Shares of PG&E Corp. plummeted on heavy volume to a record low Friday, after Citigroup warned that the latest California wildfire, which the utility may have helped start, could render them worthless. PG&E PCG-28.5% said Friday that it filed an incident report with the California Public Utilities Commission (CPUC) regarding the “Kincade” fire, which broke out near Geyserville in Sonoma County.