SAN FRANCISCO (MarketWatch) — Shares of walnut snack maker Diamond Foods are cracking under the pressure of an audit probe that’s postponed the closing of its deal to buy the Pringles chip brand.

Diamond DMND, shares sank 20% to $51.48 in afternoon trading Wednesday, following revelations that the company’s audit committee is reviewing an outside letter it received that appears to question how Diamond accounted for payments to walnut growers.

“From our perspective, the onus now lies on Diamond to prove there have been no irregularities,” wrote RBC Capital markets analyst Ed Aaron, one of at least three analysts who downgraded Diamond shares Wednesday.

Aaron lowered his rating to sector perform and cut his stock-price target to $60 from $87. Janney Montgomery downgraded the stock to neutral as did Bank of America.

Wednesday’s stock decline erases $1 billion in shareholder value since Sept. 21, when Diamond shares reached a record high of $96.13.

It also means Diamond would take on more debt from Procter & Gamble PG, +1.26% when the deal closes if its stock trades at current prices.

Diamond would assume even more debt if its stock dips below $44.61 at deal closing. In a statement late Tuesday, Diamond said the deal will close sometime before June 2012. It had been slated for this December.

The transaction was valued at $2.35 billion in stock when it was announced April 5. Diamond shareholders on Oct. 27 approved the issuance of stock to pay for Pringles.

San Francisco-based Diamond owns the Emerald Nuts and Diamond of California Nuts brands. In recent years, Diamond has been making a push to control more of the snacks aisles, with deals for Pop Secret popcorn and Kettle chips.