The images show what appears to be something applied at the base of the trees that also drifted away from the area or rainfall caused it to move in the direction of the downward slope, which apperas to have caused damage to the trees and turf. To determine what may have actually caused the damage you will need to get a history of everything that has been applied over the past 6 months to a year. The active ingredients, rates used, dates, timing, etc. will need to be known. Anyone licensed to apply pesticides must keep a record of these things and must be able to provide it when requested. If, necessary, the Oklahoma State Department of Agriculture, Food & Forestry (ODAFF) are the regulators of pesticide applicators and can provide assistance. You can find information about ODAFF services and submit a complaint at the following link under Pesticide Complaints. 

https://ag.ok.gov/pesticides/