Following the adage that “grapes don’t like wet feet,” Iowa’s wine grape harvest is expected to fall 25 to 30 percent because of near-record rainfall.

The reduction in this year’s harvest follows weak production in 2007 that was caused by a spring freeze, particularly in the southern tier of counties.

Grapes are susceptible to fungi spawned by the wet weather, said Michael White, a viticulture specialist for the Iowa State University Extension Service.

“I’ve already told a number of vineyards to pick their grapes and throw them on the ground,” White said.

Grapes are prone to developing black rot, bunch rot and downy mildew. In addition, grape growers rely on dry conditions late in the summer to drive up sugar levels and lower acid levels in the fruit. Forecasters are predicting a wetter than normal August.

Growers who are spraying fungicides in reaction to the appearance of diseased grapes probably have lost the battle, White said. That’s because disease strikes the fruit early in the growing season but does not appear until midsummer.

“We can always make up for the drop in production,” White said. “About one-half of the wine we make is coming from juice and fruit from outside the state.”

White said Iowa growers harvested about 900 tons of grapes in 2007. It is difficult to forecast the harvest from year to year because new vineyards add to the production after their grapevines are about three years old. Mature plantings, those in at least their fifth year, will produce an estimated 3.5 to 5 tons of grapes per acre.

Currently, more than 350 Iowa vineyards grow about 875 acres of grapes.