I have a lawn customer that I sprayed some Lesco liquid Prosecutor weed killer for under a Weeping willow tree. The area of the weeds sprayed was about 10x15 ,that same year the tree started dying. I always spray on this property even Canadian thistle that grows in the juniper ground cover with the same product never has kill any of it. What do you think? can this product kill a large tree around 4oz./gal. mixed.