Chris Sayer pushed his way through avocado branches and grasped a denuded limb. It was stained black, as if someone had ladled tar over its bark. In February, the temperature had dropped below freezing for three hours, killing the limb. The thick leaves had shriveled and fallen away, exposing the green avocados, which then burned in the sun. Sayer estimated he’d lost one out of every 20 avocados on his farm in Ventura, just 50 miles north of Los Angeles, but he counts himself lucky.