It's not a matter of "recourse' against anyone. Having applied pesticides for 20 years, it's more a matter of setting proper expectations at point of sale. Pesticides, fertilizers or anything applied to a tree is no guarantee of life... Does a doctor treating a patient for cancer guarantee the patient won't die, no way. If the patient does die is there a lawsuit against chemo manufacturer, nope. If there's one fact I've learned in 20 years it's that there are NO "silver bullets". No one (or more) applications of anything can guarantee control. The best policy is to be honest w/your clients, recommend your best available course of action and proceed.