ALTO PASS, Ill. — It takes much to test the mettle of grapevines in southern Illinois, where a unique blend of soils, rolling hills and weather conditions usually make for great grape growing.

As the worst drought conditions since the 1930s continue throughout the state, grape growers say they generally will harvest less this year, but it likely could be among the sweetest crops ever. It’s next year that concerns them most.

“We definitely have some damage to our leaves, especially on our younger vines,” said Paul Renzaglia, a winemaker who oversees the family-owned Alto Vineyards. His area has seen barely four inches of rain since February, some 12 inches below normal.

Alto sits within the Shawnee Hills American Viticultural Area (AVA), a region recognized by the federal government for the distinctive grapes grown and the winemaking within it. It also sits within the largest concentration of the 91 wineries throughout Illinois, which state ag officials said contributed about $319 million toward the state economy last year.

While this year’s grapes statewide may be among the sweetest because of the hot, dry conditions, it likely will not bode well for the vines and the crop next year, several growers said.

“It’s just too much heat, too much sun and not enough water, especially for younger vines to do well next year, so that’s a big concern,” Renzaglia said.

One characteristic that is helping is the type of soil where Alto is located. Renzaglia said the sandy topsoil allows for better drainage, while clay about 2-3 feet below helps water retention slightly, giving hearty vine roots access to more of what little rain has fallen in the region.

“That helps a lot for handling this drought,” he said.

At Blue Sky Vineyards in Makanda, winemaker Karen Hand said she’s worried about next year’s crop, too. “The younger, weaker vines will just continue to get weaker in these conditions,” she said.

For this year, though, some varieties of grapes are testing at much higher levels of sugar, a byproduct of less water overall, she explained.

In the central and northern portions of Illinois, growers say some spring frost has been the worst enemy so far this growing season, and continuing drought would likely reduce crop size from 25-40 percent.

Bradley Beam, winemaker at Willett’s Winery & Cellar in Manito, said he’ll likely start harvesting his seven acres of irrigated crop in about a week, two weeks ahead of past years. Frost damage in the spring likely will reduce his crop size by about one-fourth, and the heat since has led to smaller clusters of grapes.

“I wouldn’t be surprised if you saw 25 percent lower yields across the board,” said Beam, who is also an enologist at the University of Illinois Champaign-Urbana.

Further north, growers may be the worst off, said Bill Shoemaker, a viticulture specialist with U of I Crop Sciences in Chicago.

“We will need continuing rain, or I think we will have problems with ripening and the integrity of the fruit,” he said.