PG&E Corp. equipment started more than one fire a day in California on average in recent years as a historic drought turned the region into a tinderbox. The utility’s unsuccessful efforts to prevent such blazes have put it in a state of crisis.

The fires included one on Oct. 8, 2017, when nearly 50-mile-an-hour winds snapped an alder tree in California’s Sonoma County wine country. The tree’s top hit a half-century-old PG&E power line and knocked it into a dry grass field, a state investigation found. The line set the grass...