Only last year, farmers here spoke of a biofuel gold rush, and they rejoiced as prices for ethanol and the corn used to produce it set records.

But companies and farm cooperatives have built so many distilleries so quickly that the ethanol market is suddenly plagued by a glut, in part because the means to distribute it has not kept pace.

The average national ethanol price on the spot market has plunged 30 percent since May, with the decline escalating sharply in the last few weeks.

“The end of the ethanol boom is possibly in sight and may already be here,” said Neil E. Harl, an economics professor emeritus at Iowa State University who lectures on ethanol and is a consultant for producers. “This is a dangerous time for people who are making investments.”

While generous government support is expected to keep the output of ethanol fuel growing, the poorly planned overexpansion of the industry raises questions about its ability to fulfill the hopes of President Bush and other policy makers to serve as a serious antidote to the nation’s heavy reliance on foreign oil.

And if the bust becomes worse, candidates for president could be put on the spot to pledge even more federal support for the industry, particularly here in Iowa, whose caucus in January is the first contest in the presidential nominating process.

Many industry experts say the worst problems are temporary and have been intensified by transportation bottlenecks in getting ethanol from the heartland to the coasts, where it is needed most. And even if some farmers who invested in the plants lose money, most of them are reaping a separate bounty from higher prices for corn and other commodities, which are expected to remain elevated for some time.

Even so, companies are already shelving plans for expansion and canceling new plant construction. If prices fall more, as many analysts predict, there is likely to be a sweeping consolidation of the industry, and some smaller companies could go out of business.

The falling price of ethanol comes in sharp contrast to the rise in crude oil prices. Lower ethanol prices help reduce gasoline prices at the pump, where ethanol is available, but because it constitutes 10 percent or less in most blends, the impact for the consumer is marginal.