Spanish. Fortunately for me, the grower assigned to give a preview of the free farm tours Kaolin will be offering throughout Mushroom Festival weekend spoke English.

Dennis Melrath spends 40 minutes driving me around in his pickup to visit identical-looking low buildings containing Kaolin mushrooms at varying stages of growth. The process begins by combining hay, manure and water to create a medium mushrooms love but nonfarming Chester County suburbanites reportedly hate. So I was surprised not to smell anything untoward while driving into Kennett Square, or even when bringing a handful of Kaolin compost up to my nose. Melrath says it's because Kaolin is one of few growers that combines and pasteurizes the compost in tunnels instead of out in the open.

At Kaolin, conveyor belts fill the rack beds with spawn, or mushroom "seed"-infused compost, before they're covered with peat moss and plastic sheeting to get the spider-web-like growth to form "pins" that'll eventually bloom into mushrooms.

To create the perfectly round, pure white caps that Americans favor, the growing pins must then be exposed to a mix of temperatures, fresh air and water that sounds every bit as precise and difficult to achieve as Wyeth's paint blends. The farmer's art is further complicated by the recent popularity of convenience products like sliced mushrooms, which require a cap strong enough to hold up to a slicing machine. To insure that each mushroom has enough room to grow strong and round, the same beds are now harvested three times over 25 days.

Melrath offers four reasons why the mushroom industry took off in Chester County: rich topsoil, a temperate climate, a ready supply of manure substrate from local horse farms and proximity to the East Coast transportation corridor. But technological and transportation innovations have had a leveling effect, making local production increasingly vulnerable to competition, especially from abroad.

"The Chinese can get canned mushrooms here for much cheaper than I can make them," says Alonzo. As a result, Pennsylvania farmers have almost completely abandoned the canned mushroom market to concentrate on fresh fungi.

Ironically, there's not a single Philadelphia-area supermarket where you can buy the South Mill brand mushrooms grown and harvested at Kaolin (though they're easy to find in the South, home to all four of the company's distribution facilities). You can find other Chester County mushrooms in Philadelphia supermarkets, but not necessarily under local farm names.

Alonzo says the package of Dole shiitakes I found at the Super Fresh on South Street labeled Kennett Square, Pa., could have come from his farm or one of several other local farms that sell to that California-based company. Philly farmers markets and restaurants serviced by some of the smaller Kennett Square-area farms are your best bet for finding local product, he says, aside from this weekend's Mushroom Fest, of course, which offers consumers a once-a-year opportunity to buy direct.

Festival highlights include farm tours, mushroom bobbing, recipe contests, music and mushroom souvenirs (including "Shiitake Happens" boxer shorts and bottles of stinky mushroom substrate — sealed, of course). The festival even provides refrigerator truck storage to keep purchases fresh until you're ready to take them home.

(cwyman@citypaper.net)