NASHVILLE, Illinois — Agricultural weedkillers are slowly killing trees across Missouri and Illinois, scientists, state workers and landowners say. Landowners say 200-year-old oaks have gotten sickly. State conservation workers are documenting trees with curling leaves and forests with thinning canopies. Scientists have studied hundreds of trees and found widespread evidence of chemicals in their leaves. The signs are spreading across the region, from farms to conservation areas to some of Illinois’ largest forests. Many affected trees have already died or been logged, and experts and property...