The fourth quarter hasn't started out well for the stock market, and Wednesday's declines for major benchmarks were even more severe than Tuesday's. Investors increasingly fear that the U.S. economy is moving toward a recession, and they're concerned that many high-flying stocks will lose their support if overall market momentum shifts downward. In addition, some well-known companies faced significant challenges that created even larger losses in their share prices. Delta Air Lines (NYSE:DAL), United Natural Foods (NYSE:UNFI), and Acuity Brands (NYSE:AYI) were among the worst performers. Here's why they did so poorly.

Delta gets dragged lower

Shares of Delta Air Lines fell nearly 5% after the airline giant warned that its third-quarter sales might not have grown at as quick a pace as previously hoped. Delta predicted that adjusted revenue will come in about 6.5% higher in the third quarter than in the year-ago period. Investors had expected a growth rate closer to 7%, and even though Delta's estimate of earnings between $2.20 and $2.30 per share was slightly more favorable than previously anticipated, shareholders nevertheless expressed their concern. Fundamentally, though, the company saw healthy traffic levels and load factors during September, and anything short of a full-blown recession might well leave the airline looking surprisingly strong in investors' eyes.

Aircraft with Delta markings against a blue sky with thin clouds.

Image source: Delta Air Lines.

United Natural spoils

United Natural Foods saw its stock plunge 26% following the food distributor's fiscal fourth-quarter financial report. The company said that adjusted net sales were up 3% from year-ago levels after taking out the impact of last year's SUPERVALU acquisition, but earnings declined sharply year over year. CEO Steven Spinner tried to concentrate on the long run, pointing to the transformational nature of the past year. Yet calls for adjusted earnings of just $1.22 to $1.76 per share in fiscal 2020 compared unfavorably against fiscal 2019's $2.08-per-share figure, signaling that tough conditions will prevent United Natural from growing its bottom line well into the future.

Acuity sees sales slump

Finally, shares of Acuity Brands dropped 11%. The lighting and building management solutions company saw fiscal fourth-quarter sales fall almost 12% from year-ago levels, with unit volumes slumping 16% year over year. CEO Vernon Nagel blamed "sluggish market conditions" for holding Acuity back, but he was still pleased that the company managed to control costs and keep margin levels healthy. Price increases were able to offset higher tariffs to some extent. Shareholders, however, believe that if recessionary forces hit the broader economy, Acuity could see an outsize impact, and that contributed to the steep decline in the stock.